yzAo>^y^ MORPHOLOGY OF THE WHITE SHRIMP Penaeus setiferus (Linnaeus 1758)
BY JOSEPH H. YOUNG
FISHERY BULLETIN 145 From Fishery Bulletin of the Fish and Wildlife Service VOLUME 59
UNITED STATES DEPARTMENT OF THE INTERIOR FISH AND WILDLIFE SERVICE UNITED STATES DEPARTMENT OF THE INTERIOR, Fred A. Seaton, Secretary FISH AND WILDLIFE SERVICE, Arnie J. Suomela, Commissioner
MORPHOLOGY OF THE WHITE SHRIMP Penaeus setiferus (Linnaeus 1758)
BY JOSEPH H. YOUNG
FISHERY BULLETIN 145 From Fishery Bulletin of the Fish and Wildlife Service VOLUME 59
UNITED STATES GOVERNMENT PRINTING OFFICE . WASHINGTON • 1959
For sale by the Superintendent of Documents, U. S. Government Printing Office Washington 25, D. C. . Price $1 'I'he Library of Coii^'ress has catalo^'ed Fishery J^iilletiii 14o as follows
Young, Joseph Hardie, lOlS- ]Morj)holoa'y of the wliite shrimp Pcnaeus setifcfux (Liii- iiatMis 1758)' ' Washino-ton, \\ S. (lovt. Print, (')tf., V^:^\). iv, 1(>S \). illus. :\0 cm. (I'. S. Fish and Wildlife Service. Fisliery l)iilletin 14.1) ••From Fishei-y bulletin of the Fish and Wildlife Service, \()lume .I!).' P.ihliosraphy : p. ItiO-Kii).
1. Shrimps. i. Title. (Series: V. S. PMsh and Wildlife Service. Fishery bulletin, v. .")9, no. 14.">) STTn.A-i5 vol. 59, no. 145 595.;]84:') 5 Librarv of Congress The series, Fishery Bulletin of the Fish and Wildlife Service, is cataloged as follows: U. S. Fish and Wildlife Service. Fishery bulletin, v. 1- Washington, I^. S. Govt. Print. Otf., 1881-11) V. in illus., maps (part fold.) 23-2S cm. Some vols, issued in the congressional series as Senate or House documents. Bulletins composins v. 47- also nunibered 1- Title varies : v. 1^49, Bulletin. Vols. 1-40 issued by Bureavi of Fisheries (called Fish Connnission, V. t-28) 1. Fisheries—U. S. 2. Fish-cultui'e—F. S. i. Title. SH11.A25 639.200173 9-35239 rev 2* Library of Congress [r-lHe-y,,] CONTENTS Introchietion _ _ _ _ 1 PetioeiiN as comparative tnatcrial ._ 2 I. Skeletal and inuselc systems. _ __ _ _ 3 A. Protoeephalon _ _ . _ 3 1. Kyesf.ilks_ .. _ _ _ _ 7 2. Aiitenmiles _ _ 17 3. Antennae ______30 4. Labnim _ _ _ _ _ 43 B. (lnathotliorax_ . _. ______47 1. Mandibles ______60 2. ParaKnatha ______. _^ _ .. __ 65 3. Maxillae ______, _ 66 a. First maxillat^ ______66 b. Second maxillae ______, _ 69 1. Maxillipeds _ 72 a. First maxillipeds __ ___ .. . _ 72 1). S(>cond maxillipeds_ _ _ _. 74 c. Third maxillipeds 81 5. P(>reiopods 87 a. First i)er(>iopods______87 b. Fifth pereiopods _ 94 C. Abdomen 103 1. Pleopods___ _ 115 2. Tail fan 125 11. Nervous system _ 132 A. Nerves of Supraesophageal t2;aiiglion and tritocerebrum 135 B. Ventral nerve cord _ 139 III Circulatory system 140 A. Heart and pericardium 140 B. Blood vessels of the body 140 C. Appendicular blood vessels 148 D. Venous system 149 E. Respiratory system 149 IV. Digestive system 150 A. Foregut _ _ 150 B. Midgut___ _ 154 C. Hindgut 154 V. Excretory system 155 VI. I{(^productive system _ 1 55 Literature cited _ _ . 160 Bibliography ______162 ABSTRACT In this illnstmted morphology of the coininerclally important white shrimp of the (Jnlf of Mexico the muscle, nervous. circulat(n-y, excretory, reprodxictive. and respiratory systems are compared with those of the blue crab. Cdllinertc.-t. a European crawfish A.sf«c».s". the "Coon-stripe" shrimp, PandaJiiH. and other decapod crustaceans. The major portion of the comparative work deals with the muscles, since the muscle systems of a few Decapoda have l)een reported in much sireater detail than other isystems. The comparative studies of muscles and nerves indicates that the I'enaeldae I'epresent a generalized anatomical condition in the Crustacea Decapoda. thus verifying the sys tematic research in this area. Evidently the Penaeidae are relatively close to the decapod stem in the Malacostraca. Tlie generalized condition of Penaeidae is shown again and again by the repetition of functional muscle units. The same units have become simplitied in the higher decapods, having been lost, presumably, by the fusion of separate parts. Adhering to the morphological principle that the nerves tend to retain their ancient innervations despite coiilescence of parts, shifting of muscle origins, etc., and can there fore be considered as morphologically conservative, the nerves to the repeating muscle units of Pcitdcufi are found to have kept their innervations to the same nuiscles, now fused, in higher Decapoda. The comparative morphology of decapod nerves and circu latory elements is treated only to the extent that research on these systems in other decapods has been published. Several structures are found in Penaciis setiferus which have not been reported previously in the literature. A fibrous circulatory element, the capillary arbor, pene trates the distal optic ganglia. One or more hard concretions embedded in the substance of the antennal portion of the excretory gland are described. Although two pairs of muscles are associated with the labrum of Insecta, muscles have not been described in the labrum of Crustacea. The labrum of Pciiaeus has at least 12 pairs of muscles. A structure, the hindgut gland, is found in the anterior part of the sixth abdominal segment lying dorsal to the I'ectum. Its functicm is unknown. Some of the blood vessels of the heavily vascularized branchiostegal region of the carapace run parallel to the margin of the carapace, suggesting "growth rings" by their appearance. MORPHOLOGY OF THE WHITE SHRIMP PENAEUS SETIFERUS (LINNAEUS 1758) By JOSEPH H. YOUNG, Department of Zoology, Tulane University rouieiis sefifevNs, tlie white shrimp of tlie (nilf a map. the value of an anatomical study to its of Mexico, re])i'eseiits ;in ini])ortaiit ('oni])()neiit users hinges upon its accuracy and clarity. To of the coiinnercial slii'im]) catcli throiiofhout the these ends all etforts were bent, the illustrations iioi'thern, western, and southern niar NOTE.—Approved for publication April 22, 1058. Fishery Penn, all of the Department of Zoology, Tulane Bulletin 145. Report of research done under contract between University. Percy Viosca, Louisiana Wildlife Bureau of Commercial Fisheries and Tulane Universit.v (Salton- stall-Kennedv Act). and Fisheries Commission; Charles Dawson, of FISHERY BULLETIN OF THE FLSH AND WILDLIFE SERVICE tlie Hoars TMntl' Laboi-atories, Wadinalaw Island, PENAEUS AS COMPARATIVE MATERIAL S. C.; and Dr.- ^lilton P"in<»-erinan, T)e])artinent of Zooloay, Tnlane T^niversity, have all made nsefnl For ])Ui'poses of com])arative morphology, the connnents al)ont vai'ions as])e('ts of the work. T Penaeidae enjoy a unif|ue position. Evolution am indebted to Prof. TJ. 11 TToltlinis, Rijk'smnse- has l)ronght them down to us in an ai)])ai'errw>' nm van Xatnnrlijke Tlistorie, Leiden: Dr. Andre genei'alized deca])()d condition. Xaturally, we Afayrat, T>aboratoi]'e de Zooloo-ie de TEcole Xoi'- nnist vieAv with caution any attem])t to force a male Sn])erienre, Pai'is; and Jerome E. Stein. given structure or organ into the generalized Texas A. and ^f. Colleire, (lalveston, Tex., for category, for all extant animals must be highly lie1])fnl criticism of certain ])arts of the stndv. s])ecialized in specific instances, if imspecialized Heartfelt thanks a'o to the project artist, Ray in others, for life today. Furthermore, the so- mond Hollino-er of Xew Orleans, I^a., for tlie hiofh called generalized structure may be the super (juality of the plates makin^- np the anatomical ficies of a well-hidden s])ecialization. Bearing in stndy of the Avhite shrimj). Of the 13() tio-ures in mind, then, that there may be no such thing as a this work, all but 10 were tinished by ^fr. Bol- generalized structure in a modern s]:)ecies, the lin<2:er from tracino-s made by the investi AA'luMi coiupai'ed extci'imlly witli the ci'iistaccaiis rename the muscle in each case in accordance Avith iiu'iitionod ((i<2;s. 1, 2, ;>), PCIKICII.^ xctifcriix is most its specific function, which will tend to com])ound -iiiiilai- to Pdixhiliis (httmc. a carideaii prawn, and the existing confusion in morphological nomen- less like Ai § Si) I I ^ .i- §Hi ^\ 11 •^1 <.1 > '^^ < 1-k^ N 0) 3 ^ Co •d •K-*, si ^w O "^•N «1 •». \1 •--^ ~^ ^'•'^ I 11\ : ^•~ 1 s is Q t) t)« •< ll' v~ sS , V! 0 1 ^ dl' X ,M^ 1)) r 0) h WHITE SHRIMP FROM THE GULF OF MEXICO cs -C 0 s; ll 1 & (3 0 I I -43 t) -Jf? ^A/'/ .••//'',' to '•1 L. 1 ''SK-C - i \ s <; -5 « S3 Cl tf s •rst •^ a! -" •$• ••; \§ 0 * a % 0> Q "^ "^ 1 ftv, (N s U a. P ffl ,---"'^. 4 £ .- ^ ~l 5 S-^ s%^ I O t3 C !b * C -^ :(K* CJ FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE ~?3 Q :5 I u If) Q I I I 5 6JJ a o I •I 0 .51 St 6 WHITE SHRIMP FROM THE GULF OF MEXICO, The protocephaloii is clearly distinct from the The presence of a set of muscles to move the succeeding gnathothoracic and abdominal tag- corneal surface of the compound eye on the eye- inata, and in Penaeus setiferus^ and other species stalk, and of muscles to move the eyestalk about of the genus (Grobben 1917) is independently with respect to the body suggests the importance, movable. Grobben, incidentally, described the of the position of the corneal surface relative to area as the "sincipit,-' a term apparently discarded the environment. By contrast, adjustments of by Snodgrass in favor of the term, "proto- corneal position in an arthropod without eye oephalon." In the section below on the eyestalks, stalks suggests a function of head and "neck" ii consideration of the muscles which move the muscles for activities other than feeding, if we protocephalon on the gnathothorax will be found. assume any importance to the arthropod of cor A discussion of the primitive order of the head neal position adjustments. segments follows in the section on the labrum. Recently, the eyestalk nerves of a few crusta ceans have been shown to contain neurosecretory SKELETAL ELEMENTS elements which evidently proliferate hormone The protocephalon is constructed roughly on the systems controlling such processes as molting ])]an of a vertically placed hemisphere confluent (Passano 195f3), retinal pigment migration posteriorly with the gnathotlioracic hemocoele. (Welsh 1941), and chromatophore movements The skeleton is flattened dorsally in the region of (Perkins 1928). In view of the concentrated at the ocular plate (fig. 4) or lobe from which the fo tention currently being paid to matters of neuro- ramina of the eyestalks are perforated. The large, hormonal control of physiological functions in paired foramina of the antennules and antennae the arthropods, an understanding of the relation and the mesal foramen of the labrum occupy al of vision to neurosecretion appears to be near at most the entire anterior and ventral surface of the hand. protocephalic skeleton. The latter regions are The white shrimp, Penaeus setiferus^ carries its lieavily sclerotized and deeply indented between eyestalks at an angle of about 75° to the median the antennular and antennal foramina by the sagittal plane and at an angle of about 10° to 15° medial stem of the Y-shaped epistome (figs. 28, to a frontal plane at the ocular plate (fig. 4). A; 30). The top, or posterior portion of the epis- Only rarely are the eyes brought forward to lie tomal Y, divides across the anterior face of in the optic depressions of the antennules, and then tlie labral foramen and the posterior side of the but for an instant for protection or possibly clean large antennal foramina. Upon the lateral epis- ing against the long plumules surrounding the tomal bars, as shown by Snodgrass (1951), the depressions. Normally, therefore, in P. setiferus medial mandibular condyles are located. and many other species of shrimps, the eyes and stalks are widely spread and slightly upturned, a 1. EYESTALKS situation not understood by morphologists who, Most of the following section on the eyestalks working with preserved materials, have described was previously published by the writer (Young the eyestalks as projecting anteriorly. Had pre 1956). vious workers taken into account the lateral posi Tn some of the lower and in many of the higher tion of the eyestalk in the shrimps, and for that (^rnstacea, the compound eyes are set upon mov matter in the crawfishes, a certain amount of con able stalks or peduncles. Their presence at the fusion in the naming of eyestalk musculature ends of extensions has excited speculation for might have been avoided. For in fact, medial many years. Carcinologists have long discussed muscles are anterior and lateral muscles are pos the reasons for the eyestalks, thqir similarity with terior. In the interests of uniformity, however, other appendages (Caiman 1909), and the na certain of the incorrect names are here employed. ture of vision in a stalked-eyed animal, among P. setiferus is a bottom feeder. According to other things. Yet little has been written about the an unpublished observation by Charles Dawson, mechanics of the eyestalk with respect to vision. Bears Bluff Laboratories, Wadmalaw Island, Xo one has proposed any useful explanation for S. C, schools of penaeid shrimps are frequently the fine adjustments presumably available to a found on muddy bottoms. This worker describes compound eye which has as numerous oculomotor placing several P. aztecus Ives, 1891, in aquaria muscles as the crustacean stalked eye. with an inch or two of mud on the bottom, into FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE which the animals immediately burrow, except for The stalk is movable upon the short, boxlike, the eyes. Such behavior suggests that the long basal segment in the horizontal plane. Vertical eyestalks are among the organs enabling the pe- movements between the basal segment and the naeid shrimp to make use of mud for protection, stalk are restricted. With respect to the structure especially after molting. here labelled the median tubercle (fig. 4), Ander In the past, observers have described square son and Lindner (1943) and Voss (1955) state corneal facets in the eyes of several species of that shrimps of the subfamily Penaeinae have no decapod crustaceans (Huxley 1906; Caiman, distinct median tubercle on the ocular peduncle. 1909). A study of slides made of the corneal However, many of the shrimps of this group do cuticle shows that the corneal facets in the com possess large, blunt, median tubercles, similar to pound eye of Penasus setiferus are also square. those in Penaeus setiferus. Likewise, the underlying crystalline cone cells are Set between the basal segments is the ocular square in the white shrimp and total four per om- plate or lobe, a broad, roughly rectangular sclero matidium, as determined by the study of tangen tized structure which encloses laterally the medial tial sections of the eye from which the corneal parts of the basal segments (fig. 4). The ocular cuticle had been removed. Ramadan (1952) re plate is the dorsal-most region of the protoce- ports a similar situation in a species of Metape- phalon. Movements between the basal segment naeu8. In longitudinal section the ommatidium of and the ocular plate are similar in extent to those P. setiferus is like that of Astacus^ as described by between the stalk and the basal segment. Hori Bernhards (1916), with comparatively elongate zontal movements are limited to an arc of about crystalline cone and short rhabdom cells. If any 15° or 20°. thing, the cone cell is longer in the white shrimp than in the crawfish. A light pink substance MUSCLE ELEMENTS which gives the dark-adapted shrimp eye its PROTOCEPHALON MUSCLES OF THE OCULAR bright color in strong light is the tapetum (Ram REGION adan 1952). It is associated with the proximal or retinal pigment of the ommatidia. Taking origin from either the epistomal invag ination or the dorsal surface of the carapace and SKELETAL ELEMENTS inserting upon basal parts of the eyestalks are four pairs of muscles. The basal regions of the The ommatidial surfaces arise from a sclero- eyestalks will be assigned here to the dorsal part tized cup, previously named the optic calathus, or of the protocephalon. basket (Young 1956), to avoid confusion with the optic cup of the vertebrate embryo (fig. 4). The ANTERIOR PROTOCEPHALON LEVATOR MUSCLES optic calathus rests upon the elongate stalk seg FIGURES 34, 35 ment in a structural relation permitting univer sal movements, although the degree of movement The tiny anterior protocephalon levator muscles varies in different planes. are probably the muscles designated by Grobben Two points of articulation in the dorsoventral (1917, 1919) as the protocephalon levators (mus- plane allow the optic calathus considerable hori culus levator sincipitis) in a European penaeid zontal movement around the distal end of its sup and in other species of higher Crustacea. These porting stalk. These dorsoventral hinges are, muscles are difficult to make clear, either by dis however, sufficiently loose to permit vertical and section, or by illustration, since they take origin rotational calathus movements, but to a lesser ex on the carapace, on the nearly vertical sides of the tent than horizontal movements. The long stalk is rostral base. During removal of the carapace and comprised externally of several longitudinal the underlying layers of tough, fibrous epidermis sclerotized bars which are separated by pliable and connective tissue, th^se muscles are torn cuticle. Two of the bars give support to the dorso away. The anterior protocephalon levators insert ventral points of articulation (fig. 11) and others in the heavy connective tissue associated with the to less well-defined points of articulation between posterior edge of the protocephalon. Their actual the stalk and calathus, and between the stalk and levation of the protocephalon is negligible, since basal segment. they are not only minute in cross section, but WHITE SHRIMP FROM THE GULF OF MEXICO 9 / / compound ec^'G ro^rru/?7 0/V/7]<7f/0/a op//c ./Z'ec/'/a/? ca/a/hus-~\ /uberc/e d'orsa/ ,Qpr/c an/enncf/ ocu/ar jp/r/€ p/ate sea/-77en iUncy/e ^ro5 frc^/^pme f FIGURE 4.—Dorsal view of eyestalks in anterior position. Rostrum cut away to show ocular plate 10 FISHERY BULLETESr OF THE FISH AND WILDLIFE SERVICE short in length. No counterpart of the anterior and Callinectes than are the posterior proto protocephalon levator muscles was described by cephalon levator muscles. If the latter is true Schmidt (1915) in ^5?'a POSTERIOR PROTOCEPHALOX LEVATOR MUSCLES OCULAR PLATE DEPRESSOR MUSCLES FIGURES 5, 6, 34 FIGURES 5, 6, 7 The function of moving the protocephalon dor- The ocular plate depressor muscles originate on sally is performed by a pair of large muscles, the posterior surface of the epistomal invagina the posterior protocephalon levator muscles, which tion. They run anterodorsally, passing beneath originate close together at the dorsal midline of the insertions of the posterior protocephalon the carapace somewhat posterior to the origin of levator muscles, and insert broadly on the pos the anterior protocephalon levator muscles and terior edge of the ocular plate (figs. 5, 6, 7). On which run forward and downward to attach on contraction, the ocular plate depressors draw the a nearly vertical transverse plate, posterior to ocular plate posteriorly and ventrad. Eased on the postocular region of the eyestalk base (fig, 6). the attachment points of the muscles in Penaeus The muscle inserts ventrally to the insertion of setiferus, they may have given rise by partial the anterior levators. The contraction of the fusion to the anterior eye base muscles (musculus posterior protocephalon levators may also act to oculi basalis anterior) as they are found in the rotate posteriorly the eyestalk base and hence European crawfish, where the muscles are attached raise the extended eyestalks. ventrally to the epistomal region by a long tendon Possible homologs of the posterior proto and run dorsad to the edge of the eyestalk base. cephalon levator muscles are the median dorsal Cochran (1935) describes in Callinectes a pair of muscles designated as the musculus oculi basalis anterior eye base muscles which arise from a kind posterior. In Astacus^ ^chmidit (1915) found that of epistomal invagination rather like that in the these muscles arise on the median dorsal surface white shrimp, but instead of fusing as in the Euro of the carapace and are attached by short tendons pean crawfish, they diverge slightly laterad in the to the much longer tendons of other, more an blue crab in probable accompaniment w4th the teriorly placed muscles, the musculus oculi basalis general broadening of the body in the Brachyura. anterior. The anterior eye base muscles, to an- The ocular plate depressor muscles apparently glisize freely, are attached to the median dorsal are homologous with the musculus oculi basalis region of the eyestalk base (Schmidt 1915). anterior in Pandalus, the name for which Berkeley More will be said of the latter muscles below. (1928) has taken from Schmidt (1915). In The posterior eye base muscles, it should be Pandalus, these muscles are similar to those in emphasized, do not attach to the eyestalk base in Penaeus, except that they are slightly separated, Astacus, but if the assumption is made that, due whereas in P. setiferus they are close together. to the immovable protocephalon in Astacus, the PROTOCEPHALON ATTRACTOR MUSCLES attachment of the muscles to the eyestalk base has moved in that animal to the tendons of the FIGURES 5, 6, 7, 8 anterior eye base muscle, then a homology with The protocephalon attractor muscles are two the posterior levators in the white shrimp may be very large muscles which take broad L-shaped proposed. However, the extensive rearrangement origins on the carapace and run anteriorly to in of muscle attachments upon which the assumption sert on two pairs of large apodemes and on other is based weakens the proposal. parts of the protocephalon. The largest apodeme, Even more significant, muscles exist in Penaeus on w^hich the ventral-most part of the proto setiferus, as will be shown later, which are more cephalon attractor muscles inserts, arises from the likely to be the homologs of the anterior and ventral surface of the antennular foramen, posterior eye base muscles in Astacus, Pandalus, broadening posteriorly into a large vertical sheet WHITE SHRIMP FROM THE GULF OF MEXICO 11 aop // c asa/ sef/??€/?/ oesophat^ea/ ex /ernai har musc/es. of anfennu/ar for-amen pQikr/or ~^:—.•::::i-=^ ep/sfomq/ pro/ocepna/on \\ ^ fa for fevafor __ ^776^5- c/ej-—-=-•=•—"^^— oca/arp/a/e ckpre.S'Sor musc/e^s profocepf)£?/on <:7/trac/or musc/e FIGURE 5.—Dorsal view of protocephalon, carapace removed, to show muscles of postocular region. 12 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE compou/fc/eye op//c i^ar?(^//cyen ca/amu rqfafor /a/era/ cahfhus re/r-ac/or ^^miy musc/e depressor musc/es. /evafor exfer/7a/ hcrr o/ anfe/'tnu/ar ^ ep j/cT/o/ p rofo c ep/i a/on ocu/c?r p/afe o es op ncc^ec/ profocepila/on depressor . a/rracfor musc/e /VO'SC/Q FIGURE 6.—Dorsal view of left eyestalk. Dorsal cuticle and carapace removed to show muscles. r WlilTK SllKIMP KKOM TIIK GUT.F OK MEXICO 13 CO/7/POL-Dm DO una op-'ic ac/nc:?/ion'Ic ahducfor ^ rote?/or /vU'Sc/e c/br^o/ ca/afnu^s •^ ven/ra/ ca/o'/nu-s re/rac/Qr zaafnus ocu/omofor ro/cr/or branches .VL. ~^^^ ' .'-•.'-Arc/c.'.j D'-fJ- 0 j-cvf ex/<. -'ky/ oc/' a/ •C rr/.-nC!:- ^ OrC'ccep'>^'c-'r; a.''rfsrcrGf /"nc/SC/e FiGtJBE 7.—^Dorsal view of left eyestalk. Dorsal muscles removed to show branches of nerves and arteries. 468059 O—59 2 14 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE compounc/ ec/( op//C (7(7ncy//on \. \ \, ca/a//7u-s _, 'ZZi-^oca/omo-for re/racfor ^,^'''''' branc/ies .'77ujc/e— w,'n'<^an yenfra/ co/afnus re/racfor^ mi/sc/e /a/era/ ca/a/jhu^ refracfor /77 CAS-c/e-—''"^ ocu/omofor .6 ner ve^-*^* rc7/n exferna/ bar of ^cerebr/?/ 6ri7nch anfeonu/or of /atera/ ariter/or foramen orfery c/rcur77oe^opna^eaf c'onnecr//ve Jaferaf anter/or pn^focephafon £?ffracfor mu^cfe arfert^ FIGURE 8.—Dorsal view of left eyestalk. Dorsal muscles and optic tract removed to show ventral muscles and branches of nerves and arteries. WHITE SHRIMP FROM THE GULF OF MEXICO 15 of cuticular material. Slightly anterodorsal to carapace are antennal is probably in error. From the antenniilar apodeme, in the same parasagittal his iiguies the external-most muscle is properly a ])]ane. is an apodeme which invaginates from the i^emotor of the antennal scale, while the large in ventral floor of the basal segment of the eyestalk ner muscle is the protocephalon attractor muscle- and projects throngh the ventrolateral side of the The two anterodorsal parts of the protocephalon eyestalk foramen into the thoracic hemocoele. attractor muscles which find insertions in the eye- T]\\s apodeme, like that near the antennide, stalks have, in addition to attraction, the function broadens vertically. By virtue of apodemal posi of eyestalk adduction. Upon contraction of the tion, that ])art of the protocephalon attractor whole muscle, these dorsal fibers in the eyestalk iiniscle attaching n]ion the eyestalk apodeme is draw the ocular plate and attached eyestalk seg somewhat longer than is the part inserting on the ments posteriorly toward the carapace. The pos antennular apodeme. terior side of the basal segments makes contact The longest and most dorsal part of the proto with a condylic thickening on the anterior edge of cephalon attractor mnscle extends anteriorly be the carapace, at a point known as the orbital angle yond the antennular and eyestalk apodemes, to in (fig. 4), thereby swinging the eyestalks forward in sert slightly laterad in coimective tissue at the a horizontal plane into the ocular depressions on ventral surface of the basal segment of the eye the anteninules. As suggested in introductory stalk (figs. 7, 8). comments, the movement is a quick one, much To these compai'atively huge protocephalon faster than the return of the eyes to the spread muscles we may ascribe at least two functions, position. In Penaeus setiferus other muscles exist namely, (1) attraction of the protocephalon, and which function to adduct the eyes, but their effect (2) adduction of the eyes. The largest of the three is negligible when compared to that of the much inserting bodies of the muscle is the ventral-most larger protocephalon attractor muscles. j)art inserting on the antennular apodeme, de The protocephalon attractor muscles appear in scribed above. From a study of the points of ar Pandahi,s, designated by Berkeley (1928) as the ticulation between the carapace and the proto- depressor muscles c of the antennae, on grounds cei^halon, dorsally, and the mandibular segment of their attachment to the basipodites of those and the protocephalon, ventrally, the primary re structures. At the same time this worker ascribes sult of contraction of the ventral muscle body to the depressor muscles c the function of adduc would be to draw the protocephalon directly pos tion and rotation, rather than the depression of terior. The muscle was described as a proto the antennae. Berkeley's name for the muscles cephalon depressor (musculus depressor sincipitis) obviously was taken from the work of Schmidt by (xrobben (1917) in a number of Decapoda (1915) on Astacus^ in which form the antennal and in the stomato])od Squilla. This worker has depressor muscles c, while small, nonetheless de shown that the protocephalon attractors, or de- press the antennae. Although proof must wait l)i'essors, are missing in those forms, like A.stacus. upon a study of the nerves in Penaeus and Pan- in wliich the protoce]:>halon is immovably fused dalus^ Berkeley has homologized the so-called de to the thorax. The point of (irobben's discussion, pressor muscle G of Pandalus and Astacus on the that the protoceplialou is movable on the gnathal basis of their dorsolateral origins on the cara tagma in the generalized crustacean, is not pace and their insertions on the mediodorsal edge clianged by a ditferen.ce of opinion over the func of the antennal basipodite (in Pandalus) and tion of the muscle under consideration. coxopodite {iwAstamos). Furthermore, these muscles are not antennular That the depressor muscles c in Pandalus ill any way, having, as brought out by Snodgrass and the protocephalon attractors in Penaeus are (1951), origins on the carapace. They are also, homologous seems fairly certain, despite the to say the least, not strictly antennal, since a por apparent change of insertion in the former. A tion of the muscle inserts in the eyestalk. Al review of cleared and stained exoskeletons of though the protocephalon attractor muscles in Pandalus might show multiple insertions of the Penaeus hrasiUensis Latreille, 1817, may be widely muscle as in Penaeus. The homology of the pro different from those in P. setiferu-'>^ the state tocephalon attractor muscles in Peneaus with the ment of Knowles (1953) that the two muscles ly depressor muscles c in Astacus is less certain. ing just beneath the anterolateral side of the In CalJinectes, Cochran (1935) figures a pair of 16 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE ocular attractor muscles which originate on the located in the distal end of the long stalk seg carapace. Their phylogenetic relation to the ment. Phylogenetic relationships of the posterior protocephalon attractor muscles in Pen-aeu.s is basal segment adductor muscle are even more un unlikely. certain, although possibly it is the same muscle OCULAR PLATE MUSCLES as the ocular attractor muscle in Panddlu.s and A.'itacui^. The basal segment adductor muscles do Arising in the ocular plate or postocular region not appear in Callinectes. dorsal to the brain are several pairs of muscles and a muscle group. Some of these muscles in BASAL SEGMENT LEVATOR MUSCLE sert inside and some outside of the ocular plate. FIGURE f> OCULAR PLATE COMPRESSOR MUSCLES The basal segment levator muscle originates at the anterodorsal corner of the ocular plate and FIGURES 6, 7 runs ventrally to the connective tissue and apod Attached about the shallow anterodorsal groove emal cuticle on the ventral surface of the basal of the ocular plate is a group of muscles which segment (fig. 6). In the normal spread condition runs to the lateral wall of the ocular lobe (figs. of the eyestalk, contraction of the muscle tends to 6, 7), the ocular plate compressor muscles. They raise the basal segment and with it the extended function to draw the sides of the head lobe and eyestalk. ocular plate mesad, and to depress slightly the center of the ocular plate. BASAL SEGMENT MUSCLES In the functional descriptions of the muscles ANTERIOR BASAL SEGMENT ADDUCTOR MUSCLE which follow, the eyestalks will be considered as FIGURE 6 in their lifelike, lateral positions. The anterior basal segment adductor muscle BASAL SEGMENT ROTATOR MUSCLE originates on the ocular plate dorsal to the brain and attaches to connective tissue and apodemal FIGURE 6 material in the ventral part of the basal seg The basal segment rotator muscle is a short, ment (fig. 6). Contractions of the muscle turn broad structure originating on the anterodorsal the basal segment toward the ocular plate in a edge of the basal segment and inserting on the horizontal plane. anteroventral edge of the same segment. Upon POSTERIOR BASAL SEGMENT ADDUCTOR contraction, the muscle pulls the dorsal surface of MUSCLE the basal segment anteriorly, thus rotating the FiGUBE 6 entire eyestalk forward. The posterior basal segment adductor muscle in EYESTALK DEPRESSOR MUSCLES serts in the basal segment at the same point as the FIGURE 6 anterior basal segment adductors, but originates on the anterior side of the vertical transverse plate Two very small muscles, the eyestalk depressor posterior to the postocular region (fig. 6). It, too, muscles, one slightly lateral to the other (fig. 6), draws the anterior edge of the basal segment to function to draw the eyestalk ventrally. ward the ocular plate. The origins of these EYESTALK MUSCLES muscles are so widely separated that we may con clude that they have never been the same muscle. EYESTALK ABDUCTOR MUSCLE How the basal segment adductors may be homolo- FIGURE 6 gized with muscles in Pmidalus and Oallinectes^ in which forms no knowledge of muscle innervations All of the muscles of the eyestalk and optic exists, will be speculation. The ocular adductor calathus are associated with retraction and rota muscles of Astacus and Pandalus may well be the tion of the optic calathus on the eyestalk, except homologs of the anterior adductor muscles of the long eyestalk abductor muscle (fig. 6). The Penaeus^ but hardly with the ocular adductors of proximal end of the eyestalk abductor muscle is CaUinectes^ in which animal the muscles are attached in connective tissue in the ventral region WHITE SHRIMP FROM THE GULF OF MEXICO 17 of the basal segment. The muscle runs the length face of the eyestalk. One part of the muscle is of the eyestalk to insert in connective tissue near long and slender, while the others are short and tlie dorsal calatluis retractor muscle. Contraction arise from broad origins (fig. 9). The muscle is of the nniscle swings the eyestalk horizontally to inserted over a wide area on the ventral edge of a lateral ])Osition. The eyestalk abductor muscle the calathus. of Petmeu'i is very likely homologous with the ab ductor muscle described for A>>tacuH and Pandalus. MEDIAL CALATHUS RETRACTOR MUSCLE r.nd possibly with the lateral branch of the ocular FIGURE 6 , abductor muscle in Callinecte^. The medial calathus retractor muscle originates CALATHUS RETRACTOR MUSCLES on two points in the region of the median tubercle, and actually is comprised of two muscles (fig. 6). The muscles in FenaeuH which retract the optic The larger muscle originates in the median tu calathus appear to be clearly represented by the bercle and inserts in connective tissue dorsal to retractor muscles of the eyes of AstacKfi. Camharus the distal optical ganglionic mass. The smaller hartoni (Fabricius 1798), PandaluH, and Cal- muscle originates dorsal to the larger muscle and linectes. Phylogenetically, the situation in Pe- inserts on a ventromedial point on the calathus. naeus is somewhat more generalized than in the The contraction of both muscles results in medial other forms w^hich we are considering, in that sev retraction of the calathus; functioning in opposi eral of the calathus retractor muscles in Penaeu-s tion, the muscles retract the calathus in a vertical have more than one part. In addition, Penaeus plane, reinforcing the action of the dorsal and lias a number of apparently independent rotator ventral retractor muscles. muscles, none of them previously described, which function to tw4st the optic calathus about a longi CALATHUS ROTATOR MUSCLES tudinal axis through the eyestalk. FIGURES 6, 7 DORSAL CALATHUS RETRACTOR MUSCLE At least three calathus rotator muscles may be FIGURES 6, 7, 8 seen in the eyestalk of Penaeus setiferus. Rotator muscles of this type have not been described for The dorsal calathus retractor muscle arises in Pandalus, Astacus, Camharus, or Gallinectes. The connective tissue near the ventral surface of the calathus rotators bear a certain similarity to one eyestalk and attaches to the dorsal edge of the another, in that they are all superficial in position calathus. and insert in the heavy connective tissue under LATERAL CALATHUS RETRACTOR MUSCLE lying the thick cuticle of the calathus. FIGURES 6, 7, 8, 9 2. ANTENNULES The lateral calathus retractor muscle, really the The antennules, or first antennae, are said to posterior retractor, originates on sclerotized mate belong to the second segment of the body of Crusta rial along the lateral, or actually posterior, blood cea, following the eyes. In the company of the simls running the length of the eyestalk. The eyes, the antennae, and the labrum, the antennules larger portion of this muscle attaches on the are attached to the body tagma that has been term lateral edge of the calathus, the lesser part turning ed by Snodgrass (1951a; 1952) the protocephalon. ventrally and running across the ventral edge of Whereas the appendicular nature of the eye the calathus, just dorsal to the ventral retractor stalk generally has been questioned, the status muscles (fig. 9). When this muscle contracts it of the antennules as true crustacean appendages not only retracts the calathus, but rotates the cal has rarely been attacked, despite controversy over the homologies of the component segments. athus about an axis longitudinal to the eyestalk. The antennule of Penaeus setiferus is comprised VENTRAL CALATHUS RETRACTOR MUSCLE of a proximal stem divided into three basal seg ments, called the protopodite by Huxley (1906), FIGURE 9 and two distal flagella. Proponents of theories The ventral calathus retractor muscle originates such as Huxley's (1906) suggesting correspond on several sclerotized regions on the ventral sur ence of parts between the segments of the anten- 18 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE CQ/npOOr/O CC?D///ar'W c7/- DCr / ocuiomo/vr branch / / ^ / / coO'//aru a/OQr / / / ' / / If //' ei/-e.:^/c?/A .DC re refracfor . DraA"i -cereDro,'/ branch oi'/orenjf an/e/ior ar/erQ c/rcuynoe-JOp/7c?Gea/ conr}ec//\/e ex/er/7£f/ DO/' CJ/ / •'a/era/anterior anfe.o/7u/or foramen arferu FiOTiRF. 9.—Dorsal view of left, eyestalk. Dorsal muscles and optic tract removed to show brain, branches of nerves, arterial capillary supply to distal optic ganglia, neurosecretory glands, and location of anterior eyestalk pore. WHITE SHRIMP FROM THE GULF OF MEXICO 19 opf/c acnc^//or comp ouno eue^ _^ an/en-or, fa/A a/and -C ec^-e^ pore Wf rt^an ' ""~ X-Qigat)? ocu/omofor -=^ocu/omofor nerves -^=i=:r:ir orancnes cpfjc -^ Drain Ye/y c/rc^jmoejopha<:^ea/ connec/fVB ex/er/ia/ 6ar of /a/era/ anterior anfennu/ar foramen artery FIGURE 10.—Dorsal view of left eyestalk. Muscles removed to show optic tract, oculomotor nerves, neurosecretory glands, and branches of lateral anterior artery. 20 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE phology this feature of similarity would appear to simplify the process of liomologizing part to ])art, and does, except for the matter of muscle names, a problem discussed earlier. SKELETAL ELEMENTS In dorsal view the antennule is shaped in a long wedge pointing anteriorly, the broadest portion being the proximal part of the first segment at the region of attachment of the antennule with the skeleton of the body. Each successive distal seg ment gradually becomes narrower. The mesial edge of the basal segments is straight and flat where the appendage rests against the other an tennule. The first, or proximal, antennular seg ment (fig. 12) is relatively larger and more com plex than the segments distal to it. The first seg ment is attached by a large articular foramen to the protocephalon, the tagma to which the anten- nules belong. Here, dorsal and ventral points of articulation, or condyles, afi^ord the first anten FIGURE 11.—Dorsal view of left eyestalk. Diagrammatic. nular segment limited horizontal movement. The skeletal bars of the eyestalk are so arranged that Centered in the posterior region of the first an the position of the ommatidial surface is maintained tennular segment is the statocyst, the organ that whatever the angle of the eyestalk with respect to the is thought to mediate the special sense of equili long axis of the shrimp. bration (figs. 12 to 16). Earlier workers termed nule and those of the typical crustacean appendage the structure an otocyst, having ascribed to it an have not received support. Most carcinologists auditory function (Huxley 1906). The statocyst consider the parts of the antennule so extensively is constructed like an incomplete sac that has modified from the usual plan as to defy identifi dorsal and anterior openings. Hairs, presumably cation. At the same time, the clearly segmented sensory, project from the inner surface of the structure of the antennule marks it as a true ap cuticular sac into the lumen of the statocyst. The pendage, in the opinion of many workers. hairs are arranged in regular rows. The rows of The form and function of the crustacean anten hairs are confined to oddly shaped patterns lo nule usually is said to be constant among the cated at various places on the inner surface of Decapoda, relative to the extraordinary vari the statocyst. Nerves from the sensory hairs ability in the appendage occurring in the lower coalesce ventrad of the sac into a broad, flat nerve Crustacea. Nonetheless, among the decapods tract that enters the brain at a point ventral to wide differences are found. The outer or lateral the optic tract (fig. 14). flagellum is split in many of the Caridea, giving The dorsal opening of the statocyst is partially the impression that the antennule bears three covered by a fleshy, heavily setose lobe, the dorsal flagella. Among the tribe Penaeidae, the outer closing lobe (figs. 12, 15). In fresh material the flagellum is prehensile in certain genera of the dorsal closing lobe may be lifted from the dorsal Family Sergestidae. In the Solenocera each opening of the statocyst without great difficulty. flagellum is semitubular, thus forming a siphon The saccular statocyst is shut anteriorly by a thin, (Caiman 1909). The antennule is small and re curled sheet of cuticle that arises vertically by duced in the Brachyura. evagination from the ventrolateral floor of the an Despite infrequent anatomical treatment of the tennular eye depression. The sheet is designated decapod antennule, the available work suggests here as the anterior closing plate (figs. 12 to 15), that a marked uniformity exists in the appendage Neither the dorsal closing lobe nor the anterior and its parts over a broad spectrum of the Deca closing plate effect complete obturation of the poda. From the standpoint of comparative mor statocyst. As a result, water circulates through WHITE SHKIMP FROM THE GULF OF MEXICO 21 medial flage llu m^ lateral flage/lum^ . dorsal eye brush f SIDE VIEW 3rd antenna la r segmfinf-^ X-- ^^^'Sfylocerite 2nd aniennular segment- 1st antennular segments depress/or? ,--—" r- X - •— f for eye dorsal ege brushy - - - •#• \ %• \>^ anterior closing • c/orsai chs/ng plate ofstafocyst^"' o* lobe of statocysf' FiGCBE 12.—The larger figure is a dorsal view of the right antennule, the smaller a lateral view of the right antennnle. 22 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE the statocyst at all times, to the extent that the Most of the structures lining the eye depres heavy investment of hairs in the area permits. sion are heavily setose, to a degree that would In living shrimps, as has been shown by Burken- make their outlines obscure if illustrated faith road (1989), the open statocyst contains sand fully. For this reason only a fraction of the (Trains. The sand probably functions as statoliths, true covering in hair of the dorsal side of the if the statocyst is indeed equilibratory. These antennule has been shown. The presence of this statoliths are obtained by the shrimp from the sub extensive investment of hair probably can be strate throuo;h activities of the animal that are explained as a system of brushes to clean the either directed or incidental to their collection. corneal surfaces of the compound eyes. Many Cursory examination of the statocyst does not re stalk-eyed crustaceans carry the eyes laterally veal bodies that have obviously been secreted by and never for any length of time in the eye de the shrimp. Burkenroad (1939), however, finds pressions of the antennules. However, the eyes are brought frequently into and out of the de that the statocyst of penaeids contains statoliths pressions, and thus through the long hairs lin secreted by the animals. Since the statocyst is ing the depression. open to the water and to the substrate into which The second antennular segment (fig. 12), a far the shrimp is known to burrow, the possibility simpler structure than the first segment, is at exists that the statocyst contents may undergo con tached to the latter distally. The second seg tinual replacement during intermolt periods. ment articulates with the first segment allow^ing However, since the shrimp loses the statocyst with limited horizontal movements of the antennular its contents at each ecdysis, the statoliths are prob segments distal to the first segment. The sec ably replaced in large measure at the time. That ond segment is a rectangular box in shape and the animals burrow into the substrate for protec modified for the actions of the muscles wdiich it tion, statolith replenishment, and rest following contains. the molt is supported by negative information The third antennular segment (fig. 12) is a provided by observing shrimps in clean aquaria at small, square structure articulated with the sec ecdysis. Upon shedding in this unnatural environ ond segment. Movement of the third segment ment, the shrimp is unable to navigate properly and the flagella on the second segment is limited and soon perishes, even if kept alone. The stato- to an attenuated horizontal arc. No special sense cysts are found to be empty of statoliths. organ appears in the third antennular segment. The largest region of the first antennular seg Two flagella, the medial flagellum and the lat ment is the eye depression. Beginning at the eral flagellum (fig. 12), articulate independently anterior closing plate and extending to the mar with the third antennular segment. The points gins of the first antennular segment distally and of articulation between the flagella and the third to the sides is a broad, deep concavity into which segment are so arranged that the flagella may the corneal surface of the compound eye may rest move through a broad arc in the frontal plane. (figs. 12, 13). The eye depression is confined Each articulates wdth the third segment in proximally by the anterior closing plate of the slightly different horizontal planes. The lateral statocyst and the skeletal structures that surround flagellum is attached dorsad of the medial flagel the statocyst. The dorsal closing lobe projects out lum. The flagella are composed of many short slightly, dorsal to the depression. A large, wedge- articles of light construction connected by rings shaped, fleshy lobe arising in the posterolateral of thin cuticle, thus permitting bending in all region of the first segment extends anteriorly along planes. The length of the articles differs in the the lateral margin to'a poiijt. At the anterolateral two flagella, those of the lateral flagellum being corner of the first antennular segment is a small, shorter than the articles of the medial flagellum. sharp stylocerite (fig. 12), a structure common Other differences between the flagella include to the Tribe Penaeidae (Voss 1955). Along the variation in cross-sectional shape and in the mesial margin of the first segment lies the dorsal types of setae and processes projecting from the eye brush, or prosartema (figs. 12, 13), a long, articles. thin lobe arising dorsally from the proximal re Numerous studies have been made on the func gion of the segment and extending anteriorly to tion of the antennular flagella and the variety of the anteromesial corner of the segment. hairs and processes that they bear. These investi- WHITE SHRIMP FROM THE GULF OF MEXICO 23 dorsal depression eye brush / for eye \ \ iLy 1 , -.* Vv /N \ V ••\ - X \ s \ dorsal opening i anterior closing to stalocyst' plafe of state cyst KiouKE 13.—Dorsal view of proximal region of first antennular segment, right antennule. Cleared specimen showing entrance to statocyst. Dorsal closing lobe removed. 24 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE Rations show that the flagella are involved in character in the sexually mature male of Penaeus chemoreception (Doflein 1911; Balss 1913; Bell xetifenis. However, the subject needs further 1906; Lissman and Wolsky 1935; Spiegel 1927). study. A large, statistically significant number The olfactory nature of the antennular flap:ella is of male and female Penaeus setiferus shown to be related to an interesting structural modification. sexually mature by the histological methods of Tliis modification is tlie apparent sexual dimor King (1948) should be examined to prove the phism of the medial flagellum in the adult male point. In the present study 10 adult males and of Penaevs setiferus (fig. 17). The change to the 8 adult females were considered in connection medial flagellum, which is easily visible to the with the character. naked eye, results in a pronounced dorsoventral flattening, together with the appearance of nu MUSCLE ELEMENTS merous, stout processes of two sizes on the dorsal The antennule of Penaeus setiferus contains 13 margin. That these processes may be predomi muscles, as contrasted with 12 for the caridean nantly proteinaceous is suggested by their loss fol Pandalus. and 9 for the antennule of Astacus. lowing treatment of the preserved material in Cochran (1935) lists 9 for Callinectes, includ strong alkali, for as is well known strong alkali ing a double antennular promotor muscle and a degrades the glucosamine of chitin but does not double antennular remotor muscle. If these di immediately remove it, whereas the alkali-soluble vided muscle bodies are actually 2 promotor and proteins of the internal organs and cuticle are 2 remotor muscles, then the antennule of Calli- rendered soluble and washed out (Kichards 1951). nectes can be said to have 11 muscles. Other changes to the condition of adult male- ness probably occur during the same molt that ANTENNULAR ABDUCTOR MUSCLE modifies the medial flagellum. The most obvious secondary sexual character is the mesial joining FIGURE 14 of the previously free wings of the modified The antennular abductor muscle of the first an pleopod endopodities of the first abdominal seg tennular segment is attached posteriorly to an ment to form the definitive and functional pe- apodeme that arises from the ventrolateral mar tasma, or sperm-transfer organ. gin of the articular foramen by which the anten Since the antennular flagella are olfactory, the nule is connected to the protocephalon. The plane sexually dimorphic medial flagellum of the male of orientation of this apodeme is vertical. The Penaeus setiferus probably functions to enable the antennular abductor runs anteriorly a brief dis male to find the sexually mature female during tance and divides into two large branches that at the time of mating. The occurrence of secondary tach to sclerotized bars supporting the posterior sexual modifications of the antennular flagella are region of the first antennular segment in the widespread among the Crustacea Decapoda. neighborhood of the statocyst. Contraction of Meredith (1954) describes a modified outer anten the first segment abductor muscle swings the first nular flagellum in the adult male of Grangon antennular segment, and with it the distal anten vulgaris and suggests that the structure may func nular segments, outward from the mid-sagittal tion in mating. This worker finds that the plane of the body in the limited horizontal motion character is of use in identifying adult males in possible to the first segment. The antennular ab the field. The most extensive treatment of the ductor of Penaeus is homologous with the mus- subject of sexual dimorphism in Crustacea is that culus promotor I antennae of Astacws^ Pandalus^ of Eioja (1939a, 1939b, 1940a, 1940b, 1940c, 1941a, and possibly to the first segment promotor mus 1941b, 1942a, 1942b), who has described in detail cles of Callinectes^ judging from the origins, in the sexually modified medial flagellum of Penaeus sertions, and arrangements of the muscles. setiferus and several other species of penaeid However, the functions of the homologous shrimps. This worker considers the character to muscles are different, as often happens. The dif be constant in occurrence and sensory in function, ference lies primarily in the restriction of anten and closely related to the sexual activities of the nular movement created by the presence of the an- shrimps. tennal scales ventral to the antennules in Penxieus. In the opinion of the present writer, the modi In the white shrimp, the scales are relatively large fied medial antennular flagellum is a constant and, when lying in their usual longitudinal posi- AVHITE SHRIMP FROM THE GUIiF OF MEXICO 25 adductor muscle of ,abductormasc/e of medial flagellum / medial flagel/um abdacfor muscle o. abduclormuscles of lateral flagedlutn. / 3rd basal s egm ent adductor muscle of ^ abduclvr musc/e of 3rd basal segment^—- '' Znd basal segmenf add a dor muscle of ^ ^anfen n u lar dnd basal segment.^'--''' arfery antermular ^ anterior c/os/rg nerves. pi ale of stafocyst aniennuiar 1^-; addaclormuscle.'-'"'"' \\ • dorsal eye ' anfennular brush muscley^^-' abductot muscle ,i£y"^tJ * dp tie nerve to brain/ tract sta to c yst FiGUKE 14.—Dorsal view of right antennule showing dorsal muscles. Dorsal cuticle removed. 26 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE tion, their mesial marjjins overlap beneath the an- possibly of CaUinectes. However, instead of tak teniuiles. Movements of the antennules ventrad ing part in the adduction of the first antennular are tliereby prevented. In Payidahis and Astacus segment, the eye brush muscle in Penaeus serves to the first antennular seo;ments may be raised and stiffen the dorsal eye brush (figs. 12, 13). Fibers lowered about a transverse axis throuf2:h the artic of the muscle enter the eye brush at its point of at ular foramina of the antennules, in the sagittal tachment to the first antennular segment, and their plane. That Schmidt (1915) in his study of contractions presumably enhance the function of A-stacus named the muscle a '"proniotor" is open to the brush as an eye cleaner. question, Tlie nniscle might better be described as a levator of the first antennular segment. ABDUCTOR MUSCLE OF SECOND BASAL SEGMENT Berkeley (1928) and Cochran (19-S5) adopted the FIGURE 14 terminology of Schmidt for this muscle in The second antennular segment is turned away Pandalus and Callinectes, although, in Cal- from the midline on its j)oints of articulation with Jinectes the musculus promotor I antennae ap the first antennular segment by contractions of the parently moves the antennule toward the midline abductor muscle of the second basal segment (fig. of the animal. 14). This muscle originates on the anteroventral ANTENNULAR ADDUCTOR MUSCLE part of the first antennular segment at about the midpoint between the anterior margins of the seg FIGURE 14 ment. The muscle runs a short distance antero- The antennular adductor muscle of the first an laterally to insert on a posterolateral apodeme of tennular segment originates on a vertically- the first antennular segment. oriented apodeme arising from the ventromesial The second basal segment abductor muscle of region of the articular foramen between the anten Penaeus has a homologue in the musculus produc- nule and the protocephalon. The muscle courses toro I antennae of Pandalus, Astacus. and Cal- anteriorly along the mesial margin of the first an h'nertes. although the use of the term "productor" tennular segment, inserting in the cuticle at many for the action of tlie muscle in CaUinectes is ques points along the mesial edge of the first segment tionable. In the blue crab musculus produc- (fig. 14). The antennular adductor muscle func tora I antennae is said by Cochran (1935) to pull tions to turn the first antennular segment toward the second antennular segment downward, indicat the mid-sagittal line of the shrimp. ing that the muscle functions as a depressor. The musculus remoter I antennae described by ADDUCTOR MUSCLE OF SECOND BASAL SEGMENT Berkeley (1928) in Pandalus, by Schmidt (1915) in Astacus, and by Cochran (1935) in CaUinectes FIGURES 14, 15 are in all probability the homolog of the first The adductor muscle of the second basal seg segment adductor muscle in Penaeus. In the case ment originates ventrally in the anteromesial cor of the former three animals, two musculi remotor I ner of the first antennular segment and inserts on antennae have been found in the antennule of each, a small apodeme at the posteromesial corner of the a remarkable uniformity in animals as distantly second antennular segment (figs. 14,15). Contrac related as these. Penaeus^ too, has these muscles, tion of the adductor muscle turns the second anten but the dorsal-most, discussed in the following nular segment, together with the distal elements section, appears to have a function different from of the antennule, toward the mid-sagittal plane that of remotion or adduction. of the animal. The second basal segment adductor muscle of Penaeus has a counterpart in the mus DORSAL EYE BRUSH MUSCLE culus reductorg I antennae of Pandalits, Astacus, and CaUinectes. The muscle functions in Pan- FIGURE 14 dalus and Astacws to depress the distal antennular The dorsal eye brush muscle, or prosartema segments and flagella, while in CaUinectes the dis muscle, lies upon the adductor muscle of the first tal elements are raised toward the midline by the antennular segment, and is almost certainly the action of the musculus reductor2. The muscle homolog of the musculus remotor h I antennae might better have been called a second segment found in the antennules of Pandalus^ Astacus, and levator in the blue crab. WHITE SHRIMP FROM THE GULF OF MEXICO 27 abductor muscle of abductor muscles of 3rd lateral flageUun\ basal segment adductor muscle of ,sfy locertte lateral lIcjgellurn adductor muscle of ^ abduchr muscle 'nd basal segment. 2nd basal segment onlennular •depression artery. for et/e antennular an ten tor clos/'ng nerve plate of atatocyst nen/e to / r statocysty liy^J '^ opt/c 'cdorsa/ c/oslng bra in ^ tracf lobe of statocijst FIGURE 15.—Dorsal view of right antennule showing ventral muscles. 28 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE ADDUCTOR MUSCLE OF LATERAL FLAGELLUM ABDUCTOR MUSCLES OF THIRD BASAL SEGMENT FIGURE 15 FIGURES 14, 15 Arisino; from the anteromesial margin of the The second antennular segment of Penasus con first antennular segment, just dorsal to the origin tains three abductor muscles of tlie 3d basal seg of the second basal segment adductor muscle, is ment. The lateral abductor muscle originates in the long, slender adductor muscle of the lateral the posterolateral corner of the second antennular flagellum. The muscle courses anteriorly through segment and runs directly anterior to its point both the second and third antennular segments to of insertion on a small apodeme of the postero insert on an apodeme arising from the mesial edge lateral corner of the third antennular segment. of the lateral flagellum base. Upon contraction Just mesad of the lateral, or first, abductor, a of the lateral flagellum adductor the lateral flagel second abductor muscle originates broadly on the lum is turned on its points of articulation toward ventral surface of the posterior region of the the mid-sagittal plane. The muscle has a prob second segment. This second abductor muscle able liomolog in tlie musculus adductor I an courses anteriorly, parallel to the lateral-most tennae of Pandalus, although in the latter the abductor muscle, and inserts on a short apodeme muscle inserts in the ventral part of the third slightly mesad of the insertion of the lateral-most antennular segment rather than on a flagellum. muscle (figs. 14, 15). The mesial third basal seg A lateral flagellum adductor muscle does not oc ment abductor muscle originates in the postero cur in the antennule of Astacus or Callinectes. mesial corner of the second antennular segment Cochran (1935) describes a musculus adductor2 dorsal to the origin of the third basal segment I antennae for Callinectes that functions some adductor muscle. The mesial abductor runs di what like the musculus adductor I antennae found agonally to the anterior to insert on the apodeme by Berkeley (1928) in Pandalus^ hut which is not of one of the lateral abductor muscles (fig. 14). homologous with the latter muscle or with the Contractions of the third segment abductor mus lateral flagellum adductor muscle in Penaeus. A cles turn the third segment and the flagella away study of the nerves might show that the musculus from the mid-sagittal plane. Their action rein adductor2 I antennae in Ccdlinectes is part of the forces that of the first and second segment abduc musculus reductor2 I antennae (second segment tor muscles. adductor muscle of Penaeus). The lateral-most abductor muscle of the third antennular segment of Penaeus is homologous ADDUCTOR MUSCLE OF THIRD BASAL SEGMENT with the musculus productorg I antennae of Pan FIGURE 14 dalus^ Astacus^ and possibly Callinectes. In Pan The relatively large adductor muscle of the dalus and Astacus the musculus productorg 3d basal segment originates in the posteromesial swings the third segment dorsally in the sagittal corner of the second antennular segment and runs plane, while in Callinectes the muscle named by directly anterior to insert on an apodeme in the Cochran (1935) a "productor" appears to flex the posteromesial corner of the first antennular seg third segment. The second lateral third-segment ment. Contractions of this muscle turn the third abductor muscle in Penazus has homologs in the antennular segment and the flagella mesad, rein musculus abductors I antennae of Pandalus and forcing the action of the proximal antennular ad Astacus. The muscle has been lost in Callinectes. ductors. The third basal segment adductor of The mesial third-segment abductor muscle of Pe Penaeus lias liomologs in the musculus reduc- naeus does not appear in the antennule of any tors I antennae of Pandalus^ Astacus, and Cal of the crustaceans to which reference has been linectes. Cochran's (1935) use of the name "re- made here. ductor" for the muscle in Callinectes is unfor tunate. The reductor muscle of the third anten ABDUCTOR MUSCLE OF LATERAL FLAGELLUM nular segment in the blue crab appears to function virtually opposite to the muscle of the same name FIGURES 14, 15 in Pandalus and Astacus., a situation that under The abductor muscle of the lateral flagellum lines the impropriety of transferring functional has its origins along most of the mesial side of muscle names from one animal to another. the third antennular segment. The muscle nar- WHITE SHRIMP FROM THE GULF OF MEXICO 29 dorsal opening^- A. Anterior View B. Posterior View FIGURE 16.—Cleared statocyst. A. Anterior view. B. Posterior view. rows as it runs anterolaterally, ventral to the lat muscle of the medial flagellum. The adductor eral flagellum adductor muscle, to insert on an muscle courses anteriorly and inserts on an apo apodeme on the lateral edge of the base of the deme on the mesial edge of the flagellum base. lateral flagellum (fig. 15). The lateral flagellum Contractions of the muscle turn the medial flagel abductor muscle turns the lateral flagellum away lum toward the midline in the horizontal plane. from the midline in the horizontal plane. The No counterpart of this muscle in Penaeus is de muscle in Penaeus is very likely homologous with scribed for the antennule of Pandalus, Astacus, or the musculus reductor4 I antennae of Pandalus. Callinectes. : • , Astacus, and CalUnectes, although evolutional re arrangements have given rise to several changes. ABDUCTOR MUSCLE OF MEDIAL FLAGELLUM Berkeley (1928) describes the reductor 4 muscle FIGURE 14 in Pandalus as consisting of two parts, inserting on two opposite margins of the base of the lateral The abductor muscle of the medial flagellum tiagellum, and having at times an antagonistic originates ventral to the origin of the medial flagel action to one another. Such action suggests very lum adductor muscle and inserts on the medial strongly the existence of two muscles, rather than flagellum by a short apodeme located at the lat one with two functions. eral margin of the flagellum base. The muscle turns the medial flagellum outward from the mid ADDUCTOR MUSCLE OF MEDIAL FLAGELLUM line in a horizontal plane. The medial flagellum FIGURE 14 ^ •:; ri, ' s:; v ".; abductor muscle of Penaeus has been lost in the an Originating in the posteromesial corner of the tennule of the other crustaceans referred to in this third antennular segment is the small adductor study. 468059 0—59 3 30 FISHERY BULLETIN OF THE Fl^H AND WILDLIFE SERVICE 3. THE ANTENNAE of the white shrimp. Normally the scale is car The antennae, or as they are frequentl}' termed, ried in a directly anterior position, rotated the second antennae, display wide variations slightly on its longitudinal axis. The rotation is among: the Crustacea. In certain of the Copepoda such that the thin mesial margins of the scale lie and other orroups in the lower Crustacea they are somewhat ventrad of the heavy lateral margins. larfre swimminfr organs. They are modified for The effect produced is that of a ship's bow. clinging in other copepods. Frequently the an Shrimps in an aquarium were lightly secured by tenna of the male crustacean is modified as a a loop of string to the end of a rod. Lifted from sexual clasper. Sexual dimorphism of the an the bottom in this position, the animals are stim tennae is widespread in the lower Crustacea, the ulated to swim forward. Jets of water from a appendage of the adult male being more highly pump were directed at the motionless shrimps developed than that of the female. In several from various directions to see whether the an groups the structures may be extremely reduced tennal scale was used as an anterior steering de or even lost in the adult animals. vice. No compensatory movements of the scale The antenna of the higher Crustacea is fairly were observed. The organs appeared simply to uniform, although exceptions do occur. The cleave the water ahead. During normal move malacostracan antenna is said to consist of a 2- ments about an aquarium, the scales are occasion or 3-segmented protopodite, an endopodite of 2 ally spread widely, but are never kept in the or 3 segments, the distal-most bearing a flagellum, spread condition for more than a moment. and an exopodite reflected into a flat scale. The The antenna in Penaeun -sefiferi/.^ is typical of protopodite usually is comprised of the coxopo- the natant Decapoda. The antennal scale is broad dite and basipodite, but some forms may have a and strong and extends as far as the anterior tip of proximal pre- or sub-coxa in addition (Caiman the rostrum. The proto]:)odite is comprised of two 1909). Carcinologists use the expression "pe segments, a short, incomplete coxopodite and a duncle" to refer to the total number of endopodite very large basipodite. To the large basipodite is and protopodite segments, usually 5 or 6, in the articulated the exopodite, or scale, and the basal segments of the antenna. The size of the basi Malacostraca. Kepresentatives of the lower De- podite reflects its support of the heavy scale rather capoda, like Penaeus setiferus, have a 5-seg- than that of the smaller antennal segments. The mented antennal peduncle, made up of the long antennal flagellum is carried laterally, from coxopodite and basipodite in the protopodite, and which position the movements of the shrimps 3 proximal endopodite segments. The exopodite througli tlie watei- cause the flagellum to drag is scalelike. The scale is often missing or reduced alongside and some distance behind the animal. to a spine in the higher decapods and the endopo The flagellum is api)roximately twice the body dite and flagellum may be relatively small. length in the white slirimp. Functionally, the antennae have always been thought to be sensory. More specifically, the SKELETAL ELEMENTS antennal flagella are said to be centers of tactile sensation, whether pressure or simple touch is not The first antennal segment, or coxopodite (fig. known. No experimental data to support these 18), is an incomplete ring by which the antenna contentions exist. The functions of the antennal is attached to the protocephalon. The foramen of scale would appear to be varied, in accordance th'.' antenna at which the first antennal segment with its great variation in size. The scale is articulates with the protocephalon is by far the missing in the Brachyura and small in the craw- largest of the head foi-amina. At its broadest fishes and other Astacura, making its function portions, muscles insert upon the first antennal somewhat difficult to determine by observational segment. means. The organ is veiy large in the swimming The basipodite, or second antennal segment decapods of the Natantia. The suggestion has (figs. 18, 23), is a large, heavily sclerotized article frequently been made that the scale is an anterior connected firmly to the first antennal segment swimming plane in the latter group. proximally and articulating distally with the third Some attempt has been made to ascertain the antennal segment, or ischiopodite, of the endopo function of the antennal scale in the swimmino; dite, and with the antennal scale, or exopodite. At WHITE SHRIMP FROM THE GULF OF MEXICO 31 lateral rostrum^ ^2nd antennular flagellum^ \ segment medial Srdaniennalar / antenna/ flagellurri'' Segment' sca/e FiouitE 17.—Enlarged view of left antennular flagella showing sexual modification to medial flagellum in adult male. the point of articulation with the antennal scale, at right angles to its long axis, against the second the second antennal segment is deeply notched an antennal segment permits some rotation of the teriorly providing an articular foramen for the distal antenna] segments. Movements of the little large antennal scale. Strong dorsal and ventral shield-shaped fourth antennal segment (figs. 18, points of articulation, or condyles (figs. 18,22, 23) 23) against the third antennal segment are, on the other hand, extensive. The fourth antennal on the rim of the scale foramen permit the scale segment attaches laterally to the third segment considerable horizontal movement. As brought at about a 45° angle. By means of a dicondylic out earlier, the great weight and strength of the articulation, the fourth segment rotates through second antennal segment exists as support for an arc of nearly 90°, and with it the distal an the antennal scale, rather than for the long, slender tennal parts. imteniuil flagellum. The second antennal segment According to Schmidt (1915), the third anten articulates with the third antennal segment, or nal segment represents the fusion of the ischiopo ischiopodite, by a small foramen located ventro- dite and meropodite and he evidently has morpho mesially (figs. 18, 23). The extent of movement logical support for this view in Antaeus. Curi between the second and third antennal segments is ously, no trace of a division is apparent in the third very limited. antennal segment of CamTyaruH. nor does Berkeley The third antennal segment (figs. 18, 23) is a (1928) find one in the caridean Pandalus. No evi small, heart-shaped structure whose apex is di dence for the fusion of the ischiopodite and mero rected posteriorly. Its small vertical movement. podite can be seen in Penaeus setiferus. 32 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE Is i si J WHITE SHRIMP FROM THE GULF OF MEXICO 33 The fifth antennal segment (figs. 18, 23) lies on cle inserts in the first antennal segment near the the axial line distal to the fourth antennal segment external opening of the excretory apparatus (fig. and is connected to the pointed fourth segment by 21). Contractions of this short, powerful muscle a broad V-shaped surface in the proximal end of turn the coxopodite and hence the antenna mesad. the fifth segment. Two condyles in the vertical Homologs of this muscle in Af^tacv.^ and Pan'- plane located at the apex of the V-shaped surface dalus are difficult to determine without full infor permit limited horizontal movements of the fifth mation on the nerves. Allowing for functional segment on the fourth antennal segment. The rel differences in the antennae of the various crusta atively large fifth antennal segment bears the base ceans here considered, the most likely homolog of the long flagellum on its distal end. Strong of the first segment adductors in Astacus is the dorsoventral condyles allow the flagellum to turn musculus depressor a II antennae. Berkeley through an arc of more than 90° and the size of (1928) designates two medial antennal base the fifth segment is probably an evolutional re muscles as the musculus depressor a and h II sponse to the long muscles needed to operate the antennae, after Schmidt (1915). From her illus flagellum. In living shrimps, the distal j^art of trations of the antenna in Pandalus, the antennal the flagellum is carried at right angles to the fifth depressor muscle a appears to be the same muscle antennal segment, the rest of the long flexible as the first antennal segment adductor in Penaeus. flagellum floating behind the animals. Tlie anten The muscles in the latter two forms are similar, nal flagellum (figs. 23, 25) owes its flexibility to strongly suggesting an homology. The antennal its annular construction. Each small annulation depressor muscle a in Astacus is much less sug is capable of a little movement with respect to its gestive of phylogenetic similarity. neighbors. SECOND ANTENNAL SEGMENT PROMOTOR An enlargement of the flagellar rings (fig. 25) MUSCLES shows probable sensory structures. On the dorsal surface of each ring (fig. 25, A) may be seen a FIGS. 19, 22 pair of dorsal setae. On the ventral surface (fig. Far and away the heaviest musculature of the '25, B) a pair of plumose ventral setae project an anterodorsal region of the white shrimp is that teriorly from the distal portion of each annula concerned with the antennae. The dorsal-most of tion. Between the bases of the ventral setae is a these is a large, flat muscle originating in connec ventral pit. The interior of the flagellum consists tive tissue slightly laterad of the postrostrum of blood vessels and nerves. (fig. 19). This muscle, a second antennal segment promotor muscle, runs anteriorly and laterally to MUSCLE ELEMENTS insert in what appears to be a free apodeme just The skeletal parts of the antenna of Penaeus beneath the dorsal rim of the coxopodite. This setiferus are operated by 12 types of muscles, in apodeme is not connected to the coxopodite, but cluding at least 26 individual muscles. Berkeley instead (fig. 19) consists of a transverse fascia (1928) describes 15 types of muscles in Pandalus. in which the distal second antennal segment pro- Schmidt (1915) lists 18 muscle types in the Euro motor muscle originates. The presence of the free pean crawfish, the appendage containing 21 sep apodeme of the proximal second segment pro- arate muscles to carry out its complex movements. motor muscle may indicate that the muscle is in The reduced antenna of CaUinectes has only 8 mus reality a first antennal segment (coxopodite) pro- cles (Cochran 1935). The evolutional trend of re motor. The free apodeme, however, produces a duction of the number of muscles and muscle types functional second antennal segment (basipodite) has apparently been reversed in the case of the promotor and the muscle is therefore so described. crawfish antenna, in which form much more com The proximal second segment promotor is evi plicated antennal activities are displayed by the dently the homolog of the musculus promotor II living animals than in Penaeus or Pandalus. antennae in Astacus, Pandalus, and CaUinectes, although in these forms the muscle clearly at FIRST ANTENNAL SEGMENT ADDUCTOR MUSCLE taches to the dorsal margin of the coxopodite. FIGS. 21, 24 The distal second antennal segment promotor Attaching on the median rim of the coxopodite muscle extends the functional connection of the foramen, the first antennal segment adductor mus muscle group to the dorsal edge of the second an- 34 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE I 1) SO 0 •a a OS a) o 03 C OS 03 a 03 Ml a) I 0 ? ft it S WHITE SHRIMP FROM THE GULF OF MEXICO 35 ^ %) ^ ^ HJ V) ^ 0 x. V) 4i \ \ ^ ^ \ •a \i s; \ a \ S3 \ '«3 S^ >. o ^cyX , 0) ^ I TJ >a a a a 03 c3 U K 36 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE tennal segment, or basipodite. Upon contraction with the simple movements of the scale; the mus the promoter muscles raise the antennal scale and cles of the distal antennal segments of the endo endopodite segments dorsally. The muscles podite are comparatively small. actually function as levators. Without infor As a consequence of the many antennal func mation about the innervation, no homolog of the tions in different crustaceans, homologous mus distal promotor muscle is here suggested. cles have different functions and nomenclature. At least part of the second antennal segment re SECOND ANTENNAL SEGMENT REMOTOR MUSCLES motor muscle mass in Peiiaeus is undoubtedly homologous with the musculus remotor II anten FIGS. 19, 24 nae in Astacus. Pandahis^ and Callinectes. Part At least three large antennal muscles arise on also may be homologous with the musculus de the dorsolateral carapace just anterior to the pressor c II antennae in Astacus and perhaps even hepatic spine. These muscles, here considered the with a part of the large musculus depressor c II second antennal segment remoter muscles (figs. antennae in Pandalws, although certainly the 19, 24), run anterolaterally to extensive insertion major part of the antennal depressor c in Panda- areas in the lateral and posterior regions of the lus is the protocephalon attractor muscle. The second antennal segment (basipodite). The other antennal depressor muscles, a, 5, and d {d lateral- and posterior-most of these muscles might is not found in Pandalus) in Astacus and Pamda- be thought to insert on the ventral margin of the lus are not evident in Penaeus. first antennal segment, in which case the two muscles would be first segment, or coxopodite, re- SCALE ABDUCTOR MUSCLES motor muscles. Eepeated dissections in the area FIGURES 19, 20, 24 indicate otherwise, however, and for this reason the muscles are assigned to the basipodite. The Taking origin from large areas to the posterior, posterior-most remotor displays a definite torsion ventral, and medial region of the second antennal as it passes from its lateral point of origin to a segment (basipodite), the proximal scale abductor posterior and even slightly medial insertion area. muscles (figs. 19, 20, 24) insert on the lateral mar While the function of all three remotor muscles gin of the antennal scale (exopodite), lateral to the is actually the depression of the large scale, that dorsoventral scale condyles. In addition to the of the posterior remotor muscle may also include huge ventral scale abductor, at least two and prob adduction of the scale, together with depression. ably three small-scale abductors (figs. 20, 24) are Since the antennal scale of Penaeus bears a found in the second antennal segment of Penaeus. major portion of the w^eight of water striking the A long, distal scale abductor muscle (fig. 24, 5), anterior end of the animal, large remotor (de originating in the distal part of the scale, runs pressor) muscles are needed to maintain the scale proximally along the lateral margin of the scale in position. to insert on the lateral margin of the basipodite foramen, lateral to the axis of the scale condyles. The homologies of the second antenna segment When the scale abductor muscles contract, the remotors is made confusing by the functional large scale is swung laterally some distance. The muscle nomenclature. A comparison of the func functional reason for this movement is not clear. tion of the antennae in the crawfish and Penaeus Shrimps in an aquarium occasionally spread the shows wide differences. The crawfish antennal scales, at times in association with cleaning activi segments are constructed to permit extension ties of the head appendages and a sudden flushing movements of the comparatively short, stiff flagel- out of the gill chamber. lum. One kind of extreme of this modification has been achieved in the antenna of PaUnurus, in The proximal scale abductor muscles of Penaeus which form movability is combined with great are probably liomologous with the second antennal size and power for the protection of the animal. exopodite abductor muscles a, b. and c in Astacus As has been suggested, the antennal movements and with the single exopodite abductor in Pan of Penaeus are comparatively limited by virtue of dalus. The scale is reduced in Callinectes. segmental architecture, particularly in the seg Schmidt (1915) does not show a distal scale abduc ments of the protopodite. The heavy muscula tor muscle in Astacus such as exists in Penueus^ and ture of carapace origin is in reality associated Berkeley (1928) makes no mention of the muscle WHITE SHRIMP FROM THE GULF OF MEXICO 37 1 =3 § It . o C3 13 0) a _ a ^ Hi «H be u •JH OS t< PL, s \ s X \ C3 \ \ \ t-3 \ s s ^ X X X X ^1 5 ^ K ^ 1^ 38 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE fhgellum^ 'Scake •^^ rf! ;#^^$$> s--,\ \\ ;^$;- xx.\\~- promcfor muscles of ,c/orsa/point of 2nd antennai segment^ / scate art/cu/afton I 1/ rota tor muscle of '^ Zndantennat s^menf protocephalon " re motor muscle of attractor muscte ^ Zridantennat segmenf FIGURE 22.—Dorsal view of right antenna showing antennal scale and protopodlte muscles. Carapace removed. WHITE SHRIMP FROM THE GULF OF MEXICO, 39 in Pandalu^. However, l^erkeley illustrates a dis meropodite muscle in Pdiuhdii.s is more nearly tal scale muscle which she designates as the mns- similar to the situation in Penaem^ than in Anta- culus adductor exopoditis h II antennae and cus. In both Astacus and Pandalus^ Schmidt which from the standpoint of its arran<2:ement ap (1915) and Berkeley (1928) illustrate a muscle, pears to be the distal scale abductor muscle in the musculus reductor ischiopoditis II antennae, Poiaeus. A careful revie^Y of the insertion of said to oppose the action of the meropodite Berkeley's exopodite adductor muscle might show muscles a and h. No similar muscle has been tliat it is in reality an abductor muscle. found in Penaeus^ although the shrimp may have a functional analog in the fourth antennal ssgment SCALE ADDUCTOR MUSCLES adductors. FIGURES 20, 21, 24 FOURTH ANTENNAL SEGMENT ADDUCTOR Two types of scale adductor muscles (figs. 20, MUSCLE 21, 24) are found in the antenna of Penaeus. At FIGURE 24 least two scale adductors originate on the medial wall of the second antennal segment ventral to the The fourth antennal segment adductor muscle excretory pore (fig. 24, B)^ and run diagonally to (fig. 24) originates in a broad fan on the ventral insertion points on the ventral and medial margins surface of the basipodite. The muscle runs an- of the scale foramen. Their insertions are mesad terodorsally, narrowing to its apex at its point of the axis of the scale condyles. The distal scale of insertion on a small, movable article presumed adductor muscle (fig. 24, /?), like the distal scale to be a part of the fourth antennal segment. abductor, is located in the body of the scale. It Certainly a muscle originating on the basipodite originates in the distal region of the exopodite and inserting on the fourth antennal segment is and runs caudad parallel to the distal scale abduc unusual. Controversy could be avoided by as tor to insert on the margin of the basipodite fora signing the small, movable article to which this men mesad of the scale articles. Upon contraction, muscle inserts to the third antennal segment; the scale adductor muscles move the antennal scale however, the movable article appears to be widely inward toward the median line of the shrimp, in separated from the third segment and instead is opposition to the action of the scale abductors. lateral to the fourth segment and clearly con nected to it. When the fourth segment adductor THIRD ANTENNAL SEGMENT ROTATOR MUSCLE muscle contracts, the movable article is drawn posteroventrally with the result that the fourth P^GURES 20, 21, 24 antennal segment is rotated upon the third seg Arising on the dorsomedial rim of the first an ment and turned a short distance mesad. As tennal segment (coxopodite) foramen and running such, the fourth segment adductor represents in ventro-medially, the third antennal segment rota part an opposing muscle to the third segment tor muscle (figs. 20, 21, 24) inserts on an apodeme rotator muscle. The homology of this muscle is located on the lateral margin of the third antennal uncertain. segment (figs. 23, 24). Contractions of the muscle accomplish the movement described in the section FOURTH ANTENNAL SEGMENT ABDUCTOR MUSCLE on the skeletal elements, namely, lateral rotation of the third antennal segment and the antennal FIGURE 24 parts distal to the third segment. Originating from a broad area slightly poste The homologs of this muscle in the other rior to the origin of the fourth segment adductor crustaceans referred to are not clear. The best muscle, the fourth antennal segment abductor possibility in Astacu-s is one or both of the mero- muscle (fig. 24) runs to an insertion on the same podite muscles a and b. On the basis of area of small movable article of the fourth antennal seg inseition, the most likely homologs is the mus- ment to which the fourth segment adductor at culus meropoditis «, although the third segment taches. The fourth segment abductor is much rotator in Penaeus and the meropodite muscle a larger than the fourth segment adductor. Upon in Astacus have different origins, the latter being contraction the fourth antennal segment abductor in the basipodite of the crawfish antenna. The reinforces the action of the fourth segment ad- 40 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE antenna / , onfenna/ scaie^ / t/aae//um \ ;/ / \ :— / \ i- / \ '~ / s / \ / \ / \ -•'*.. / s , • l .\ ^ s ••-•••-'.•, / \ '• y, • . ; •-.' V \ N . / ••-/, \ • -• • •''', \ • .• ' /. \ • / ,'/ . ; • •• \ '.-• \ N 1 \' - .- . •'- \ •. ' i ': •'.'/ /) th ^'?/-;/<2 /-7/-9^7 / • ' .' * ' A^L _.,-..; / "-t-Tn anrennai seg men t^ segmer/r •i \' fv , fx •/••• / r N -i fe/ ve/Ttra/ / po/n/-/'•y'!/'/-)/•/- of! A /3rd antenna/ scale arf/cu/af/on segmeni Jsf antenna/ segrr/enf \ M /^•\ 2net an tennaf y^ "Gxcret'ori/ segment'^'' por-'~ FIGURE 23.—Ventral view of right antenna sliowing skeletal elements. J WHITE SHRIMP FROM THE GULF OF MEXICO 41 antenna/ --antennal sca/e- flagellum sca/e abdacfor ^ antenna I muscle neri/e sca/e adduchr flagellum flexor tnusc/e 1 muscles scale adductor ^,antennal muscles .-"^ artery rotator muscte of 3rdantBtinat segmefff scale abductor muscles ^ ^^-"excretorcf pore , _ -—adducfvr muscle nmiofor musc/es of tbfantenna I of 2nd antenna/ segment segment ^antennaI nets/e yenfralpoint of flagellum flexor scale articulation musc/e promotormuscles of ^ V Jm\^'''t.'^~'^ flagellum extensor 5th aniennal segment^ ^^ il \^ %I / -V,./ '": i/ aclduclormusc/e of '. "'~~'* JI/i^ ^..^protnotormuscles of 4th antennal s^metit^ M 4th antennaI segment -/H)^ — cKcretory pore abc/uclormuscle of Islantennal 4/ti antenna I segmen /t - - - segment >cale abductor '•^ rotator muscte ot muscle-' 2ncl antennat segment 3rd antemat segment FIGURE 24.—Ventral view of right antenna showing ventral muscles, nerves, and blood vessels. Ventral cuticle removed. A. Superficial ventral elements. B. Dorsal elements. 42 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE seta '*<*^i»wgps'^Ty^-»'i>«^ A. Lateral Vie w ventral ^ventraf seta-s pit B. Ventral View FIGURE 25.—Enlargement of proximal segments of antennal flagellum showing sensory structures. ductor muscle in rotating the fourth segment on narrow apical tips on an apodeme of the fourth the third segment. In opposition to the fourth antennal segment. Contractions of the fourth an segment adductor, the fourth segment abductor tennal segment promoters move the fourth seg turns the fourth antennal segment laterad a short ment and the distal antennal parts anteriorly. distance. No homolog of this muscle is liere How these muscles are represented in the other advanced. crustaceans referred to in this paper is uncertain. FOURTH ANTENNAL SEGMENT PROMOTOR FIFTH ANTENNAL SEGMENT PROMOTOR MUSCLES MUSCLES r FIGURE 24 FIGURE 24 The fourth antennal segment promotor mus The fifth antennal segment promotor muscles cles (fig. 24) are situated within the body of the (fig. 24) are comprised of a tuft of at least four third antennal segment. At least three of these small muscles restricted to the fourth antennal muscles occur in Penaeus. The short, thick pro- segment. The muscles originate on the lateral motors originate throughout the ventral surface and posterior margin of the fourth article and of the third antennal segment and insert at their attach to an apodeme arising from the posterior WHITE SHRIMP FROM THE GULF OF MEXICO 43 (groove of the fifth antennal sefjnient. Contrac labrum is a lobe or sac suspended over the mouth tions of the fifth segment promotors move the fifth from a sclerotized region of the head known as the antennal segment anteriorly in a limited way. epistome (the hexapod clypeus). Crustacean mor- The homolo^ of these muscles in Afifanrs is phologists ordinarily do not consider the labrum very like the mnscnhis extensor propoditis II an an appendage. Most workers consider the labrum tennae. Berkeley (1928) does not find the muscle an unpaired structure. Some students of Crus in Panda!us. tacea place the epistome as the ventral element of a preoral, premandibular segment, behind the eyes, FLAGIOLU'M P]XTENSI()X MUSCLES antennules, and antennae, in the order of their oc FlGIJRK 24 currence in many adult crustaceans. Others have The fifth antennal segment of Penaeuf< contains even assigned the epistome to tlie sternum of the two flagellum extensor muscles (fig. 24), occupy antennal or mandibular segments, giving the lab ing the medial half of the segment. One of the rum an utterly indefensible postoral position. flagellum extensors originates on the medial side The position of the epistome and labrum in of the fifth segment and runs distally to insert on adult arthropods is variable. In some crusta the large extensor apodeme on tlie medial side of ceans, like the adult isopods and amphipods, and the flagellum base. The second extensor takes ori in most insects, the epistome is anterior or facial gin on the posteroventral region of the fifth seg (Snodgrass 1951). The labrum thus is ventral or ment, runs distally, and inserts on the large exten anterior to it. However, in most crustaceans, sor apodeme on the flagellum. These muscles some chilopods, and a few insects the head seg bring the base of the flagellum directly anterior to ments have tlirust forward, overgrowing the the proximal antennal segments. One or both of epistome anteriorly. The result is a secondary these muscles is undoubtedly homologous with the ventral position of the epistome and labrum in antennal dactylopodite extensor muscle as shown some arthropods. In point of fact, the labrum is by Schmidt (1915) in Astacus and by Berkeley the anterior end of the arthropod. I am in full (1928) in Pandalus. The related muscle, if any, agreement with Snodgrass (1951) in this view. in Gallinedes is uncertain without adequate in Furthermore, the labrum is here considered not formation about the nerves. only the most anterior part of the arthropod, but also the anterodorsal "upper lip" of these forms, FLAGELLUM FLEXOR MUSCLES and as such the dorsal part of the first segment. FIGURE 24 Considerable support for this interpretation has Three flagellum flexor muscles in the fifth an been adduced by Ferris (1947) and Henry tennal segment of Penasu^H turn the antennal fla (1948a), based on the study of the labral nerves. gellum to its normal position at riglit angles to the If we accept the view of various workers, in proximal antennal segments. The largest and cluding Henry (1948a), that the tritocerebrum of ventral-most of these muscles originates broadly arthropods is in reality the first ganglion of the along the proximal groove of the fifth segment and ventral nerve cord, whatever its fate in the adult, inserts on the flexor apodeme on the flagellum tlien we are bound to regard structures innervated base. Dorsal to the large muscle, two flagellum by tritocerebral nerves as primitively anterior. flexor muscles insert on the same flexor apodeme Since the labrum of PenaeMs is clearly innervated on the flagellum. The dorsolateral flagellum flex by a pair of nerves from the tritocerebral ganglia or originates in the lateral corner of the fifth seg (figs. 27, 76), similar to the situation in other ment, the dorsomedial flexor originating in the Crustacea and Insecta (Henry 1948a, 1948b), then medial corner of the fifth antennal segment. At the labrum is segment 1 in Penaeus. least one of these muscles in Penaeus is the homo- No evidence is here advanced to suggest that log of the musculus flexor dactylopoditis II anten the labrum is a reduced appendage. Indeed, this nae in Astacus and Pandalus. Whether homo- ancient "upper lip" was probably never an arthro- logs exist in Callinectes is not known. appendage in tlie true sense at any time in its history. The labrum is, however, very likely 4. LABRUM paired. In support of this is the morpliology of The labrum is the final component of the pro- the nerves and muscles. The labral nerves are tocephalon to be considered. In all arthropods the paired and arise from the clearly bilateral trito- 44 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE cerebral ganglia. Each nerve enters the sac of gous with the musculus oculi basalis posterior in the labrum and innervates only those sensoria and PandaluH, Astaem, and probably in CalUnectes. muscles in half the labrum from the medium Giving the name, epistomal stators, to these sagittal plane laterad. In Pcnaeiifi no nerve can muscles may be adding confusion to the morpho be seen crossing the median sagittal plane. Fur logical scene, since these muscles are undoubtedly thermore, the embryonic labrum of many higher the musculus attractor sincipitis described by arthropods (Johannsen and Butt, 1941; Young Grobben (1919) in the stomatopod Squilla mantis 1953) develops as a paired structure, with paired and the muscle attracteur du synciput illustrated coelomic sacs. by Mayrat (1955, 1956a, 1956b) in Praunus fieQc- U0SU8 O. F. Miiller. The "sincipit" (Mayrat spells SKELETAL ELEMENTS it "synciput") of Grobben (1917) is of course the FIGURES 26, 27 protocephalon of Snodgrass (1951) as applied to the Crustacea. No great objection is offered here In Penaeus setiferus the labrum is a soft, lightly to designating the muscles in Squilla and Praunus sclerotized sac attached between the widely spread as the synciput attractors. The muscles are in posterolateral bars of the epistomal Y (fig. 28, A). deed synciput or protocephalon muscles appar It may be noted in passing that these lateral epis ently functioning in certain forms to draw the tomal bars are morphologically anterior, al protocephalon posteriorly. However, the same though due to rearrangements of the head seg muscles in Penaeus do not attract the protocepha ments, discussed above, the position of the epi- lon. Furthermore they insert on a specific region stome is reversed in the head of many crustaceans. of the protocephalon, the epistome, and so deserve The anterior or medial bar of the epistome, as special a name as possible. The problem can dividing the antennal foramina, curves anterodor- be resolved by a study of the nerves, for if the sally to form a deep ventral pit, best seen in lateral epistomal stator muscles belong to the epistome, view (figs. 28, J.; 30) of a cleared anterior skeleton they should be innervated by epistomal or clypeal cut along the median sagittal plane. To the epis nerves. tomal invagination, or apodeme, is attached a LABRAL MUSCLES pair of muscles to be discussed below. FIGURE 27 The labrum is shaped to fit between the antennal bases anteriorly and the incisor and molar sur One of the most astonishing features of the faces of the mandibles posteriorly (figs. 26, yl, 5). anatomy of Penaeus setiferus is the musculature Various auricles and lobes project from the labral of the labrum. In the generalized insect labrum, surface to enhance its function as an aid in feeding. the structure is moved by two pairs of extrinsic A toothed structure, the posterior feeding process muscles, the anterior and posterior labral muscles (figs. 26, 27), projects directly into the mouth arranged for production and reduction. To the aperture. intrinsic labral compressor muscles of insects may be assigned various functions. In contrast, a re MUSCLE ELEMENTS view of general and special accounts of the anat EPISTOMAL STATOR MUSCLES omy of Crustacea has shown no reference to labral muscles in this class. Yet the labrum of Penaeus FIGURES 5, 6, 30, 34 (fig. 27) is operated by at least 12 pairs of in Originating on the dorsal surface of the cara trinsic muscles, bilaterally situated, and at least pace, lateral to the posterior protocephalon levator 1 intrinsic muscle running across the entire lobe. muscles, and converging on the anterior side of the From their arrangement, the labral muscles ob epistomal invagination is a pair of small muscles viously distort the labrum in all sorts of ways in which are named in the present work the epistomal the function of the organ as a tongue. In addi stator muscles (figs. 5, 6). The name derives from tion, at least 2 pairs of extrinsic muscles insert on the fact that contractions of the muscles would the edge of the labral foramen to move the entire appear to hold the epistomal invagination in posi organ. No attempt has been made here to assign tion during the contraction of other muscles in the functional muscle names to the individual labral area. The epistomal stator muscles are homolo muscles. WHITE SHRIMP FROM THE GULF OF MEXICO 45 /eft /ncrrfcZ/hu/cr/'- cifcumo&sophac/eo'/ connecf/ve ''mar}c//6/& poster/or- I process FIGURE 26.—Labrum. A. Lateral view of labrum from the left side showing relation of labrum to mandible, li. Ventral view of labrum showing posterior feeding process. 468050 O—59 4 46 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE /efr /ey^/^o'/ ga/7ff//o/7 co/?/7Ga f/y&. /(ji>r-a/ posten'or- feedinff • process gastric • /ahra/ ml/L gcfng//of7 \ / f,— I I • fabra/ nerve ! r B \ /e^f c/rcamo FIGURE 27.—Lateral views of labrum from the left side showing muscles and nerves. A. Cuticle removed to show superficial lateral muscles of left side. B. Interior muscles of labrum. WHITE SHRIMP FROM THE GULF OF MEXICO 47 Although not shown in the illustrations of the (^ompared to the ventral skeleton of Astacura labrum, material of a p;landular nature is found and Brachyura, that of Penaevs is very lightly in median ventral regions of the structure. Gland K'lerotized (Snodgrass 1952; Huxley 19()(i). In cells in the crustacean labrum have been described the crawfishes, the median sternal elements are in the past. rigidly fused together, except for the sternum' of B. Gnathothorax the last thoracic segment, and thus provide a rigid keel from which the pleurosternal arms arise. The j'Z'uatliothorax will be considered here as Pleural (laterotergal) and sternal apodemes arise those segments following the protocephalon in from the invaginations between the fused arms of volved primarily Avith feeding and walking. The two adjacent segmental units. Similar apodemes hitter are not truly separable as body tagmata in occur in the same locations in the ventral skeleton tlie Crustacea, since after the mandibles and max of Pendens, but the ventral sternal element of illae, varying numbers of walking legs may be each segmental unit is separated from its neigh adaj)ted for feeding (Snodgrass 1951). Walking bors by a transverse slit of thinly developed legs ada])ted for feeding are referred to as maxil- cuticle (fig. 28). The slits permit movements be lipeds. In tlie Crustacea Decapoda, the gnath tween segments along the anteroposterior axis of othorax is comprised of the mandibles, paragna- the thorax, even though the lateral pleurosternal tlia, 2 pairs of maxillae, ?) ])airs of maxillipeds, arms are fused. and 5 pairs of walking legs. In the present study, The ventral skeleton of the gnathothorax in tlie gnathal segments will include the maxillipeds, Penaeus (figs. 28, 30) consists of a series of and to the thorax will be assigned the five walk sclerotized units which are slightly movable with ing legs, without implying any morphological respect to one another, but not articulated (fig. rigidity to the division. Above and to the sides, 28). Each unit bears the paired foramina and the gnathothorax is protected by the large dorsal muscle apodemes of the jointed appendages at shield, or carapace. Nearly all trace of segmenta tached thereto. The typical segmental unit con tion has disappeared from the dorsal regions of tains contributions from two sources: The tergum, the gnathothorax and the carapace. The few re in the form of the dorsal tergum, and the vertical, maining sutures and markings of the carapace are laterotergal pleural plates (Snodgrass 1952), not well understood in the Crustacea; consequent dorsal to the leg bases, and the sternum which ly systematic nomenclature which has grown comprises the ventral region between the leg bases. The dorsal condyles of the coxopodites are situ up around these devices is highly artificial. ated on the laterotergal plates while the ventral The gnathothorax is constructed in the form coxopodite condyles are sternal. In the anterior of a rather special box arranged to provide both region of the gnathothorax of Penaeus^ the rigidity and movability. The immobile carapace pleural plates lie horizontally to become the roof is heavily sclerotizecl for protection of the internal of the gill chamber, similar to the arrangement in organs and support of muscle origins, and ex Caviharus Jongulus Girard as shown by Snod tends ventrad in a deep fold of the tergum to cover grass (1952). completely the laterally placed gills (figs. 30, 31). The delicate nature of the ventral skeleton in The lateral carapace is called the branchiostegite, PenaeuH is even more noticeable in lateral view since the structure forms a chamber for the gills. (fig. 28, A). From here the pleurosternal arms The deep fold of the rigid carapace forming the may be seen to bifurcate in the pleural region bianchiostegite permits movement between the and those of each segmental unit tend to unite carapace and the architecture of the ventral dorsally in the form of a reverse-curve, or ogee, gnathothorax. The whole is reminiscent of a arch of the architect. Again, compared to the modern sedan in which a rigid body above is at composite pleura of the crawfishes (Snodgrass tached to a chassis able to respond to imperfec 1952), those of PenaeuH appear to have been re tions in the road surface. Upon the dorsal cara tained in a somewhat more generalized condition, pace orginate numerous important muscles, in with 7 or 8 pleura clearly distinguishable from cluding some of those of the protocephalon ap one another (figs. 28, 30). pendages, the mandibles, maxillae, gastric mill, Huxley (1906), Caiman (1909), and other and dorsal and ventral abdominal muscles. students of Crustacea often refer to the system 48 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE C3 I 1 WHITE SHRIMP FROM THE GULF OF MEXICO 49 f « % •§ 5. 0 a. S -^ 8 -5 s '•fc "10-Q ^ g =^ 50 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE I •ij / > / '•' \. 11 -^ I ^ / *-) 03 a a 37 o ^ c \ \ \ \ \ s \ \ \\ \ " \ \ I. \, I is 0 i 1 I p •§ is- 1 I f^ ^ .^ 5 I WHITE SHRIMP FROM THE GULF OF MEXICO 51 o in 1 o I I i Ah -5? (^ 52 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE of pleiirosternul iiiviio:infttioiis or apodeiiies in its mechanical action will be considered when the decapods as the endophragmal system. In Asta- scaphognathite muscles are described, page 69. cura and Brachyura tliese apodemes fnse inter The tightly packed gills or branchiae rise nally to form complex endoskeletons consisting roughly dorsad from their points of origin on the of septate structures in the thoracic sejjments leg bases and pleura (fig. 31). Each gill consists above the ventral nerve cord. The sternal apo of an axial circulatory rachis from which the in demes fuse in the midline to form the sternal dividual gill filaments branch. Details of gill furca in the Insecta. In the fjnathothorax of structure will be given below in the section on Penaeus the laterotergal and sternal apodemes respiration. Interspersed among the gills are six are light and do not fuse, consequently no endo- flat, setose, bilobed structures, the mastigobran- phragmal system is found, unless we consider the chiae or epipodites. The lateral margins of the transverse mandibular apodeme (see endosternite, mastigobranchiae may be seen upon removal of fig. 38) an endophragm of some sort. This the branchiostegite (fig. 31), but they are best structure will be considered more fully in the seen if the gills are removed (fig. 32). If a shrimp treatment of the mandibles. whose branchiostegite has been removed is cooled Although the ventral sternal elements of the so that the body processes are reduced but not penaeid gnathothorax are not coalesced into a stopped, the long, fine setae of the mastigobran rigid keel as in Astacura, the sternal plates do chiae may be seen to beat in phase with the move broaden from the anterior to the posterior ends ments of the scaphognathite, or gill bailer, thus of the gnathothorax, and abruptly so in the last suggesting that the epipodites play a part in the three thoracic segments. The sternal plates of the water flow and cleaning of the gills. last two thoracic segments are particularly modi The older literature of Crustacea abounds in fied in the female to receive the spermatophore so-called branchial formulae, the formal study of from the male (figs. 28, B; S9). These structures gill origins. In the decapods the generalized situ will be discussed in detail in the section on the ation is 4 gills for each side of the segment (fig. reproductive organs, page 155. 32) ; 1 arises from the laterotergal, pleural plates (pleurobranchia), 2 from an articular element Gills dorsal to the coxopodite (arthrobranchia), and 1 The gills may be exposed by cutting away the from the coxopodite (podobranchia) (Caiman 1909; Snodgrass 1952). The epipodite (mastigo- branchiostegal region of the carapace along the branchia) arises from the coxopodite. As shown dorsal-most reaches of the inner lining of the by Caiman (1909), the use of the gill origins as brancliiostegite, where the lining joins the lat- evolutional landmarks is limited by the practical erotergal plates. The gills are thereby found to difficulty of distinguishing between the pleuro- occupy a chamber (fig. 31), open to the outside branchiae and arthrobranchiae in different ventrally and posteriorly by a narrow slot between species, since ontogenetic changes of gill origin the leg bases and thoracic wall on the inside and are frequently seen. In fact, evidence exists sug the extreme margin of the branchiostegite on the gesting that all the gills develop from the ap outside. The chamber is closed dorsally by the pendages, rather than from the limb bases or body branchiostegal fold. The chamber is rather shal wall. Apparently in Penaeus the podobranchiae low transversely in its broad, posterior region, develop embryo]ogically from the mastigobran but becomes narrow anteriorly and is made much chiae. deeper in the region of the second maxilla by the As may be seen in figure 31, the branchial for lateral and horizontal reflection of the latero- mula for Penaeus setiferus consists of 1 tiny ar tergal, pleural plates. This narrow, deep, an throbranchia on the first maxilliped (fig. 42) ; 1 terior chamber is thus a funnel, closed dorsally podobranchia, 1 mastigobranchia, and 2 arthro by the pleural bridges, medially b}^ the vertical branchiae on the second maxilliped; 1 mastigo pleural wall, laterally by the branchiostegite, and branchia, 2 arthrobranchs, and 1 pleurobranch on ventrally by the large, flat coxopodite exites (figs. the third maxilliped, and the first, second, and 29, E; 42) of the first maxilliped. Inside the third pereiopods; 1 arthrobranchia and 1 pleuro funnel resides a pump, the scaphognathite of the branchia on the fourth pereiopod, and 1 pleuro second maxilla (figs. 29, D: 31; 41). Details of branch on the fifth walking leg. In a European WHITE SHRIMP FROM THE GULF OF MEXICO 53 C cS o I I o 0) 54 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE Species of Penaeus^ Caiman (1909) describes a of the nerve cord and the ventral (subneural) mastigobranchia on the first maxilliped and 1 ar- artery. These muscles are thin and fan shaped, throbranch and 1 pleurobranch on the second broad anteriorly and narrowing to posterior at maxilliped. Otherwise the branchial formulae of tachments to thin connective tissue fasciae. In the two penaeids are the same. Astacus the lateral and ventral pleurosternal apo- demes fuse above the ventral nerve cord to pro MUSCLE ELEMENTS duce the mesophragm of the endoskeletal system. Of the numerous muscles attaching throughout The superficial ventral thoracic muscles of the the gnathothoracic region, many belong to the crawfish attach to these mesophragms. Berkeley mouthpart and thoracic appendages and will be (1928) finds that the ventral thoracic muscles in taken up when the latter are discussed. Others are Pandalus attach to the endophragmal para- protocephalon muscles and have already been con phragms, the lateral fusion product of the pleural sidered. Still others are associated with the ali and sternal apodemes. As stated above, Penaeus mentary canal and heart and will be dealt with in has neither paraphragm or mesophragm; how^- the sections concerned with these organ systems. ever, one would expect the superficial ventral The many remaining muscles are either small su muscles to attach to the little pleurosternal apo perficial lateral and ventral muscles of the thorax, demes near the limb foramina. Contrary to such or large dorsal and ventral muscles. Some of these expectations, the superficial ventral thoracic are morphologically thoracic muscles, and some muscles in the white shrimp attach to small apo are morphologically abdominal muscles. In the demes on the pleurosternal brachia. The result is functional sense, the foregoing muscles may be to place these muscles ventrad of the nerve cord. classified as abdominal musculature, for all of the Their function of drawing the ventral thoracic large muscles taking origin on extensive areas of segments together is probably the same as in the dorsal and ventral thoracic skeleton represent Pandalus and Astacus. the major muscular attachments of the abdomen In Penaeus 7 pairs of superficial ventral tho to the thorax. Substantial movements, espe racic muscles are evident compared to 6 for Asta cially in the dorsoventral plane, are possible be cus and Pandalus. Schmidt (1915), however, tween the thorax and abdomen and it is these finds tw^o superficial ventral thoracoabdominal muscles which mediate the movements. muscles in Astacus compared to one in Penaeus and Pandalus. SUPERFICIAL LATERAL THORACIC MUSCLES ANTERIOR THORACIC MUSCLES FIGURE 33 FIGURES 33 TO 36 Stretching between the dorsal reaches of the By far the largest ventral muscles of the tho pleural brachia are at least five superficial lateral rax are the lateral and median anterior thoracic thoracic muscles. These muscles are extremely muscles (figs. 33 to 36). The anterior-most mus thin and weak. xVpparently they give rigidity to cle of these is the lateral anterior thoracic muscle the thin cuticle of the area during lateral move 1 (figs. 33 to 36). This muscle originates by a ments of the shrimp along the anteroposterior large, lateral oval in the region just posterior and axis. Schmidt (1915) and Berkeley (1928) do slightly dorsad of the hepatic spine. In dorsal not describe these muscles in Astacus and Parida- view (fig. 35) the muscle may be seen to run pos- lufi. The possibility that the epimeral attractor teroventrally to join the other anterior thoracic muscles in Astacus and Pandalus are the super muscles on the ventral surface of the thorax. In ficial lateral thoracic muscles of Penaeus is slight. this area all the anterior thoracic muscles are in terconnected to segmentally arranged fasciae. VENTRAL MUSCLES , The other lateral anterior thoracic muscles (figs. 34, 35), Nos. 2, 3, 4, and 5, may best be seen SUPERFICIAL VENTRAL THORACIC MUSCLES in ventral view (fig. 36). These muscles take FIGURE 36 origin from the ventrolateral fascia of each tho The superficial ventral thoracic muscles (fig. racic segment and run posteriorly into the ab 36) are situated slightly laterad of the ventral dominal musculature with the other anterior tho nerve cord. Their median parts are also ventrad racic muscles. WHITE SHRIMP FROM THE GULF OF MEXICO 55 -5 I ^•8 to >!^ ^ ^ i; S? ^ in 5. II ^ Ik •5- I 9 V Vs I •3 X % I o •A\\ ly it- S<>i I \\ ' '• "ix a •a \ 5 ^ ^ e 5 I II OS i 1 I" I-5 if: fi i (/) J) llll 2 o 0* o o ^ s III «- k ^ I § ts S .0 g-^ > 0 56 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE ^ v-J1s 4u *. In \ \ \ ^1 \ \ 1^ •a a OS •a a) a & ft b o 0} G d in .a 1^1 III WHITE SHRIMP FROM THE GULF OF MEXICO 57 a P o s^ 5S Ma -d a "^"^ ~ a C3 0? o 11' 4-) ao -$5 ew O W ^ •^ » "3 s^ %• O Q1 1* CO 58 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE Tlie median anterior thoracic muscles (fio;. 36) carapace adductor muscle (Schalenschliessermus- also oriirinate in se^mentally a]Tano;ecl fasciae on kel) in species of Penaeus. Palaemon. Leander, the ventral surface. Joined by the other anterior Pandalus, Galathea, and Athanas. thoracic muscles, the median anterior thoracic muscles pass caudad into the abdominal muscles. DORSAL MUSCLES Tlie anterior thoracic muscles play an impor DORSAL THORACOABDOMINAL MUSCLES tant pai't in tlie powerful flexion of the abdomen on the thorax made by the white shrimps when FIGURES 34, 3.^ the animals withdraw suddenly from dauii-er. Inserting on the anterodorsal terguni of the first The anterior thoracic musculature is similar to abdominal segment and running forward and that in FandaluH and A^taeu^, except that in the down to the laterotergal brachia of the thorax crawfish these muscles are somewhat smaller. are four pairs of long, slender muscles, the dorsal The anterior thoracic muscles are fully homolo tlioracoabdominal muscles (figs. 34, 35). In dor gous in all the forms mentioned here. sal view (fig. 35) the area of attachment may be seen on the dorsal part of the first abdominal seg VENTRAL HEAD LIGAMENTS ment. The muscles divide around the heart and FIGURES 37, .38 ]iepato])ancreas as they go to the lateral wall of The ventral head ligaments are small structures the thorax. Each muscle originates on its own attached between the lateral win^rs of the epistome pleural arm, suggesting that the muscles each be and the mandibular endosternite (fig. 37). Ap long to specific thoracic segments. Upon contrac parently they hold the endosternite in position an tion the dorsal thoracoabdominal muscles extend teriorly. Schmidt (1915) calls these ligaments the the first abdominal segment with res])ect to the ventral head muscles in Astacus. Grobben (1917), thorax, in opposition to the action of tlie anterior however, denies the presence of muscle fibers in the thoracic muscles. structures and suggests the name ligament for mus The dorsal thoracoabdominal muscles are evi cle. Berkeley (1928) describes the organs as ven dently the same muscles as those designated as the tral head muscles in Pandalus. dorsal thoracoabdominal muscles in Pandalus and Astacu'u although they appear to be relatively CARAPACE ADDUCTOR MUSCLE larger in size in the caridean shrimp. FiGUKKS 33. 34. 37. 38 LATERAL THORACOABDOMINAL MUSCLES The carapace adductor muscle originates on the carapace just ventrad of the most ventral part of FIGURES 34, 35 the protocephalon attractor muscle (fig. 33). The From an area laterad of the heart and the dor origin point of the carapace adductor is slightly sal thoracoabdominal muscles, the lateral thoraco dorsal to the horizontally turned pleural plates, abdominal muscles (figs. 3-1, 35) originate along above the gill pump, or scaphognathite. The mus a diagonal line just above the dorsal edge of the cle runs direct!}^ mesad to insert on the postero- inner branchiostegal fold and run ventrally and dorsal midline of tlie endosternite (fig. 37). The caudad to junctions with abdominal muscles. carapace adductor muscle functions as the major Very likely the thoracoabdominal muscles are in position retainer of the endosternite. It may also reality abdominal muscles. Unlike the anterior play a part in necessary distortions of the cara thoracic muscles, the lateral thoracoabdominal pace associated with feeding, molting, and tlie like. muscles are not seginented. At least four pairs of The carapace adductor muscle appears in many these muscles covering a broad area laterally are crustacean groups (Grobben 1917). Schmidt found in Penaeus. Functionally, the lateral thora (1915) describes it in Astacus as the musculus coabdominal muscles are involved in the flexing of dorsoventralis posterior and Berkeley (1928) has the abdomen, reinforcing the action of the an adopted his terminology for the muscle in Panda terior thoracic muscles. Berkeley (1928) suggests lus danae. Grobben (1917) considers the cara that contractions of the muscles on one side may pace adductor a useful phyletic character, because bend the abdomen laterally in Pandalus, and such of its frequent occurrence, and we are indebted to a movement may also take place in Penaeus. The this worker for the name. Grobben described the lateral thoracoabdominal muscles of Penaeus are WHITE SHRIMP FROM THE GULF OF MEXICO 59 .1 , -J" I 4. § S2 V) / ^1 \ •j^ 1» lb >1 • I A '^ 7 / 4. e8 ^ 5 -i-I •k-^ 1' 1 ( i,"- 1,1 1 !.;^ x« 1 ' / t! C ,^ cn 1 « ^ 5t X ;^\ & ^'fe. s^ 3 .' '- !" - / I • ^'•'' a >l>b y 60 I 0 1 % -"'•'' ,'i '•- •'' •' « .-•' t I • /• ! «' » H «i 4. ii -' \ \ \ , o 7 ^i- o -i- >^ Si •'^ A >- •S ft 4- o •^ U^?;<^5^ -L^ «: I 1' .• II , 1," 1 " 1 >. \, '• 'H to /if . tin 05 fs I^ it ^<5^ 1^5 y IS I 9) 5 1 1^ I 60 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE imdoubltedly lioniolofrous to the nmsculi laterales its ventromedial incisor and molar surfaces and tlioi'acoabdomiiiales of Astacuf< and Pandahi-'i. anterior palp, may l)e seen in ventral view (fig. "29, mandible). Tlie incisor surfaces consist of 2 APPENDAGES OF THE GNATHOTHORAX or 3 sharp ridges used in cutting and tearing food, The generalized limb of Crustacea is thought to Avhereas the flat molar surfaces are for grinding. consist of a basal protopodite, made up of a prox Both kinds of surface are heavily impregnated imal coxopodite and a distal basipodite, bearing with a hardening substance, possibly calcium car two rami, the exopodite and endopodite. This bonate, in the form of crystalline stones molded simple arrangement has been retained in the pleo- into the cuticle. The stones may be removed by pods of Crustacea, notably the Natantia. The dissection. The tubular part of the mandible, the typical gnathothoracic limb of Penaeus^ if one can body, is said to be the coxopodite, whereas the be said to exist, has the basal coxopodite and basip palp is thought to be part of the endopodite (Cai odite, the latter with a well-developed endopo man 1909). The subcylindrical body of the dite and a reduced exopodite. The coxopodite of mandible is distinctly divided into a large basal the white shrimp is attached to the limb base by portion and a distal lobe, immovably connected to means of diametrically placed condyles, the axis the basal part. In the mandible of Anaspides, of the condyles varying in accordance with the the basal part is called by Snodgrass (1952) the location and function of the appendage. The coxopodite and the distal lobe an endite of the basipodite is attached to the coxopodite by di- coxopodite. It is interesting to note that the condyles whose axis is at right angles to that of distal gnathal lobe is movable in some arthropods, the coxal condyles. Distally, the endopodite is notably the Diplopoda, and similar to the maxil divided into the typical five articles: the ischiop- lary lacinia of insects. odite, meropodite, carpopodite, propodite, and The mandibular palp, a part of the endopodite, dactylopodite. The coxopodite may give rise to extends anteriorly from the body of the mandible lateral epipodites and in some cases gills. in the form of a flat, setose lobe shaped to fit The exopodite may be modified to long, frond- around the labrum laterally and anteriorly. The shaped filaments (second and third maxillipeds) larger part of the broad palp arises from a basal or reduced to a small finger projecting from the segment, the latter being narrow at its posterior basipodites (fig. 31). The gnathal appendages, junction with the mandible and broad anteriorly. excluding the second and third maxillipeds, tend The mandible of Penaeus is attached to the ven to be modified substantially from the typical plan tral skeleton by a medial condyle located at a point given above. on the posterior arm of the epistome just laterad of the labrum (fig. 28), and by thin cuticle be 1. MANDIBLES tween the mandible and the lateral extension of SKELETAL ELEMENTS the posterior epistomal arm. The mandible has also a lateral condyle on the side of the carapace. Of the gnathothoracic appendages, the mandi Among taxonomists the external manifestation of bles are perhaps the most difficult to understand. this lateral mandibular condyle is known incor For one thing the dorsal and internal manifesta rectly as the hepatic spine. Snodgrass (1951) tions of this and other gnathal segments are coa erroneously shows two mandibular condyles in lesced or obliterated. In addition, the mandible is Penaeus on the laterally spread, posterior bars of complicated by the presence of a true endoskeleton the epistome, and does not mention the lateral (figs. 37, 38), lying transversely in the gnatho- mandibular condyle. This is curious since, as thorax above the nerve cord and supporting the Snodgrass (1935, 1951b) has shown in earlier gastric mill. Nevertheless, Snodgrass (1951b) ad work, a significant phyletic series is evident from duces strong evidence in support of the evolution the study of the arthropod mandible, based upon of the mandibles from a typical limb on the basis the evolutional response of the musculature to re of a comparative study of the skeleton and muscles striction of mandibular movements. In the course of the arthropod mandible. of the work, Snodgrass makes the fact amply The sclerotized parts of the mandibles in clear that in no case is the site of the primary Penaeus are relatively simple. The strongly (lateral) condyle ever to be found on the epis sclerotized incomplete tube of the mandible, with tome. Rather, the generalized arthropod man- WHITE SHRIMP FROM THE GULF OF MEXICO 61 left oe3opha(jHJi> cite •umoesophagea/ connect/ve mi/sc/es i /// / abc/ucfor' /// /// head musc/e /// //gamenfs /// 0 '/' 'mi'iy, ,.. -" mand/bufcfr abafacfor musc/e I. ' dorso-ventra/ tnotidibahr •'•V- tensor t fnusc/e :.?i..j anterior . jl mand/buhr ;:/ carapace adductor adductor musc/es rm/sc/e poster/or dorso-ver/traf mand/bij/cj/' / dorso-ventraf max/'/fary adductor / mandibuiar \ tensor muscti!? ^ tfgament musc/e FIGURE 37.—Dorsal view of mandibular musculature. 468059 0—59——5 62 riSHEBY BULLETIN OF THE FISH AND WILDLIFE SERVICE /efr oesop/?ac/i/s / fr/focerehra/ c/rcamoesoffha^<9a/ / co/vm/ssute m anc/ihu/af / mand/'bi-f/ar abdcfctor ahdacfor mt'/sc/e, rm/sc/e " venttd/ /?ead o'orso- ventra/ //^merits mandibular tensor rnu-safe, anterior fas a a (endostoniite) I : / ' I / anf'er/or apodeme of mond/hu/ar •,./// poster/or adductor k mandiiicdar mofsc/es ' adductor \ \ musde \ \ \ \ poster/or \ carapace mand/bu/ar \ scaphognarhd& addifctot- adductor \ resp/ratar// musc/e • mfjscte musctes FIGURE 38.—Dorsal view of mandibular musculature, dorsal-most muscles removed. WHITE SHRIMP FROM THE GULF OF MEXICO 63 dible articulates with the mandibular segment at gulus Girard, in speaking of the structure. In the a single point laterally, an arrangement which course of a histological study of muscular and permits the varied movements of the organ seen skeletal elements in various Crustacea, Debaisieux at this stage of its evolution. (1954) emphasizes that the mandibular endo The monocondylic mandible is found in all the skeleton is an endosternite, fully homologous -in mandibulate arthropods except some higher Crus his opinion with that of Arachnida. tacea and the pterygote insects (Snodgrass 1935). No further evidence is offered in the present In the latter forms, a secondary condyle on the study for the homology of the endosternite of epistome is added mesad of the primary lateral Arachnida with the mandibular endoskeleton of condyle. Mandibular movements now become re Crustacea. However, from the work of Debaisieux stricted to those about the dicondylic axis. With (1954) on the structure in Crustacea, we can be simplicity, furthermore, comes strength. Snod fairly certain that the mandibular endoskeleton of grass (1935, 1951b) has described the evolutional all crustaceans mentioned above are homologous. simplification of mandibular musculature which In response to the plethora of names for the man follows in groups developing the dicondylic dibular endoskeleton, the present writer sees no mandible. objection to the terms "endosternite" or "endo A comparison of the mandibular hinges and sternum" given by Debaisieux (1954) and Snod musculature of the white shrimp with the account grass (1952). These names are used here as of Snodgrass indicates that Penaeus represents a equivalent to one another and to the expression, transitional form; for, although the shrimp man "mandibular endoskeleton." Whether an endo dible is dicondylic, its musculature is strongly skeleton and an endosternite are morphologically reminiscent of the monocondylic musculature in equal is not made clear in the literature. apterygote insects, myriapods, and lower Crus As a final word on the composition of the endo tacea. skeleton, it will be recalled that Schmidt (1915) The endoskeleton of the mandibles, mentioned describes two small medial muscles between the above, is a thick tendon situated between the mesophragms of the endosternite, the endo- mandibles and upon which the heavy ventral ad phragmal compressor muscles. Grobben (1917) ductor and abductor muscles insert (figs. 37, 38). denies the existence of muscle fibers in this mate The substance of the endoskeleton is extremely rial in the crawfish and establishes that the area tough connective tissue, not sclerotized. Such is composed of connective tissue. cuticular components of the structure as may exist are not hard. The mandibular endoskeleton MUSCLE ELEMENTS of Crustacea Malacostraca has been variously in MANDIBULAR ABDUCTOR MUSCLES terpreted. In forms having a well-developed endoskeletal system in the thorax, the mandibular FIGURES 33, 37, 38 element is considered to be a part of that system. At least three mandibular abductor muscles In Astacus, Schmidt (1915) refers to the struc (figs. 33, 37, 38) are found in Penaeus. The ture as the endoskeleton, specifically, the head smallest is the most anterior. This muscle arises apodeme. Snodgrass (1935), referring to a simi in connective tissue in the anterior region of the lar structure in the mandibles of a diplopod, uses gnathothorax, lateral to the esophagus and dorsal the term median ligament. Whether this worker to the circumesophageal connective, and runs pos attributes the median ligament to the endoskele teriorly and ventrad to insert on the anteromedial ton is not clear. Berkeley (1928) designates the part of the mandibular endoskeleton. Contrac same material in Pandahis as the anterior fascia. tions of these slender muscles aid in opening the She evidently considers it to be endoskeletal in mandibles. No counterpart of the muscle is de nature, and the fusion product of the mandibular scribed for the other crustaceans to which ref and first maxillary segments. Grobben (1917) erence has been made. apparently believes the mandibular tendon to be The large and important mandibular abductors endoskeletal, naming it the median transverse originate in connective tissue on the laterotergal mandibular tendon. Snodgrass (1952) refers to plates (fig. 33) and run posteromedially to inser an intergnathal ligament in Anaspides tasmaniae tion areas on the mandibular endoskeleton (figs. Thomson and an endosternum in Carribarus lon- 37, 38). Their contractions serve to open the 64 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE gnathal elements of the mandibles. These two of the dorsal carapace by a narrow, thin, apodeme mandibular abductor muscles are fully homolo between the dorsal edges of the protocephalon at- gous with the major and minor mandibular ab tractor muscle and the gastric mill (fig. 35). The ductor muscles in Astacus and with the mandibu apodeme runs anteroventrally to a thin, fan- lar abductor muscle in Pandalus. Presumably shaped muscle inserting on the dorsal surface of the homology holds for the major and minor the endosternite. It crosses over the tendon of the mandibular abductors of Callinectes. dorsoventral maxillary tensor muscle medially. The muscle is closely appressed to the fibers of the DORSOVENTRAL MANDIBULAR TENSOR MUSCLE dorsoventral maxillary tensor, but may be sep FIGURE 37 arated readily from the latter in the sagittal plane. The dorsoventral mandibular ligament (or The long tendon of the dorsoventral mandibular muscle) aids in retaining the endosternite in tensor muscle (fig. 37) originates in connective position. tissue on the dorsal carapace by an extremely thin band mesad of the origin of the antennal promotor The structure was found in a species of Penaeus muscle. The thin apodeme runs ventrad to the and of PoXaemon and named by Grobben (1917). muscle body which inserts on the anterior part of Since the dorsoventral mandibular ligament ac the mandibular endoskeleton, slightly laterad of tually ends in a small muscle, Grobben's name the small mandibular abductor muscle. The mus should be replaced with the name, "dorsoventral cle pulls the mandibular endoskeleton dorsad, pos mandibular muscle." A muscle in Pandalus^ des sibly as a minor adjustment of the mandible in ignated by Berkeley (1928) as the musculus dorso- feeding. The muscle was named by Grobben ventralis anterior 2, is probably the dorsoventral mandibular ligament. (1917), who described it in a number of crusta ceans, including species of Penaeus^ Palaemon^ POSTERIOR MANDIBULAR ADDUCTOR MUSCLE Leander^ Pandalus^ and Nebalia. The description by Berkeley (1928) of Pandalus danq,e does not in FIGURES 30, 33, 34, 35, 37, 38 clude the dorsoventral mandibular tensor muscle, Taking origin in an elongate ovoid on the dorsal although it is almost certainly present in that carapace is a large, wedge-shaped muscle, the pos species. The muscle is missing in Astacus. terior mandibular adductor muscle (figs. 35, 37). Growing narrower as it passes ventrad, the pos ANTERIOR MANDIBULAR ADDUCTOR MUSCLES terior adductor attaches to a broad apodeme (figs. FIGURES 33, 34, 37, 38 29, B; 30, apodeme, mandibular adductor muscle). The anterior mandibular adductor muscles (figs. The adductor apodeme arises from the posterior 33, 34, 37, 38) are the largest occupants of the margin of the semitubular mandibular body. The subcylindrical mandibular body. At least three apodeme is so placed that a dorsal pull of the pow muscles are evident in Penaeus^ although a study erful muscle brings the gnathal lobe to the mid of the nerves may show that the muscle groups line. The posterior mandibular adductor muscles are actually parts of the same muscle. The adduc are widely represented in the Arthropoda. tors originate laterally throughout the body of MANDIBULAR PALP FLEXOR MUSCLE the mandible and insert extensively over the tis sues of the mandibular endosternite. Contractions FIGURE 29 of the anterior mandibular adductors draw the The muscles operating the mandibular palp are gnathal lobes of the mandibles together. The ac situated either in the gnathal lobe of the mandible tion is direct and efficient. The anterior mandibu or in the basal segment of the palp. The distal lar adductor muscles of Penaeus are the homo- lobe of the palp contains no muscles. The mandib logs of the musculus adductor anterior mandibu- ular palp flexor muscle (fig. 29) originates in the lae in Astacus and Pandalus. proximal region of the basal palp segment near DORSOVENTRAL MANDIBULAR LIGAMENT the foramen between the gnathal lobe and the palp base. The muscle runs distally, becoming broad FIGURES 33, 34, 35, 37 and flat, and inserts on the posteroventrad margin The dorsoventral mandibular ligament (figs. 33, of the distal palp lobe. In action, the mandibular 34, 35, 37) originates in the thick connective tissue palp flexor turns the distal lobe ventrad. An ap- WHITE SHRIMP FROM THE GULE OF MEXICO 65 parently similar muscle in the palp of Astacus is called by Schmidt (1915) a palp flexor also. Berkeley (1928) describes both a palp flexor and a palp extensor in the body of the mandible of Pandahis. MANDIBDLAE PALP ADDUCTOR MUSCLES FiGUEE 29 The proximal, and larger, mandibular palp ad ductor muscle originates on the posterior wall of the gnathal lobe of the mandible and inserts on the medial margin of the basal palp foramen. The smaller palp adductor originates dorsad of the insertion of the larger palp adductor on the medial margin of the gnathal lobe foramen. Con tractions of these muscles turn the palp segments toward the midline. The proximal mandibular FIGURE 39.—Paragnatha. A. Anterior view. B. Posterior view. palp adductor muscle is probably homologous with the musculus flexor a mandibulae of Astacus and either the palp flexor or extensor of Pandalus. mandibular entities. In a comparative study of arthropod nerves, Henry (1948a) assigns the 2. PARAGNATHA paragnatha to the mandibular segment by virtue of their innervations. She invariably places the The paragnatha (figs. 29, 39) are two rounded paragnathal nerves in a position posterior to those lobes suspended from small foramina in the ven of the mandibles. tral skeleton immediately posterior to the gnathal Eecently, Chaudonneret (1955, 1956), in a de lobes of the mandibles. Their cuticle is very thin, tailed study of the gnathal nerves of Orconeotes except for a slightly thickened ridge along the linwsus (Eafinesque) { — Oambarus affinis Say), posterior surface of each paragnath. No intrinsic takes issue with Henry (1948a) and advances the muscles are found in the body of the paragnath in idea that the paragnathal nerves are in fact an Penaeus; however, a small muscle, the para- terior to those of the mandibles and entirely dis gnathal muscle (fig. 39), inserts on the lateral tinct from the mandibular nerves. On other margin of the paragnath. The paragnathal mus grounds, furthermore, this worker suggests that cle moves the paragnath laterally and anteriorly the paragnathal foramina are anterior to the man against the gnathal lobe of the mandible. dibular foramina and holds that their relative Classically, the paragnatha have been inter positions with respect to the mouth are constant preted as a secondary development of the mandib in the Malacostraca. In the opinion of Chau ular segment, on grounds of their embryonic de donneret (1956), the facts make difficult the inter velopment and because of their innervation by pretation of the paragnatha as either epithelial mandibular nerves. Also, the view that they be lobes or parts of the mandibles. Instead, this in long to the maxillae has been expressed. The idea vestigator thinks that the paragnatha may belong that the paragnatha are reduced true appendages to a reduced premandibular, paragnathal segment has in general been discounted, despite the pres homologous to the insectan superlingual segment. ence of movable terminal lobes in the paragnatha If the paragnatha are indeed premandibular of Tanaidacea (Crustacea). Snodgrass (1935) and homologous to the superlinguae, an interpre mentions the similarity of the insectan super- tation which Snodgrass (1935) seems to consider linguae to the crustacean paragnatha. This possible, certain aspects of the morphology of the worker feels, apparently, that there is no evidence insect hypopharynx will need review. Careful that the paragnatha are appendicular or that the study of the paragnathal nerves in Penaeus tends superlinguae and paragnatha are homologous to support the view of Chaudonneret (1956) that structures. He does indicated (Snodgrass 1952) this nerve is slightly anterior to the mandibular in a study of Gambarus, that the paragnatha are nerve. However, the gross anatomy will have to J 66 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE be verified by histological work before a decision arising from the brachia between the first and on this interesting point can be made. second maxillary foramina. This apodeme pene trates the substance of the endosternite. The 3. MAXILLAE promotor muscle runs ventrolaterally to insert in 3a. First Maxillae the lateral lobe of the coxopodite. Upon contrac tion, the muscle turns the first maxilla forward SKELETAL ELEMENTS and upward. The first maxilla coxopodite pro- The first maxillae articulate with the ventral motor muscle in Penaeus is homologous with the gnathal skeleton at relatively large foramina musculus promotor I maxillae of Pandalns, As- slightly caudad and laterad of the paragnathal tacus^ and Callinectes. Berkeley, Schmidt, and foramina (fig. 28, ^). The medial lobes of these Cochran state that the promotor muscle origi accessory feeding organs fit closely to the pos nates on the head apodeme, or endosternite, in terior surfaces of the paragnatha (fig. 30), and the above three crustaceans, whereas in Penaeua thus project anteroventrally over the mouth from the area of origin of the promotor and other lat the gnathal framework. eral muscles is not directly on the endosternite. Although the first maxillae is true appendages, they are much modified for functional ends. The COXOPODITE REMOTOR MUSCLE OF FIRST body of the first maxilla is produced into several MAXILLA lobes and a flagellum (figs. 29, 40). The flat FIGURE 40 medial lobes are the proximal coxopodite and the distal basipodite. The medial edges of these lobes Taking origin on the sternal apodeme some dis are covered with stiff hairs or spines, those on the tance ventrad of the origin of the coxopodite pro- basipodite margin being especially strong. The motor muscle, the first maxilla coxopodite remoter spines function to hold food particles. Laterally, muscle (fig. 40) passes laterad to the lateral lobe the coxopodite is produced into a rounded lobe of the coxopodite. Contractions of the muscle from which a tuft of large, plumose, sensory setae draw the first maxilla posteroventrad. The cox project. Endites of the basipodite, including an opodite remotor in Pendens is the homolog of the anterior three-jointed flagellum, extend antero- musculus remotor a or 6 I maxillae of Pdndcdus^ laterally from the basipodite. Schmidt (1915) Astatcus, and Callinectes. In the latter three considers the endites the endopodite. Various lobes of the basipodite endites bear sensorial forms, two remotor muscles are described. hairs. A single large seta projects anteriorly LATERAL COXOPODITE ADDUCTOR MUSCLE OF from the base of the flagellum. FIRST MAXILLA MUSCLE ELEMENTS FIGURE 40 The musculature of the first maxilla in Pendens The first maxilla lateral coxopodite adductor appears substantially similar to that of other muscle (fig. 40) is a long, slender muscle originat Decapoda. Groups of muscles function to bring ing on the laterotergal wall and running antero the spinous gnathal margins of the appendage to ventrally to a point of insertion on the medial re the midline in feeding. Other muscles open the gion of the coxopodite. Contractions of the muscle opposing gnathal parts and make various posi raise the first maxilla and turn the gnathal lobes tion adjustments. Peneaus appears to have at towards the midline. Berkeley and Schmidt least 10 muscles and muscle groups in the first maintain that the lateral coxopodite adductors in maxilla, against 9 each for Pandalus^ Astacus^ and Pandalus and Astdcus originate on the lateral Gdllinectes. carapace. Despite this difference, the lateral cox opodite adductors in the latter forms are homol COXOPODITE PROMOTOR MUSCLE OF FIRST ogous with those of Pendens. Cochran (1935) MAXILLA finds in Callinectes a muscle termed by her the FIGURE 40 posterior adductor muscle. This muscle is a pos The first maxilla coxopodite promotor muscle sible homolog of the lateral coxopodite adductors (fig. 40) originates on a large sternal apodeme in Pen/ieus. WHITE SHRIMP FROM THE GULF OF MEXICO 67 coxopod/te bas/poc///& ienc//fe r/7usc/es tries/a/ coxopoc//fe f endifes oF addifcfor' bas/poc/ife coxopoa^/fe ,'' arifer/or leva/or coxopod/f& musc/e adducfor musc/e coYopoc/i'f-e coxopod/fe depressor obc/ucfor- rr)USc/& musc/e coxopod/f6' promofor , miJsc/0 '•' /af&ra/ coxopoc/ff'e? / coxopoa//fe r&mot'or- / adductor tn/Jsc/e' /nusc/e FIGURE 40.—Dorsal view of right first maxilla. Dorsal cuticle removed to show muscles. 68 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE MESIAL COXOPODITE ADDUCTOR MUSCLE OF remotor muscles originate. It inserts on the cox FIRST MAXILLA opodite. The angle of the muscle attachment FIGURE 40 is such that its contractions raise the first max illa. The same muscle as the coxopodite levator Arising from the large sternal apodeme of the in Penaeus appears in Astacus^ Pandnlus., and coxopodite promotor muscle, the first maxilla Callinectes. mesial (= medial) coxopodite adductor muscle (fig. 40) inserts in the coxopodite. The muscle COXOPODITE DEPRESSOR MUSCLE OF FIRST functions to turn the first maxilla to the midline, MAXILLA thus bringing the opposing gnathal lobes together. FIGURE 40 The first maxilla mesial coxopodite adductor mus cle in Penaeus is the same muscle as the musculus The first maxilla coxopodite depressor muscle adductor medialis coxopoditis I maxillae in (fig. 40) originates on the base of the sternal Astobcus and Pandalus. CalUnectes does not ap apodeme mentioned above. The muscle runs to its pear to have this muscle. insertion on the coxopodite so that its contractions draw the first maxilla ventrad, thereby lowering ANTERIOR COXOPODITE ADDUCTOR MUSCLE OF the gnathal lobes away from the mandibles and FIRST MAXILLA paragnatha. The coxopodite depressor muscle ex FlOUBE 40 ists in the other crustaceans referred to above. The first maxilla anterior coxopodite adductor ENDITE ADDUCTOR MUSCLE OF FIRST MAXILLA muscle (fig. 40) originates on the laterotergal plate adjacent to the origin of the lateral coxopo FIGURE 40 dite adductor muscle. The muscle passes antero- Intrinsic to the endite of the basipodite is a ventrally, diverging from the lateral adductor, group of muscles, the first maxilla endite adductor and inserts somewhat anteriorly of the insertion muscle (fig. 40), which pass across the proximal of the lateral adductor. Together with the neck of the gnathal lobe of the basipodite to the medial and lateral coxopodite muscles, the an base of the endite flagellum. The muscle bends terior adductor closes the gnathal lobes of the the flagellum mesad. The endite adductor muscle first maxilla. The anterior adductor is not evi is common to the first maxilla of Penaeus, Panda dent in Astacus or Pandalus^ but does appear in lus, Astacus, and Callinectes. Oallinectes. DORSOVENTRAL MAXILLARY TENSOR MUSCLE COXOPODITE ABDUCTOR MUSCLE OF FIRST MAXILLA FIGURES 33, 34, 35, 37 FIGURE 40 This muscle originates by a broad, fan-shaped The first maxilla coxopodite abductor muscle apodeme in the connective tissue of the dorsal cara (fig. 40) originates with the anterior and lateral pace just laterad of the gastric mill (fig. 35). The adductor muscles on the laterotergal wall. The apodeme runs ventromedially to the small dorso- muscle runs anteriorly and inserts in the lateral ventral maxillary tensor muscle (fig. 37) lying lobe of the coxopodite. Upon contraction, the laterad of the muscle of the dorsoventral mandibu muscle pulls the first maxilla away from the mid lar ligament and closely applied to it. Schmidt line, opening the gnathal lobes. The coxopodite (1915) described the muscle as the anterior dorso abductor muscle is found in Pandalus^ Astacits, ventral muscle in Astacus and Berkeley (1928) and Oallinectes. has followed this terminologly in her work on Pandalus danae. Grobben (1917) studied the COXOPODITE LEVATOR MUSCLE OF FIRST MAXILLA muscle in species of Penaeus, Palaemon, Leander, and Pandalus, and concluded that, on grounds of FIGURE 40 its innervation by nerves of the first maxilla, the The first maxilla coxopodite levator muscle muscle should be renamed the "dorsoventral max (fig. 40) is attached to the sternal apodeme upon illary tensor muscle." The name given by Grob which the medial adductor and the promotor and ben is adopted here. WHITE SHRIMP FROM THE GULF OF MEXICO 69 3b. The Second Maxillae composition of the second maxilla needs clarification. The second maxilla is one of the most extensive ly modified appendages of higher Crustacea. The MUSCLE ELEMENTS structure serves a double function. The medial The principle function of the muscles of the sec lobes participate in feeding while the large lateral ond maxilla is the operation of the scaphognath part pumps water over the gills. Despite its com ite as a gill pump. As mentioned above, the gill plexity, the second maxilla is so remarkably uni pump lies in a narrow channel through which the form in structure and musculature throughout a water is moved that passes over the gills. If the broad phyletic spectrum of higher Crustacea that> body processes of a shrimp are reduced by chill as Caiman (1909) has shown, the appendage ing, and the branchiostegite is cut away, the gill is of limited value in the study of crustacean pump may be observed in slow action. Two func evolution. tionally interrelated but distinct oscillations of the SKELETAL ELEMENTS scaphognathite occur. The more obvious is that taking place about the horizontal, lateral axis The second maxilla projects laterally and ven- through the gill pump, and by which the scaphog trally from its attachment point on the ventral nathite is tipped back and forth, or rotated on skeleton. The large foramen enters a deeply its axis. The less obvious oscillation is the dorso- sculptured coxopodite from which two small, ventral movement of the lateral margin of the medial gnathal lobes arises (figs. 29, D; 41). scaphognathite about the long axis of the struc Distal to the coxopodite lies the complex basi- ture. The marginal undulation tends to ramify podite bearing two larger, medial gnathal lobes the former oscillation with respect to water pump and an anterior endite (figs. 29, D; 41). Spines ing. on the gnathal lobes aid in holding food. Lateral Water is drawn into the pumping chamber by to the basipodite lies the flat, indented scaphog- a forward tipping of the anterior end of the gill nathite, or gill pump (fig. 41). The folds and pump to the floor of the chamber. At the same grooves of the scaphognathite represent areas of time the posterolateral margins of the pump are muscle attachment and of articulation. raised to the top of the chamber. The postero Many different interpretations of the compo lateral margins of the pump are now brought nents of the second maxilla are encountered in the ventrad at which time the anterior end rises, and literature. In Astacus^ Schmidt describes the the whole organ rolls anteriorly along the floor gnathal lobes as partly coxal and partly basal, as of the chamber, forcing water out of the cephalic had been done in the present study of Penaeus. aperture of the pumping chamber. However, the former considers the basipodite Conflicting opinions about the skeletal nomen endite the endopodite, and is followed by Berke clature of the second maxilla have given rise to ley in Pandcdus. Caiman (1909) refers to the some confusion in the naming of the muscles. In endite as a palp. The position of Cochran (1935) addition, small but important differences in the in her study of Gallinectes is rather inconsistent. number, arrangement, and in particular the func When describing the second maxilla of the blue tions of the second maxillary muscles of Penaeus crab, this worker calls all of the gnathal lobes are apparent when the second maxilla of the white endites of the coxopodite, and terms the basip shrimp is compared to that of Astaciis, Pandalits, odite endite the endopodite. By contrast, in a and Callinectes. The differences are of sufficient description of the mouthparts of a number of magnitude to make difficult the homology of all the muscles in Permeus with those in the three crustaceans included as a subsection of the study crustaceans mentioned without knowing the de of the blue crab musculature, Cochran labels the tails of comparative innervations. Under pain of median gnathal lobes as basal and the anterior causing further confusion of names in the litera lobe as an endite of the basipodite, as has been ture, the present writer renames the muscles of done by the present writer in Penaeus^ even the second maxillae of Penaeus in accordance with though the second maxillae of Callinectes and their functions. Penaeus are very similar. To say the least, the 70 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE scaphognafhitv endif'e of I bas/pai//te //fas/pof///e-y (g//l bai/eri basi/jod/ie. / scapfjog^nathfte ,'^!GfVe adc/ac/o^ \ musc/e \ enJ/rc / adc/ucfor coxopoc/ife fpfjsc/e poster/or ' \, .'•-''btrs.''f.-oc///e scaphOi/ncff-fy/te ^ abc/i/c/v/^ rotator ,|^^ masc/e mifsc/e '* anfero-i/enfraf scapfyognaffi/fe rohJfcf' f77fjsc/es posfem-ypnfra/ ( / venfra/ scapbognaf/i/fe ' -/. scapbognafb/'fe nitufor / adc/i/cfer musc/e / musc/e postero-abrsa/ do/^a/ anffffo - dorsa/ scaphognoth/fe scapbogaafhife scapbogfiaffy/te rofator adc/ucfor musc/e' riGUKE 41.—Dorsal view of second maxilla. Left side, Intact appendage. Right side, dorsal cuticle removed to show muscles. WHITE SHRIMP FROM THE GULF OF MEXICO 71 BASIPODITE ADDUCTOR MUSCLE OF SECOND embedded in the substance of the endosternite as MAXILLA is the first maxillary apodeme. The postero FIGURE 41 ventral scaphognathite rotator runs posterolater al ly to a point of insertion on the ventral surface This small muscle originates on the medial of the scaphognathite. This muscle brings the margin of the coxopodite and runs to the proxi posterior tip of the gill pump ventrad and re mal region of the gnathal lobe of the basipodite. motes the whole structure. The muscle is evi The second maxilla basipodite adductor muscle dently the homolog of the musculus respiratorius turns the basipodite lobes toward the midline. e II maxillae in Astacus and the other forms The only muscle in Astactts in a similar position referred to. is part of the musculus depressor II maxillae. DORSAL SCAPHOGNATHITE ADDUCTOR MUSCLE BASIPODITE ABDUCTOR MUSCLE OF SECOND OF SECOND MAXILLA MAXILLA FIGURE 41 FIGURE 41 This muscle originates on the apodeme of the The basipodite abductor muscle runs from the second maxilla and runs ventrolaterally to insert posterior rim of the coxopodite to the proximal on the dorsal surface of the scaphognathite. The region of the basipodite gnathal lobes, laterad of dorsal scaphognathite adductor muscle turns the the insertion of the basipodite adductor. Con posterolateral margin of the gill pump dorsad. tractions of the basipodite abductor open the Although far from clear, the muscle may be the gnathal lobes. Like the basipodite adductor, the same muscle as Schmidt's musculus respiratorius basipodite abductor may be the same muscle as h II maxillae in Astacus. part of Schmidt's musculus depressor II maxillae in Astacit^. POSTERODORSAL SCAPHOGNATHITE ROTATOR MUSCLE OF SECOND MAXILLA ENDITE ADDUCTOR MUSCLE OF SECOND FIGURE 41 MAXILLA The posterodorsal scaphognathite rotator muscle FIGURE 41 originates on the dorsal part of the sternal The endite adductor muscles (fig. 41) arises in apodeme of the second maxilla and passes later the proximal region of the basipodite gnathal ally to the dorsal surface of the scaphognathite. lobes and passes laterad to insert on the lateral In action, the muscle aids the posteroventral ro margin of the basipodite endite. Its contractions tator muscle in remoting the whole scaphogna turn the endite mesad. The endite adductor of thite, but opposes the posteroventral rotator Penaeus is fully homologous with the endopodite by lifting the posterior tip of the gill pump. adductor muscle of Astacus, Pandalus, and Gal- The posterodorsal rotator in Penaeus is almost linectes. certainly the homolog of the musculus respira torius d II maxillae of Pandalus^ Astacus^ and POSTEROVENTRAL SCAPHOGNATHITB ROTATOR Callinectes. MUSCLE OF SECOND MAXILLA FIGURE 41 VENTRAL SCAPHOGNATHITE ADDUCTOR MUSCLE OF SECOND MAXILLA The posteroventral scaphognathite rotator muscle, in company with several of the so-called FIGURE 41 respiratory muscles, originates on a large, sternal The ventral scaphognathite adductor muscle apodeme arising from the sternal brachia between originates in the ventral region of the coxopodite the second maxilla and the first maxilliped. The and inserts on the scaphognathite ventrad of the dorsal, or distal, portion of this apodeme lies close insertion of the posteroventral rotator muscle. to the dorsal end of the large sternal apodeme upon The muscle turns the lateral margin of the gill which muscles of the first maxilla are attached, pump ventrad, in opposition to the action of the but the second maxillary apodeme is not deeply dorsal adductor. 72 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE ANTERODORSAL SCAPHOGNATHITE ROTATOR SKELETAL ELEMENTS MUSCLES OF SECOND MAXILLA The skeleton of the first maxilliped is comprised FIGURE 41 of a series of lightly sclerotized lobes or plates. The anteroventral scaphognathite rotator mus The appendage articulates with the ventral skele cles have separate origins and may well be two ton by a transversely elongate foramen. The different entities. The posteriormost of the two foramen enters the fused coxopodite and basipo- is a long muscle extending from the ventral dite (protopodite, fig. 42). Projecting anteroven apodeme to an area well out in the anterior region trally from the protopodite is a set of three of the scaphognathite. The more anterior rotator gnathal lobes, the large, thick distal lobe being originates in the coxopodite and passes to a broad the endite of the coxopodite (fig. 42). Together, area of insertion in the anterolateral region of the the endites of the maxillipeds are cupped against gill pump. This muscle may be the counterpart the anterior mouthparts. Heavy spines directed in Penasus of the scaphognathite flexor muscle of mesad from the edge of the endites help to hold Astacus^ Pandalu^^ and Gallinectes. The poste food. Laterad of the coxopodite endite and ex riormost rotator of this pair in Penaeus appar tending anteriorly is a slender lobe bearing on its ently is not homologous with any of the muscles in medial surface the rudiment of the endopodite the other crustaceans referred to. These muscles and the jointed flagellum of the exopodite. The bring the anterior tip of the scaphognathite exites of the coxopodite, lying laterally, are two dorsad and promote the whole structure. large, flat sheets that close the gill pump cham ANTERODORSAL SCAPHOGNATHITE ROTATOR ber on the ventral surface. Posteriorly, a small, MUSCLE OF SECOND MAXILLA flattened gill may be seen. This gill is said to be FIGURE 41 an arthobranchia, but on embryological grounds it might be as easily a podobranch. The anterodorsal scaphognathite rotator mus The components of the first maxilliped are in cle takes origin on the endosternal apodeme of the terpreted in different ways by different authors. second maxilla and runs anteroventrally to an The structure called the protopodite in Penaeus area of insertion mesad of the anteroventral rota is termed the coxopodite in Astafms by Schmidt. tor muscles. The muscle reinforces the action of Also, Schmidt considers the coxopodite endite the the anteroventral rotators in raising the anterior basipodite, and refers to the coxopodite exites as tip of the scaphognathite and promoting the epipodites. If, as has been discussed earlier, the structure. As nearly as can be determined, the coxopodite exites develop from podobranch pri- anterodorsal rotator in Penaeus is the musculus mordia, then no objection to the term "epipodite" respiratorius a II maxillae in Astacus, Pandalus^ can be offered. In passing, we may note that the and CaUinectes. first maxilliped of Penaeus bears two coxopodite endites compared to one in the first maxilliped of 4. MAXILLIPEDS Pandalus^ Astacus, and CaUinectes. 4a. First Maxilliped MUSCLE ELEMENTS Like the second maxilla, the first maxilliped is a highly modified appendage having a dual func Compared to the musculature of the second max tion. Its strong gnathal lobes and sensory flagel- illa, that of the first maxilliped is very light. The lum participate in feeding, while its fiat, lateral muscles function almost entirely in feeding, since lobes and small arthrobranchia play a part in the part played by the exites in breathing is most breathing. Superficially, the first maxilliped ap ly passive. Wider variation in the functions of pears to be as widely modified from the plan of the muscles of the first maxilliped in different the typical appendage as is the second maxilla, crustaceans makes necessary a variety of muscle but the muscles indicate otherwise. And whereas names, but most of them can behomologized. The the second maxilla is a relatively stable phylo- first maxilliped of Penaeus has 12 muscles, against genetic entity in the Crustacea, the form of the 9 for Astacus, 13 for Pandakis, and 11 in Cal- first maxilliped is variable. Unectes. v WHITE SHRIMP FROM THE GULF OF MEXICO 73 ex/tes of exopo(//Ye coxopoc///& / enc//'fe of coxopoc//fe profopo o'/fe "protopoa/ife promotof ••'fhrobf-'anchia /7?usc/e FiGUBB 42.—Dorsal view of left first maxilliped. 74 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE PROTOPODITE PROMOTOR MUSCLES OF FIRST the posterior exite, and run ventromedially to a MAXILLIPED point of insertion on the lateral margin of the FIGURES 42, 43 coxopodite endite. Their contractions draw the eudite laterad, in opposition to the coxopodite ad The protopodite promoter m^^scles (fig. 43) orig ductors. The endopodite reductor muscle of Asta inate on tlie small sternal apodemes (para- cus is a possible homolog of the coxopodite ab phragms) arising from the sternal brachia be ductor in Penaeus. tween the first and second maxillipeds. The long er, medial promotor inserts on an apodeme on the EXITE ATTRACTOR MUSCLES OF FIRST anterior wall of the protopodite and the lateral MAXILLIPED promotor inserts in connective tissue ventrad of FIGURE 43 the medial promotor. Contractions of the promot- ors turn the first maxilliped forward and dorsad The first maxilliped of Penaeus contains at least about a transverse axis. The protopodite promot two exite attractor muscles which pull the cox ers of Penaeus are very likely homologous with opodite exites caudad. The muscles originate on the lateral and medial promotor muscles of Asta- apodemes of the sternal brachia and insert on cus, CaUinectes, and Pandalus. the medial margin of the posterior coxopodite exite. The epipodite attractor muscle in Astacus PROTOPODITE LEVATOR MUSCLES OF FIRST is very likely the homolog of the exite attractors MAXILLIPED of Peiiaeus. FIGURE 43 EXOPODITE ADDUCTOR MUSCLE OF FIRST Penaeus has at least three protopodite levator MAXILLIPED muscles. All of them take origin on a small sternal FIGURE 43 apodeme overhanging the medial margin of the The fiagellum, or exopodite, of the first maxil maxillipedal foramen. They fan out as they pass liped is moved towards the midline by means of laterally to insert in connective tissue in the lateral a short muscle, the exopodite adductor muscle, part of the coxopodite. Contractions of the pro which originates at the base of the exopodite and topodite levators lift the dorsal edges of the coxop runs distally within the structure. The homolog odite exites dorsad. At least a part of this mus in Astacus is the exopodite adductor muscle. cle group is homologous with the levator muscle of Pandalus, Astacus, and Callinectes. EXOPODITE ABDUCTOR MUSCLE OF FIRST MAXILLIPED COXOPODITE ADDUCTOR MUSCLE OF FIRST MAXILLIPED FIGURE 43 FIGURE 43 The exopodite abductor muscle originates at the distal end of the exopodite adductor and runs Taking origin in the proximal region of the distally in the fiagellum. Its contractions tend to coxopodite, the coxopodite adductor muscle passes straighten the fiagellum, thus turning the struc ventrad along the medial wall of the coxopodite ture laterad. The muscle in Penaeus is in all to insert on the coxopodite endite. Upon contrac probability the fiagellum muscle of Astacus. tion, the muscle turns the endite mesad. The cox opodite adductor in Penaeus appears to be the 4b. Second Maxilliped maxillipedal depressor muscle of Astacus and The structure of the second maxilliped is much Pandalus, and possibly one of the small unknown more like that of the typical arthroappendage muscles in the coxopodite of Callinectes. than the anterior gnathal appendages already COXOPODITE ABDUCTOR MUSCLES OF FIRST treated. The typical number of appendage arti MAXILLIPED cles are found, albeit those of the basipodite and ischiopodite are fused. A large, flagellar exopo FIGURE 43 dite is developed. A notable difference is that Two coxopodite abductor muscles exist in the some of the endopodite articulations of the second first maxilliped of Penaeus. The muscles origi maxilliped permit far more extensive movements nate in the lateral part of the coxopodite, near than do the hinges of the anterior gnathal limbs. WHITE SHRIMP FROM THE GULF OF MEXICO 75 coxopoc//fis / F/a(/e//i/m of exite / exofio€///e> exopO(//fe , coxopoc/Z/e musc/e exopocy/fe / coxopotJ/te aa/c/fjctor / aa/c/ucfor rr}(Jsc/e / musc/e coxopoc/ift^ yprotopoc/ife ^ /ei/afor /77i/sc/es mc/sc/es coxopoc/f'f-e ~pro -f'opoc///e exiff? pro/opoc//fe ^ aftracfor f promo'for I rnusc/es frujsc/es /7(?rre FIGURE 43.—Dorsal view of left first maxilliped, dorsal cuticle removed to show muscles. 76 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE Tlie second maxilliped is an active participant in neatli the mandibles. Stilf spines on the medial the process of feeding. side of the meropodite opposing those on the lateral side of the dactylopodite produce a func SKELETAL ELEMENTS tional cliela or claw. The coxopodite (fig. 44) of the second maxil MUSCLE ELEMENTS liped projects ventrad from a region of the ven tral skeleton thta is much more heavily sclerotized The musculature of the second maxilliped of than are the corresponding areas anteriorly. The Penaeus is substantially similar to that of Panda foramen of the second maxilliped is surrounded lus. Astacus, and Callinectes, although Penaeus by several large sternal and laterotergal apodemes has a larger number of discrete muscles. The sec upon which muscles originate. The whole struc ond maxilliped of Penaeus contains 14 types of ture is distinctly heavier than is that of the an muscles including 23 muscles. Astacus has 15 mus terior gnathal appendages, in keeping with the cle types with 17 muscles. The second maxilliped greater movability and strength of the second of Pandalus has 14 types of muscles and 16 dis maxilliped compared to that of the anterior acces crete muscles, whereas Callinectes has 16 muscle sory mouth-parts. Projecting laterally from the types and 17 muscles. The classical muscle nomen coxopodite is a small gill, the podobranchia (fig. clature has been changed slightly here as elsewhere 44), and a mitten-shaped mastigobranchia, or to conform to the appendage article in which the epipodite. (See fig. 32.) The articular membrane muscles insert. dorsal to the coxopodite bears two arthrobran- chiae. COXOPODITE PROMOTOR MUSCLE OF SECOND The short, curved carpopodite is connected by MAXILLIPED dicondyles between the meropodite and the pro- FIGURES 44, 45 podite placing the propodite laterad of the distal Taking origin on a paraphragmal apodeme on end of the meropodite. The propodite is a short, the lateral pleural wall, the coxopodite promoter square article containing muscles operating the muscle passes mesad to insert in connective tissue heart-shaped dactylopodite on its distal end. The on the medial wall of the coxopodite. The muscle dactylopodite lies laterad of the proximal part of turns the coxopodite, and with it the distal seg the meropodite, its apex nearly touching the ments, anterior and dorsad. The musculus pro- ischiopodite. The condyles between the carpopo motor II pedis maxillaris of Astacus^ Pandalus, dite and propodite and between the propodite and and Callinectes is homologous wdth the coxopodite dactylopodite are rotated 90° from the axis of the promotor muscle of Penaeus. condyles between the meropodite and the carpo podite. Thus the movements of the distal seg COXOPODITE REMOTOR MUSCLE OF SECOND ments are at right angles to those of the proximal MAXILLIPED segments. FIGURES 44, 45 The coxopodite articulates with the basipodite The coxopodite remotor muscle originates on a (fig. 44) by dorsoventral condyles which permit paraphragmal apodeme above the posterolateral extensive lateral movements. The coxopodite and margin of the foramen and runs to an insertion basipodite are said to be fused in Pandalus and in connective tissue on the posterior wall of the Astacus. To the basipodite is articulated the exop- odite, a long, annular, plumose flagellum (fig. coxopodite. Its contractions turn the coxopodite 44) that extends anteriorly and then curves grace caudad and the distal elements ventrad. The fully laterad. The ischiopodite (fig. 44) is im coxopodite remotor muscle in Penaeus is fully movably fused to the basipodite in Penaeus, as homologous to the second maxilliped remotor in CaUinectes, but a fine line of light cuticle clearly muscle in the other crustaceans mentioned. distinguishes the two articles. The meropodite BASIPODITE LEVATOR MUSCLES OF SECOND (fig. 44), the longest article of the endopodite, is MAXILLIPED attached to the ischiopodite so as to allow the distal segments limited movements from side to side FIGURES 44, 55 as well as up and down. This article projects an The basipodite levator muscles originate at two teriorly from the ischiopodite to a position be- different points. The medial portion is attached WHITE SHRIMP FROM THE GULF OF MEXICO 77 carpopod/te, /pro/3oa'/f0 / idacfu/opoc/ife I I rnetopod/fe^^ 'is chiopoc//fe exopod/fe / has/podife Cf/ageffum) ^ g/// i podchrar7ch/a) wusc/e COXOpod/te pro mo tor musc/e ' .nofye coxopod/fe 1 ' ' 11/ baB/'pod/fe / basipod/t& ,'/ levator ^^ depressor \ muscie / musc/es cQX opod/t& FIGURE 44.—Dorsal view of left second maxilllped. 468059 0—59 6 78 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE propod/te cfacfy/opod/te I dacP(//opO(///'0 /pf^opocZ/'fe proc/ucfv/- musc/& /nasc/e, ' m/Jsc/& propoc//fe ^ 'o''0cfip/opoc///e producfof \ musc/e^ \ ca/^piopod/fe carpopoa//te yfv&fop od/fe abc/ucfor tiieropodife musc/es reduc for muscles ^,'' ^^'' , ,^ tneropod/fe producfor musc/e ccfrpopoc//re __^^ exopod/'te ac/c/ucfor- crdafucfof tm^sc/e rnusc/es exopoc/ife exopod/'te abdi/ciof- mc/sc/e /'sch/opocZ/fe •• • artery coxopod/te prumoto/^ musc/o caxopod/Ycf bas/pod/f& \^ 'das/pod/te 'coxopod/fe /e^afor' \ depressoh- promo/v/^ musc/ei FIGURE 45.—Dorsal view of second maxllliped, cuticle removed to show muscles. WHITE SHRIMP FROM THE GULF OF MEXICO 79 to a large sternal apodeme overhanging the an- the depressor muscles a and & in Pandalus, Asta teromedial part of the foramen. The muscle cus, and Callinectes. runs beneath the coxopodite promotor and inserts together with the lateral basipodite levator on the EXOPODITE ABDUCTOR MUSCLES OF SECOND MAXILLIPED anterior margin of the basipodite foramen, slightly laterad of the dorsal coxobasipodite FIGURE 45 condyle. The lateral levator muscle originates on Two exopodite abductor muscles exist in the the posterior wall of the coxopodite and runs second maxilliped of Penaeus. The medial ab ventrad to join the medial levator. The muscles ductor is extrinsic, originating broadly on the me raise the distal articles and also abduct them. The dial wall of the basipodite and inserting on the muscles are homologous with the levator muscles posterior edge of the exopodite base. The extrin a and h of Pandalus and Astacu^, and with the sic abductor turns the exopodite laterad. Intrinsic single levator in Callinectes. to the exopodite is a long abductor muscle which originates by a fine tendon on the base of the BASIPODITE DEPRESSOR MUSCLES OF SECOND MAXILLIPED exopodite and extends distally for some distance in the exopodite flagellum. Its contractions bend FIGURES 44, 45 the flagellum laterad. The intrinsic exopodite The second maxilliped of Penaeus contains four abductor muscle in Penaeus is homologus with basipodite depressor muscles. The largest is the the exopodite abductor muscle of Astacus and medial depressor, a short, strong, semicylindrical Callinectes., and the extrinsic abductor is proba muscle which, in a manner of speaking, lines the bly the same muscle as the flagellum abductor in medial curvature of the coxopodite. This muscle Astacus. originates from the medial margin of the dorsal EXOPODITE ADDUCTOR MUSCLES OF SECOND coxopodite foramen and inserts with the other de MAXILLIPED pressors on the ventromedial margin of the coxo podite, mesad of the ventral condyle between the FIGURE 45 coxopodite and basipodite. Two other basipodite The second maxilliped of Penaeus has at least depressors arise, the larger from the posterior three exopodite adductor muscles. The extrinsic margin of the coxopodite foramen, the smaller exopodite adductor originates in a broad fan from the posterior wall of the coxopodite. The across the dorsomedial wall of the basipodite fourth depressor muscle originates on the ventro and inserts on the anterior edge of the exopodite lateral wall of the coxopodite, beneath the coxo base. When it contracts, the exopodite is turned podite remotor, and runs across the coxopodite to towards the midline. Within the exopodite are join the other basipodite depressors. The con two exopodite adductors which function to tractions of the depressor muscles turn the basipo dite and thus the distal maxillipedal elements straighten out the exopodite flagellum, in oppo ventrad. In addition, as a consequence of their sition to the action of the intrinsic exopodite ab insertion mesad of the condylic axis, the depressor ductor muscle. No homolog of the extrinsic exop muscles turn the basipodite and the distal elements odite adductor muscle is evident in Astacus or towards the midline. Callinectes. Berkeley (1928) illustrates an ex In fact, from the arrangement of the basipodite trinsic adductor of the exopodite, but does not discuss the muscle. The intrinsic exopodite ad levators and depressors with respect to the basipo ductors of Penaeus apparently have no homologs dite condyles, the true function of the muscles in the crustaceans referred to here. may be as abductors and adductors, rather than as levators and depressors. The latter names, of MEROPODITE PRODUCTOR MUSCLE OF SECOND course, derive from Astacus in which the coxopo MAXILLIPED dite and basipodite of the second maxilliped are fused. On functional grounds, then, Schmidt's FIGURE 45 names are at least partially incorrect when ap The types and arrangement of the second max plied to Penaeus. The basipodite depressor mus illiped endopodite muscles are remarkably uni cles of Penaeus are nonetheless homologous with form in many higher Crustacea, in accordance 80 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE with the evolution of this versatile accessory CARPOPODITE ADDUCTOR MUSCLE feeding mechanism. The meropodite productor OF SECOND MAXILLIPED muscle originates on the medial side of the ischi- FIGURE 45 opodite, near the exopodite adductor, and inserts on a small apodeme on the dorsal surface of the The carpopodite adductor muscle runs from the meropodite. The muscle lifts the meropodite and proximal end of the meropodite to an adductor other distal elements towards the ventral surface apodeme on the medial side of the carpopodite. of the mandibles. The same muscle appears in The muscle is less powerful than the carpopodite Pandalu.s, Astacus, and Callinectes. abductors. The adductor turns the carpopodite mesad, but as a result of the distal hook, the MEROPODITE REDUCTOR MUSCLES OF SECOND gnathal surface of the dactylopodite is moved MAXILLIPED laterad. The carpopodite adductor of Penaeus is fully homologous with the carpopodite adductor FIGURE 45 in the other crustaceans to which reference Penaeus has two meropodite reductor muscles. has been made. The medial reductor originates on the medial wall of the ischiopodite, ventrad of the meropodite PROPODITE PRODUCTOR MUSCLE OF SECOND MAXILLIPED productor, and inserts on an apodeme on the ven tral margin of the meropodite. Another reductor FIGURE 45 muscle originates on the anterodorsal surface of The propodite productor muscle is a short, the ischiopodite and passes to the same apodeme thick structure originating on a proximal, ven as the medial reductor. The muscles turn the tral part of the carpopodite, and running to a meropodite ventrad. The meropodite reductors broad apodeme on the ventrolateral side of the of Penaeus have partial counterparts in the sec propodite. Its contractions turn the propodite ond maxilliped of Astacus, Pandalus, and Callin and dactylopodite ventrad. The propodite pro- ectes, but in the latter only one reductor has been ductors in Penaeus are homologous with the described. propodite productor in Astacus, Pandalus, and Callinectes. CARPOPODITE ABDUCTOR MUSCLES OF SECOND MAXILLIPED PROPODITE REDUCTOR MUSCLE OF SECOND MAXILLIPED FIGURE 45 FIGURE 45 Two carpopodite abductor muscles are found in the second maxilliped of Penaeus. The smaller The propodite reductor muscle occupies the dorsal part of the carpopodite in the shape of a of these originates on the dorsal side of the merop fan. The broad portion originates in the proximal odite and inserts on the common abductor region of the carpopodite. The muscle becomes apodeme on the lateral edge of the carpopodite. narrow as it inserts on a little apodeme on the The larger abductor takes origin on the lateral dorsal side of the propodite. The muscle turns side of the meropodite, proximally. The muscle the propodite and dactylopodite dorsad, about the runs out along the lateral side of the meropodite horizontal axis through the condyles. The propo and attaches to the abductor apodeme on the dite reductor muscle of Penaeus is represented in carpopodite. The two muscles turn the small the second maxilliped of the three crustaceans to carpopodite laterad. However, with respect to which frequent reference has been made. the gnathal surface of the dactylopodite, the carpopodite abductors cause functional adduc DACTYLOPODITE PRODUCTOR MUSCLE OF tion, in consequence of the hooked shape of the SECOND MAXILLIPED endopodite. The large carpopodite abductor FIGURE 45 muscle is the same muscle as the carpopodite ab The dactylopodite productor muscle is a small, ductor in Astacus, Pandalu^, and Callinectes. fan-shaped muscle that originates proximally in The small abductor in Penaeus is not described in the propodite and inserts on the ventral margin of any of the foregoing crustaceans. the dactylopodite. It serves to straighten the WHITE SHRIMP FROM THE GULF OF MEXICO 81 dactylopodite and turn the distal article ventrad, liped and all of the walking legs. A large, taper in opposition to the dactylopodite reductor. The ing, annulated exopodite articulates with the same muscle as the dactylopodite productor in basipodite laterally. Penaeus is found in Pandalus^ Astacus, and Cal- The ischiopodite is fused to the basipodite im linectes. movably, although the line of demarcation is clear. The ischiopodite is the longest article of the third DACTYLOPODITE REDUCTOR MUSCLE OF SECOND maxilliped. The meropodite is connected to the MAXILLIPED distal end of the ischiopodite by two condyles FIGURE 45 whose axis permits both flexion of the distal articles towards the midline and reduction of the Attached between the proximal part of the pro- distal elements. The movements at this joint are podite and the dorsal edge of the dactylopodite is extensive. Due to the bending at this joint the the small dactylopodite reductor muscle. Its func shrimp is able to grasp food with the third maxil tion is to turn the dactylopodite dorsad, opposing liped. The carpopodite articulates with the distal the action of the productor. The muscle is homol end of the meropodite by two condyles. The axis ogous with the dactylopodite reductor of Panda- of these dicondyles is vertical, permitting the lus, Astacus^ and Callinectes. carpopodite to flex on the meropodite. The axis through the condyles of the joint be 4c. Third Maxilliped tween the carpopodite and the propodite is also The third maxilliped is the first accessory feed vertical allowing the propodite to be flexed upon ing appendage which lacks the jawlike character the carpopodite. The small dactylopodite is simi istics of the anterior gnathal limbs. Far more larly articulated with the propodite. than any of the other appendages, the third max- The distal elements, beginning with the ischio illipeds function to grasp large food particles podite, bear rows of stout spines on their medial passed up by the chelate legs and hold them next sides for holding food particles. The exopodite to the mouthparts for further reduction in size has long, plumose setae. The third maxilliped and for swallowing. Structurally, the third max has a branchial arrangement similar to that of illiped is closely similar to the pereiopods. the first three walking legs. From the pleural plate arises a pleurobranchia. Two arthro- SKELETAL ELEMENTS branchiae project from the dorsal articular mem brane, and a bilobed mastigobranchia arises from The body hemocoel is confluent with that of the coxopodite. the third maxilliped by means of a ventral skele tal foramen whose fringes are strongly sclerotized MUSCLE ELEMENTS and from which phragmal apodemes project over The musculature of the third maxilliped of the opening. The sternal plate between the Penaeus is typical of that seen in the walking legs. foramina is wider at this point than that between Some variation, however, is evident when the third the anterior mouthpart foramina. The heavily maxilliped of different crustaceans is compared, sclerotized coxopodite articulates with the ventral especially in the musculature of the exopodite and skeleton by a dorsal, laterotergal condyle and a the distal articles of the endopodite. In Penaeus^ ventral sternal condyle, the typical situation in most of the basipodite and coxopodite muscles the white shrimp limb. The axis through the con originate on the laterotergal, pleural plates rather dyles is about 45° from the vertical, with respect than on phragmal elements as in the anterior to the transverse plane. Since the distal elements gnathal appendages. Pleural origins of these are anterior, movements about these coxopodite muscles are typical of all the posterior append condyles raise and lower the appendage, as well ages, thoracic and abdominal alike. as promote and remote it. The basipodite articu The third maxilliped of Penaeus is operated by lates with the coxopodite by typical dicondylic twenty muscles comprising 12 functional types. connections, the axis of the condyles being hori That of Astacus contains 17 muscles, including zontal. Thus the basipodite accounts for most of 14 types. The third maxilliped of Pandahis is the depression and levation of the distal elements, somewhat modified, containing 13 muscles of 9 an arrangement that is common to the third maxil functional types, whereas the same appendage in 82 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE CalUnectes has 16 muscles grouped into 15 muscle BASIPODITE DEPRESSOR MUSCLES OF THIRD types. MAXILLIPED COXOPODITE PROMOTER MUSCLE OF THIRD FIGURES 46, 47, 48 MAXILLIPED Considerable variation from the pattern of the FIGURES 46. 47, 48 depressor musculature of Penaeus is seen in the third maxillipeds of Astacus, Pandalus, and Cal The coxopodite promotor muscle is a strong, Unectes. Schmidt and Berkeley describe two de lozenge-shaped muscle (fig. 47, ^) originating on pressors of the third maxillipeds of Astacus and the pleural plate along the anterodorsal margin Pandalus, whereas Cochran shows one depressor in of the muscle. The insertion is on the anterior CalUnectes. By contrast, Penaeus has at least margin of the coxopodite (fig. 46). The promotor four basipodite depressor muscles in the third turns the coxopodite forward and, since the distal maxilliped, just as in the second maxilliped. parts extend directly anterior, the latter are raised The lateral basipodite depressor takes origin against the other mouthparts. The promotor mus on the posterolateral margin of the coxopodite cles of Pandalus^ Astacns, and CalUnectes are the (fig. 47) and inserts, together with the other de counterparts of the coxopodite promotors in pressors, on the large apodeme on the posterior Penaeus. rim of the basipodite. Interior to the lateral basip COXOPODITE REMOTOR MUSCLES OF THIRD odite depressor is a long, two-part depressor MAXILLIPED muscle (fig. 47, B) originating on the laterotergal plate mesad of the coxopodite remotor muscle. FIGURES 46, 47 This depressor joins the short lateral depressor - The third maxilliped of Penaeus contains two on the basipodite depressor apodeme. Immedi coxopodite remotor muscles. They are much more ately mesad of the long depressor lies a small, powerful than the coxopodite promotor muscles flat basipodite depressor muscle (figs. 46; 47, A) which they oppose. The remotors originate on the which originates on a medial apodeme of the the laterotergal plate dorsad (fig. 47) and insert maxillipedal foramen and inserts on the basipo on the posterior margin of the coxopodite. They dite apodeme. The most internal basipodite turn the coxopodite caudad and thus the distal depressor muscle (figs. 46, 48) is a semicylin- elements ventrad as well as laterally. The larger, drical structure originating on medial phragmal lateral coxopodite remotor of Penaeus is fully apodemes and inserting on the posteromedial edge homologous with the remotors in the three crusta of the basipodite. ceans to which we have referred. The total action of the basipodite depressor muscles is relatively powerful. By their contrac BASIPODITE LEVATOR MUSCLES OF THIRD tions the basipodite and distal elements of the MAXILLIPED third maxilliped are turned ventrad. The homol FIGURES 46, 47 ogies of these muscles with those of Astacus, Pandalus, and CalUnectes are not entirely clear. The two basipodite levator muscles that have The depressor muscle a of Astacus and Pandalus been found in Penaeus have dift'erent origins. The is the same muscle as the innermost, medial basip short, stout lateral levator takes origin on the odite depressor muscle in Penaeus. Undoubt anterolateral margin of the coxopodite and inserts edly, the depressor muscles a^ and b in Astacus, and on the anterior margin of the basipodite. The «!, ^2 and h in Pandalu^s have counterparts in the larger basipodite levator lies internal to the third maxilliped of Penaeus, but their exact rela smaller muscle. The origin of the former is on tionships are difficult to determine. the laterotergal plate and its insertion on the an terior rim of the coxopodite. The levators pull EXPODITE ABDUCTOR MUSCLES OF the anterior side of the basipodite upward and THIRD MAXILLIPED w4th it the distal maxillipedal elements. Three levators of the third maxilliped are described in FIGURES 46, 47 Pandalus^ Astacus, and CalUnectes. Part of this Similar to the arrangement in the second maxil group is very likely homologous w^ith the levator liped, the third maxilliped exopodite of Penaeus pair in Penaeus. is moved by 2 exopodite abductor muscles, 1 ex- WHITE SHRIMP FROM THE GULF OF MEXICO 83 doc t/j /opoc// fe _ •• cfac fy/opodifig f/oxor propoc//f'C' _ ^propoc/if^' f/e^xof carpopoa'/f(? ~~ __ mpsc/es Ofopocy/re 'Ccrrpopoc/ite re'a'Mofor rnusc/e! tni/sc/e mer-cjpocj/i'fe — . ^ yischiopod/fe carpopoc/i 'fe nef've proc/acfor musc/e rrrfv/^/'es - . rneropoc/ife f/exof exopo(//fe ^, djasrpoc//fe _,,' exapoc//fe exopocZ/fe '&xopocJ/'fe acJcfucfo/^ abductof- muscles •^' rpusc/e r'oxopa(J/f(? '^"^Sasipodite promotop depressor P7ijsc/e musc/es branchf'ae "-'''' ''coxopodlie \ bas/poofr't& coxopocJ/te \ ^bas/pod/fe refnofof^ \ /evafor rmtsc/es ^ musc/e' mi/sc/es FiGUKE 46.—Dorsal view of left third luaxilliped, dorsal ciitifle removed to show muscles. 84 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE Goxopocf/fe ^/bas/poc///-e /coxopod/fe pramofor depressor r&/7?ofvr mi/sc/es mi/sc/e m6/sc/e /??i/sr/e musc/& mi/sc/cs eyopoc//'fe adductor /masr/ffoAra/7ci^/if musc/e Cep/pod/fe) exopod/'te aMucfor musc/e exopod/f-e '' /sc/^/opad/te i)osipod/fe •Tihas/pod/fe /eva/or depressor musc/es musc/es arfer(/ ^ neri/e 'caxopod/'Pe remofor masc/es meropod/fe i 'e'xopod/fe e-xopoc/tte das/pod/As' f/exor' / adductor orAductor depressor rnusc/e' muscte musc/e /??i/sc/es FIGURE 47.—Lateral view of leg base of left third maxilliped. Cuticle removed to show muscles. A. Superficial lateral and exopodite muscles. B. Lateral muscles, exopodite removed. WHITE SHRIMP FROM THE GULF OF MEXICO 85 trinsic and. 1 intrinsic to the structure. The due to the arrangement of the condyles, ventrad. wedge-shaped extrinsic abductor (figs. 46; 47, B) The ischiopodite-meropodite junction is the major originates on the ventromedial wall of the basipo- functional joint in the third maxilliped. The dite. The muscle tapers to a point, laterally, in meropodite flexor muscle of Pena^us is fully ho serting on the posterior margin of the exopodite mologous with the meropodite flexor muscle of foramen. Contractions of the exopodite abduc Astacus and Callinectes. The third maxilliped tor turn the exopodite laterad. Intrinsic to the endopodite of Pandalus has lost parts by fusion, flagellar exopodite is a long abductor muscle (fig. making difficult the homologies of its muscles. 46) which originates in the proximal region of the exopodite and runs distally along the lateral CARPOPODITB PRODUCTOB MUSCLE OF THIRD side of the flagellum. Its contractions increase MAXILLIPED the lateral curvature of the flagellum, thus turn ing it laterad. The extrinsic exopodite abductor FiauBE 46 muscle of Pendens is represented under the same The carpopodite productor muscle takes origin name in Astacus and Callinectes. The exopodite over an extensive area along the lateral side of the of Pandalus is extremely reduced. The intrinsic meropodite (fig, 46) and inserts on an apodeme exopodite abductor probably appears as the exop of the carpopodite. Its contractions straighten the odite flagellum muscle in Astacus and Callinectes. carpopodite with respect to the meropodite. The EXPODITE ADDUCTOR MUSCLES OF muscle is represented in the third maxilliped of THIRD MAXILLIPED Astacus and apparently in Pandalus. The produc tor muscle is referred to as an extensor by Cochran FIGURES 46; 47, B in Callinectes due to a difference in condylic Two exopodite adductor muscles in PeTiaeus orientation. turn the exopodite flagellu^ mesad. The conical extrinsic adductor originates on the dorsomedial CARPOPODITE RBDUCTOR MUSCLE OF THIRD wall of the basipodite, dorsad of the extrinsic MAXILLIPED exopodite abductor muscle (fig. 46), and inserts FIGURE 46 on the anterior rim of the exopodite foramen. The intrinsic exopodite adductor is comprised of Opposing the action of the carpopodite produc a pair of small muscles originating on the pos tor muscle is the carpopodite reductor muscle. terolateral side of the flagellum base and attach This muscle originates in the proximal region of ing a short distance distally on the medial and the meropodite and passes distally along the me dorsal wall of the flagellum. The extrinsic ad dial side of the meropodite to insert on a ventro ductor has a counterpart in Callinectes^ but not medial apodeme of the carpopodite. Upon con in Astacus. The little intrinsic adductor muscle traction, the muscle turns the carpopodite and the in Penaeus possibly may be the exopodite flagel distal articles ventrad. The carpopodite reductor lum abductor muscle of Astacus and Callinectes, mubcle of Penaeus is the same muscle in Astacus, although this is doubtful. Pandalus, and Callinectes, although in the latter Cochran describes it as a flexor. MEROPODITB FLEXOR MUSCLE OF THIRD MAXILLIPED PROPODITE EXTENSOR MUSCLE OF THIRD MAXILLIPED FIGURES 46, 47 The meropodite flexor muscle is a long, spindle- FIGURE 46 shaped muscle arising in the basipodite and pass Originating for some length along the lateral ing distally along the lateral wall of the ischiopo- margin of the carpopodite, the propodite extensor dite to an apodemal insertion on the proximal end muscle inserts on a laterally located apodeme of of the meropodite. The apodeme of insertion is the propodite. The muscle extends the propodite located ventromedially. When the meropodite and dactylopodite directly anterior to the carpo flexor contracts, the meropodite and distal articles podite. The same muscle is found in all the crus of the endopodite are turned sharply mesad and. taceans to which reference has been made. 86 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE bas/pocf/f-e I /jas/poc//fe / caxopoc/i/'e' promofo/' masc/es musc/es /rwsc/e Vv hOCfs/pocZ/fe I c/e'p/^&SSO/- mi/sc/es artery ^ fien^e meropod/fe ' f/exor ^ . - ' 'Aas/bcxf/Ye /scfyfopoc//fe^ artery^ bas/poc/ife depressor /77usc/es nert/e - -... apoeshf77& meropoc//fe has/poc/ff'e f/exor dopr&ssor masc/e '' r??i/sc/e FIGURE 48.—Lateral view of leg base of left third maxlUlped. A. Lateral and medial muscles, li. Medial muscles. WHITE SHRIMP FROM THE GULF OF MEXICO 87 PROPODITE FLEXOR MUSCLES OF THIRD great. The first three pereiopods bear small che MAXILLIPED lae w^ith sharp cutting edges. The last two walk FIGURE 46 ing legs are subchelate. Small exopodites are found on all five pairs of pereiopods. The joint The third maxilliped of PenaeuH is unique by between the basipodite and ischiopodite is movable comparison with the same appendag:e of Panda- in the walking legs of Penaeus. The coxopodite lus^ Astacu.s, and CalUnectes in having two pro- and basipodite comprise the protopodite to w'hich podite flexor muscles. The larger originates in an endopodite with the typical five articles is at the carpopodite proximally and passes distally on tached. Associated w4th the pereiopods are the lateral side of the article to its insertion on an branchiae. With the third maxillipeds, the first, apodeme of the propodite. The smaller propodite second, and third pereiopods have a bilobed mas- flexor is a little triangular muscle (fig. 46) which tigobranchia (epipodite) arising from the coxop- has a common insertion with the larger flexor. odites, 2 arthrobranchiae on the dorsal articular The flexor muscles turn the propodite mesad on membrane, and 1 pleurobranchia on the latero- the carpopodite. The larger propodite flexor tergal plate. The mastigobranchia and one ar- muscle is found in all the crustaceans referred to. throbranchia is missing on the fourth pereiopod and the fifth pereiopod has only a pleurobranch. DACTYLOPODITE FLEXOR MUSCLE OF THIRD MAXILLIPED Due to pronounced serial similarity of parts in the walking legs, only the first and fifth pereiopods FIGURE 46 will be considered in detail here. The dactylopodite flexor muscle arises in the proximal portion of the propodite and is attached 5a. First Pereiopod to an apodeme of the dactylopodite. The,muscle The first pereiopod is considered as an example turns the dactylopodite toward the fnidline. of a chelate limb. It is ordinarily treated as the Dactylopodite flexors appear in the third maxil first walking leg. Whereas in the Reptantia the liped of Astacus and Oallinectes. The distal arti first pereiopods are modified into huge chelate cles are fused in Pandalus and the muscles thus chelipeds, those of the Penaeidae, while chelate, lost. The third maxilliped of Penaeus has no are similar in size to the remaining four pairs of dactylopodite extensor muscle, a structure de walking legs. In point of fact, the first pereiopod scribed in Astacus and Oallinectes. of PenaeuB is not a functional walking leg. That is to say, none of the body weight is supported on 5. PEREIOPODS the appendage. The first pereiopods are carried In the Crustacea Decapoda the last five pairs horizontally, directed anteriorly, ventrad of the of thoracic appendages are usually referred to as third maxillipeds, and function to pass food par walking legs, or pereiopods. Although their ticles to the latter. length, size, and functional modifications are var iable in the group, the walking legs are all funda SKELETAL ELEMENTS mentally alike in structure. Exopodites, usually The strongly sclerotized coxopodite articulates small, are either present or absent. The large pro- wdth the ventral skeleton by dorsoventral dicon- topodite-endopodite is almost always composed of dyles. The axis through the condyles is such that the typical seven appendage articles, although fu when the coxopodite swings forward it also ap sion of the basipodite w^ith the ischiopodite has oc proaches the midline. The rearward motion, on curred in some groups. Some or all of the pereio the other hand, turns the coxopodites aw^ay from pods may be chelate. Incidentally, Dougherty the midline. The angular attitude of the dicon- (Steinberg and Dougherty, 1957) objects to the dylic axis of the first pereiopod is thus raised spelling of the word "pereiopod" and offers good laterally with respect to the frontal plane and reasons for dropping the i in American usage. rostrad with respect to the transverse plane. Pro The common and perhaps incorrect spelling is ceeding caudad, the angular attitude of the axis used in the present w^ork. through the coxopodite condyles rises laterally In Penaeus, the pereiopods are all relatively with respect to the frontal plane in conjunction long and slender as befits a lightly sclerotized form with the shift dorsad of the dorsal condyle and whose body weight on the walking legs is not the increased width between the ventral sternal FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE condyles. To the coxopodite is attached the COXOPODITE PROMOTOR MUSCLES OF FIRST strong, curving tube of the basipodite. The axis PEREIOPOD of the condyles between the coxopodite and ba FIGS. 49, 50, 51 sipodite is rotated 90° from the axis of the con dyles connecting the coxopodite and the ventral The first pereiopod of Penaeus possesses at least skeleton. Primarily by means of the basipodite, two coxopodite promotor muscles. They origi the distal elements of the pereiopod are raised and nate by broad regions along the anterodorsal lowered. A sharp process projects distally from margin of the pleural plate belonging to the seg the distomedial portion of the basipodite. A ment and insert on the anterior rim of the cox small, fingerlike exopodite bearing long setae opodite. Their contractions turn the coxopodite projects laterad from the basipodite. forward. Thie coxopodite promotors of Penaeus The ischiopodite is hinged to the basipodite to are represented in Astacus^ Pandalus^ and Calli permit limited reduction of this article, and also nectes. some rotation of the distal elements due to the COXOPODITE REMOTOR MUSCLE OF FIRST oblique angle of the condyles. Like the basipo PEREIOPOD dite, the ischiopodite bears a sharp spine medially. The meropodite, one of the longer articles of the FIGS. 49, 50, 51 first pereiopod, articulates with the distal end of The strong, spatulate coxopodite remotor mus the ischiopodite, allowing some flexion and ex cle originates on the posterodorsal part of the tension, together with a little reduction, of the laterotergal plate and attaches to a small apodeme distal segments. The most movable joint in the on the posterior margin of the coxopodite fora first walking leg is that connecting the meropodite men. The muscle turns the coxopodite rearward and the carpopodite. The axis of the joint is hori and in so doing brings the distal elements of the zontal, and the carpopodite is thereby capable of pereiopod ventrad, in opposition to the coxopo deep flexion on the meropodite. An antennal dite promotors. The movement is evidently cleaning brush composed of comb-like setae re weaker than that of promotion, since the single sides in a distomedial depression of the carpopo remotor is much smaller than the promotor dite. muscles. The homologies of the coxopodite re The chela is freely movable on the carpopodite. motor muscle of Penaeus with the remotor muscle The propodite component is made up of a base of Callinectes are fairly obvious. Ast WHITE SHRIMP PROM THE GULF OF MEXICO 89 •iacty/opoo/ife, chemarecepfor f/jt'ts / d pfopod/te ,che/a carpop(?d/te^ mdacfor carp>opoc//fe extensor meropod/f-e /77usc/e /?7f/-x?pod/ff9 ischiopodite -—. _ f/exor- •scfy/opod/ye f-educ^r exopodite - musc/e coxopod/'fe coxopod/fe __ pi-TDmofaf /77usc/e ventra/ ~ ba3ipod/t& nen/'e cofd - musc/e bosipoa'/te ibcfs/poc//'f& depresso/^ c/eprssso/' mijsc/e /77i/sc/e FIGURE 49.—Dorsal view of first pereiopod. Left side, intact appendage. Right side, dorsal cuticle removed to show muscles. 90 FISHERY BULLETIN OF THE FISH AND "WILDLIFE SERVICE coxopod/te promofor ^ Repressor mi/se/es mi/sc/e coxopchJife coxopod/hf re m(J for rnusc/e exapcc///e ^^ has/poc/ife mi/sc/e -y /eya/o/- masc/es exopo€//'/e arterif iSa-s/pocf/te fevafof- hasfpocZ/fe depressor mi4sc/e \ I coxopod/te f-Tefrjofor /pt/sc/e arfcrij.^ ''masf/ffO^r-arfcA/a ner\/&r oas/pud/te (epi'pod/fe) FIGURE 50.—I.ateral view of leg base of left first perelopod. A. Superficial lateral muscles. K. Some lateral muscles removed to show deeper muscles. WHITE SHRIMP FROM THE GULF OF MEXICO 91 coxopoc/ffe has/poiy'/re coxopoc///e mofsc/es i rr)i/sc/e Aas/poc//fe '6as/poc//fe depressor- /77i/sc/es artery 'oara/'/c/?/a (ep/poc//fe) coxopod/'tf -— nerve at/'ery bas/po FIGURE 51.—Lateral view of leg base of left first perelopod. A. Lateral and medial muscles. B. Medial muscles. 92 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE The lateralinost levator (fig. 50, A) is a short, odite depressor muscle in Penaeus. The rela strong muscle taking origin on the lateral edge tionships in Penaeus to the seven depressor mus of the coxopodite and inserting on an anterolateral cles of the blue crab are difficult to ascertain. The apodeme of the basipodite. Mesad of this levator depressor muscle h in the third pereiopod of Pan- lies a long, curving basipodite levator (fig. 50, A, dalus looks much like the medial, fourth basip B) which originates on the pleural plate and joins odite depressor muscle in the first pereiopod of the former levator on the basipodite levator apod Penaeus. eme. The third basipodite levator is a short, rounded muscle situated mesad of the previously EXOPODITE MUSCLE OF FIRST PEREIOPOD mentioned levators (fig. 50, J.). The third levator FIGURE 50 originates in the coxopodite and inserts on the anterior rim of the basipodite. The medial, fourth The little, spindle-shaped exopodite muscle levator, also originating on the coxopodite, inserts originates on the wall of the basipodite, mesad, on the anteromedial edge of the basipodite (fig. 51, and passes across the basipodite to a point of in A). The basipodite levator muscles turn tlie basip sertion at the base of the exopodite. Presumably odite and the distal leg article dorsad. its contractions move the exopodite, which, while While the three walking leg levator muscles very reduced, is connected to the basipodite by an found in Astacus and CaUinectes are undoubtedly articular joint. Nothing similar to the exopodite homologous with some or all of the basipodite muscle is described in the first pereiopod of the levators in Penaeus^ the details of their relation other crustaceans referred to. Berkeley describes ships are uncertain. an attractor of the mastigobranch in the pereio- pods of Pandalus., but illustrates the muscle as a BASIPODITE DEPRESSOR MUSCLES coxopodite component. OF FIRST PEREIOPOD ISCHIOPODITB REDUCTOR MUSCLE OF FIRST FIGURES 49, 50, 51 PEREIOPOD The first walking leg of Penaeus has four basip FIGURE 49 odite depressor muscles. Two of the depressors The ischiopodite reductor muscle originates originate in the coxopodite and two on the pleural over an extensive area on the dorsomedial surface wall. The lateralmost member of the group of the basipodite. The muscle inserts on a prox originates on the lateral side of the coxopodite imal ischiopodite apodeme located on the ventro (fig. 51, A) and passes ventromedially to insert medial rim of the ischiopodite. The reductor on the common basipodite depressor apodeme. pulls the ischiopodite and with it the distal arti The longer, second depressor originates dorsad, cles ventrad. The ischiopodite reductor muscle of on the pleural wall, and runs ventrad to the com Astacits is the same as that of Penaeus. The mon apodeme. Mesad of the second basipodite muscle is missing in the blue crab, in w^hich the depressor lies a short third depressor of the basip basipodite and ischiopodite are fused. odite which is attached between the dorsomedial rim of the coxopodite and the basipodite apodeme. MEROPODITE FLEXOR MUSCLE OF FIRST The fourth basipodite depressor muscle lines the PEREIOPOD medial side of the coxopodite. Unlike the other FIGURE 49 depressors, the fourth basipodite depressor inserts The meropodite flexor muscle is a two-part for some length along the posteromedial margin structure whose short fibers insert at an angle on of the basipodite. an elongate apodeme projecting from the ventro The basipodite depressor muscles turn the basip medial edge of the meropodite, proximally. The odite ventrad, thus accomplishing body support apodeme divides the muscle approximately in on the limbs. Considerable variation in basip half on its long axis. The muscle fibers originate odite musculature exists. The depressor muscle about a wide area of the ventral and medial sur a of Astacus is very likely the homolog of the face of the ischiopodite. Their contractions turn long, second basipodite depressor of Penaeus^ the meropodite mesad and to a lesser extent ven- whereas the depressor muscle h of Astacus is trally. The meropodite flexor muscle apparently probably homologous to the lateral, first basip is subdivided into two parts in Astacus, in which WHITE SHRIMP FROM THE GULF OF MEXICO 93 Schmidt describes them as reductors. A single CHELA REDUCTOR MUSCLE OF FIRST meropodite reductor is shown in the study of PEREIOPOD CalUnectes. The muscle is unopposed by an ex FIGURE 49 tensor or productor in Penaeus and the other forms considered here. Directly ventrad of the chela productor muscle lies the chela reductor muscle, a structure also CARPOPODITE EXTENSOR MUSCLE OF FIRST attaching by several slips to a long apodeme. The PEREIOPOD reductor apodeme arises from the ventral rim of FiGUEE 49 the propodite. The chela reductor muscle turns the chela ventrad. The same muscle appears in The fibers of the carpopodite extensor muscle the first pereiopod of Astacus and CalUnectes^ and take origin over very nearly the whole of the lat in the second pereiopod of Pandalus^ as the pro eral side of the meropodite. The apodeme upon podite reductor muscle. which they insert extends proximally from the lateral edge of the carpopodite and runs almost CHELA EXTENSOR MUSCLE OF FIRST PEREIOPOD the length of the meropodite. The carpopodite FIGURE 49 extensor straightens the carpopodite on the mero podite in a horizontal plane. The muscle is rep As mentioned above, the joint between the car resented in Astacus and CalUnectes as the carpo popodite and propodite affords free movements podite abductor muscle. of the latter on the former. The chela extensor muscle straightens the chela with respect to the CARPOPODITE FLEXOR MUSCLE OF FIRST carpopodite in the horizontal plane. The muscle PEREIOPOD originates in the distolateral part of the carpopo FIGURE 49 dite and inserts on an apodeme projecting prox imally from the lateral margin of the propodite. Somewhat like the carpopodite extensor, the Exact counterparts of the chela extensor are miss carpopodite flexor muscle is a long muscle in ing in the first pereiopod of Astacus and Cal serting on a long apodeme. The muscle origins Unectes. Berkeley describes, however, 2 extensors occur about the medial and ventral surfaces of the and 2 flexors of the propodite in the second pereio meropodite. The carpopodite flexor apodeme pod of Pandalus. Homologies of these muscles arises from the proximal margin of the carpo without information about their innervations is podite, mesad of the dicondylic axis. A pull on not feasible. this apodeme turns the carpopodite and chela deeply on the meropodite, for this joint is a free DACTYLOPODITE ADDUCTOR MUSCLE OF FIRST one. The same muscle is found in the first perei- PEREIOPOD opod of Astacus and CalUnectes^ but named the FIGURE 49 carpopodite adductor muscle. The dactylopodite adductor muscle originates CHELA PRODUCTOR MUSCLE OF FIRST throughout the medial part of the pod-shaped pro PEREIOPOD podite and inserts on a heavy apodeme of the dac FIGURE 49 tylopodite. Contractions of this large muscle close the dactylopodite on the distal gnathal process of The fibers of the chela productor muscle arise the propodite. The dactylopodite adductor mus from lateral, proximal, and medial regions of the cle is found in the first pereiopod of Astacus and dorsal part of the carpopodite and insert on the CalUnectes. long productor apodeme of the propodite. Since the joint between the carpopodite and propodite DACTYLOPODITE ABDUCTOR MUSCLE OF FIRST PEREIOPOD is a free one, tension on the productor apodeme tends to straighten the chela with respect to the FIGURE 49 carpopodite and even to levate the chela. The Functioning in opposition to the dactylopodite propodite productor muscle of Penaeu^s is found in adductor muscle, the fibers of the dactylopodite much the same form in Astacus and CalUnectes^ abductor muscle originate about the lateral sur under the name of propodite productor muscle. faces of the propodite and insert on the abductor 4G8059 0—59 7 94 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE apodeme attached to the lateral margin of the dite. The propodite is not so long as the carpopo dactylopodite. The muscle opens the jaws of the dite. The slender, tapering dactylopodite makes chehi. The dactylopodite abductor muscle of contact with the propodite by a simple dicondylic Penaeus is fully homologous with the muscle of joint. Chemoreceptor tufts and other sensory and the same name in the tirst pereiopod of Astacus mechanical setae are arranged in rows on the and CaLlinectes. The first pereiopod chela of Pan- dactylopodite and to some extent on the propodite. dalu^ is reduced. MUSCLE ELEMENTS 5b. Fifth Pereiopod The arrangement of the muscles of the fifth The fifth pereiopod has been chosen as an ex pereiopod is typical of the true supporting limbs ample of a nonchelate limb. It is truly ambula of the thorax. The ambulatory muscles are light tory, slender and very long, much longer than the by comparison with those of the heavy reptant first pereiopod, mainly due to pronounced length crustaceans and the terrestrial arthropods. In re ening of the meropodite and carpopodite. Dorsad sponse to the support function of this appendage, of the fifth pereiopod projects a single pleuro- the depressor musculature of the basipodite is es branchia. pecially well developed. The only anatomical ac count of the fifth pereiopod of a crustacean known SKELETAL ELEMENTS to the present writer is that of Cochran on Cal- The coxopodite (fig. 54) is a rounded box whose linectes. However, the fifth leg of the blue crab is lateral corner projects dorsally to a point. This the swimming leg and hence its muscles, particu point makes contact with the dorsal condyle of larly the basal ones, have diverged from the typi the article. The ventral condyle is located on the cal pattern. A remarkable uniformity of endopo- sternal plate between the limbs. The arrange dite musculature between Penaeus and Callinectes ment of these coxopodite dicondyles permits this still remains, however. The fifth pereiopod of article great freedom of movement. A large Penaeus is operated by 24 muscles grouped into 13 region of thin cuticle, the articular cuticula (fig. functional types. 54), lying caudad of the coxopodite, presents little resistance to posterior movements of the seg COXOPODITE PROMOTOR MUSCLES OF FIFTH ment. A second articular cuticula associated with PEREIOPOD basipodite movements is found on the lateral sur FIGURES 52, 53, 55 to 57 face of the coxopodite. Two coxopodite promotor muscles are found in The strong basipodite (fig. 54) is hinged to the the fifth pereiopod of Penaeus. The smaller lat ventral surface of the coxopodite in the typical eral promotor originates by its broad, dorsal, fan- way, allowing for extensive movements of this shaped portion on the laterotergal plate of the seg article and of the distal segments in the vertical ment (figs. 55, 56) and inserts on the anterior rim plane. With the dorsolateral surface of the basip of the coxopodite. Lying medial to the latter mus odite is articulated a small exopodite. The cle is a larger promotor muscle mass originating ischiopodite (fig. 52) makes an oblique connec somewhat dorsad of the smaller promotor on the tion with the basipodite. The joint is slightly pleural region. Contractions of these muscles movable. The ischiopodite may be bent ventrad turn the coxopodite forward on its condyles. on horizontally located condyles. The articula tion point between the short ischiopodite and the COXOPODITE REMOTOR MUSCLES long meropodite is a transverse one capable of OF FIFTH PEREIOPOD limited motion. By means of the joint the mero FIGURES 52, 53, 55, 56 podite may be turned ventrad. The carpopodite articulates with the meropo The fifth pereiopod of Penaeus has two coxo dite by means of a complex "knee" joint made up podite remotor muscles. These are large, flat of a pair of heavy condyles so arranged that the muscles (fig. 55) situated beneath the thin ma joint has considerable freedom of action. Simi terial of the articular cuticula (fig. 54). Both larly oriented, but not so freely movable, is the originate by broad margins on the pleural plate articulation between the carpopodite and propo- and insert on the caudal margin of the coxopodite. WHITE SHRIMP FROM THE GULF OF MEXICO 95 merc>paa'/'f& cjc//'f'e? • p/'rypoc//f& ".-arpop. I yfpnf/'cr/ I cord 5ff>pen?/opocf isc/?/opoc//fe basipoc//fi=' /dacfi//op(yc/.//e' (?xopoc//f&., coxopoc///^ promofor- m/Jsc/es has/poc/ife coxopoc/ite \ supetFicia/ aepressor- remofor- \ Ventr-cr/ /nasc/e musc/e FIGURE 52.—Dorsal view of left fifth pereiopod showing protopodlte muscles. 96 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE •carpopoc/f'fB', , p/-typoc//fe / p/-'opac///iS' /7?asc/e , ''propocZ/Ve ca-z-pqax/Zf^ , ''c/acff/Zopod/ie -f/GXOK f77(/Sck ca/tpopoa//fw ^ /<±3cfe//opocJ/f& cfbc/ucfor- musc/e P7i/sc/e meropoc/f'fe ^ /scfy/'opoc//f& cord meropoc/ife S&jp&rep/opod musc/e has/poc//fe- "dacfy/opodife /sc/?/'opo(y/p(Pr0c/ucfo/^ / /\P' 'tdacfa/- masc/e coxopoof/fe bas/pod/fe caxopod/'f^ depressor remotor promofo^, / /•nc/sc/e FiQUEE 53.—Dorsal view of left fifth pereiopod. Dorsal cuticle removed to show muscles of protopodite and endopodite. WHITE SHRIMP FROM THE GULF OF MEXICO 97 The coxopodite remotor muscles act with some tral skeleton. All of the foregoing five muscles power to turn the coxopodite rearward. insert on the basipodite depressor apodeme. The sixth and seventh depressor muscles (fig. 58) arise COXOPODITE DEPRESSOR MUSCLE on phragmal material on the medial side of the OF FIFTH PEREIOPOD ventral skeletal foramen and insert for some FIGURE .58 length along the posteromedial rim of the basipo dite. The function of the basipodite depressor The coxopodite depressor muscle arises from muscles is to turn the fifth pereiopod ventrad, phragmal material on the medial margin of the providing support for the body. ventral skeletal foramen entering the coxopodite and runs to the medial margin of the coxopodite ISCIHOPODITE REDUCTOR MUSCLE OF (fig. 58). Apparently the muscle is able to lift FIFTH PEREIOPOD slightly the medial margin of the coxopodite, and for this reason the structure has been named the FIGURE 53 coxopodite depressor muscle. The ischiopodite reductor muscle has multiple origins over a substantial area on the dorsal and BASIPODITE LEVATOR MUSCLES OF FIFTH PEREIOPOD medial parts of the basipodite. It inserts on an apodeme projecting from the ventral surface of FIGURES 55, 56 the ischiopodite. The muscle bends the ischiopo Four basipodite levator muscles are seen in the dite ventrad slightly and to some extent rotates fifth pereiopod of Penaeus. The two lateral leva it, due to the oblique angle by which the basipo tors constitute together a broad fan (fig. 55) dite and ischiopodite are connected (fig. 52), The originating along the caudal margin of the coxo muscle is not opposed by any other muscle. podite. They become narrow as they run to their insertions on the lateral rim of the basipodite. MEROPODITE REDUCTOR MUSCLE OF Two longer levators lying mesad of the lateral FIFTH PEREIOPOD levators originate in the dorsal apex of the coxo FIGURE 53 podite and run to heavy apodemal material com Arising by multiple origins on the dorsal and mon to the levator muscles. The basipodite leva tor muscles raise dorsally the basipodite and with medial half of the ischiopodite, the meropodite it the distal elements of the limb. reductor muscle fibers insert on a small apodeme on the ventral surface of the long meropodite, BASIPODITE DEPRESSOR MUSCLES OF proximally. The meropodite reductor bends the FIFTH PEREIOPOD meropodite ventrally a short distance. FIGURES 52, 53, 55 TO 58 CARPOPODITE ABDUCTOR MUSCLE OF The most important muscles of the fifth perei FIFTH PEREIOPOD opod of Penaeus are the depressors of the basipo FIGURE 53 dite. Seven basipodite depressor muscles exist in the limb. The first and second depressor muscles The fibers of the carpopodite abductor muscle are lateral (fig. 56). They take origins on areas originate over most of the dorsal half of the mero of the caudal margin of the coxopodite and insert podite and insert on an extremely long apodeme on the large common depressor apodeme of the running nearly the whole length of the meropodite basipodite. The third depressor (figs. 55, 56) is along the midline. The length of pull of this a large fan which originates broadly on the latero- muscle and of its opponent, the carpopodite ad tergal plate of the body segment. The fourth ductor, is very great. The long apodeme to which basipodite depressor (fig. 57), also fan-shaped but it attaches arises from the proximal end of the narrower than the third, lies just mesad of the carpopodite, lateral to the condylic axis. The third depressor muscle. It, too, has a broad ori muscle thus turns the carpopodite to a position gin on the pleural area. The fifth depressor is in which the axes of the meropodite and carpopo a rather small muscle (fig. 58) which takes its dite are in line. The carpopodite abductor mus origin on medial phragmal material of the ven cle easily could be described as an extensor. 98 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE coxopoc//te c/orsa/ co/7c/^/e ' arf-/ci//ar of coxopoc//f& cu/'/cu/a arf/ca/ar cuf/cu/a. I exopod/fe iscMopoa^/te- ^ h<7s/poc//fe FIGURE 54.—Lateral view of leg base of left fifth perelopod. WHITE SHRIMP FROM THE GULF OF MEXICO 99 coxopoc//ta i)as/poc//te , coxopoc//fe> promotor' /i rerrfofuf masa/€?s j 1 /77i/sc/es bas/poc/'tf? n7(jsc/e's exapoc//fe. arfe/-!/ bos/fiod/te rne/^opoc//fe nen/e' musc/e "ischiopoc//fe FIGURE 55.-Lateral view of leg base of left fifth pereiopod. Lateral cuticle removed to show superficial lateral muscles. 100 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE coxopotjrh? baa/podf'te coxopocy/As prt?momr c/epresso^' /"C/r/o/o/-' /m/sc/a rr/U3c/e basipodife levator mofscias arttjri/ 'bas/'pocf/'fe depressor neri/e \ mempoof/fe / \ \ musc/e' fscrniopoa, fy 6as/poc//fe FIGURE 56.—Lateral view of leg base of left fifth pereiopod. Some superficial lateral muscles removed. WHITE SHRIMP FROM THE GULF OF MEXICO 101 coxopocJ/tf? I jba's/poc///'& I c/epr&ssor- /776/£c/(? /7?t::/sc/€' arfert/ nerye. jbae/poa/'/'fe depr&ssor /sc/?/opoc//'f& bas/poc//fe^ rr?iysc/es FiGtTBE 57.—Lateral view of leg base of left fifth pereiopod. Lateral muscles removed to show medial muscles. 102 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE coxopoc//t& has/pocf/te c/epr&^so/^ ' a/opresso/- musc/e /T/i/sc/e ' ar/erg net-wB has/poo//fe c/epr&sso/^ /ru/sc/es is c/}i'opodite I />as/poc///e FiouKE 58.—Lateral view of leg base of left fifth pereiopod. Medial muscles. WHITE SHRIMP FROM THE GULF OF MEXICO 103 CARPOPODITE ADDUCTOR MUSCLE OF DACTYLOPODITE FLEXOR MUSCLE OF FIFTH FIFTH PEREIOPOD PEREIOPOD FIGURE 53 FIGURE 53 In like fashion to the carpopodite abductor, the The dactylopodite flexor muscle bends tlie dac carpopodite adductor muscle fibers oritjinate on tylopodite upon the propodite. Like the exten'sor a large area of the meropodite, but on the ventral muscle, the flexor lias multiple origins upon the half of the article. The muscle inserts on a long medial surface of tlie propodite. The muscle in apodeme arising from the proximal portion of serts on the long flexor apodeme of the dactylo the carpopodite, medial to the axis of the dicon- podite. ' dyles. Carpopodite adductor muscle contractions serve to turn the carpopodite toward the body, and C. Abdomen in fact deeply on the meropodite. The muscle Unlike tlie head and gnathothorax, the abdo might better be considered a flexor of the carpo men is almost entirely devoted to the propulsion podite. of the white shrimp. Except for slender com ponents of the gut, the gonads, and the nervous PROPODITE EXTENSOR MUSCLE OF FIFTH PEREIOPOD and circulatory systems, the space within the ab dominal skeleton is filled with muscles, most of FIGURE 53 them concerned with the powerful flexion of The arrangement of the propodite muscles in which the animal is capable. The abdomen con the carpopodite is very similar to that of the sists of six segments, all of wliich bear append carpopodite muscles within the meropodite. The ages, and a posterior telson wliich does not. The abdominal segments are attached to one anotlier propodite extensor muscle originates over much by deep folds of thin articular cuticle which allow of the dorsal part of the carpopodite and inserts each segment great freedom of movement with on a long apodeme projecting proximally from respect to its neighbors and with the thorax. the base of the propodite. The apodeme is so con Intersegmental connections in the abdomen are of nected to the propodite that a pull on it extends several types. The simplest and perhaps most the propodite with respect to the carpopodite. movable is that between the thorax and first ab dominal segment (fig. 59). Here, cuticular folds PROPODITE FLEXOR MUSCLE OF FIFTH PEREIOPOD of great depth reinforced by heavy muscles in ternally make ventral flexion possible between FIGURE 53 these body tagmata. Tlie junction is without The propodite flexor muscle opposes the action special, restrictive condyles, allowing extensive of the propodite extensor muscle. The fibers of lateral movements of the abdomen on the thorax at this point. the flexor muscle arise from the ventral surface of the carpopodite and, like those of the extensor, The junctions of the first and second, and of the attach to a long apodeme of the propodite. This second with the third abdominal segments (fig. apodeme arises from a position opposite to that 59) are identical. Motion at these joints is limited of the extensor apodeme. Contractions of the to flexion and extension by lateral condyles of simi propodite flexor muscle flex the propodite upon lar design. In contrast, the joint between the third and fourth adbominal segment differs from the carpopodite, those between the first three segments. This con DACTYLOPODITE EXTENSOR MUSCLE OF FIFTH nection is much simpler. The condyles are rather PEREIOPOD loosely connected, affording flexion and extension FIGURE 53 and a certain amount of lateral motion. The fourth, fifth, and sixth abdominal segments arti The dactylopodite extensor muscle originates culate with one another by means of two pairs of along the lateral side of the propodite and inserts identical condyles similar in their rigidity to those on a long apodeme arising from the proximal end between the first, second, and third segments, but of the dactylopodite. The muscle straightens the different in structure (see enlargements of con- dactylopodite on the propodite. dylic structure, detail of articulation, fig. 59). 104 FISHERY BULLETIN OF THE PISH AND WILDLIFE SERVICE 42 ^ 1 S ^ ^in, ->f S' ft ^; 'It^5 -C;-; <•-t^ ^-.,'' -'<<^i •St \ - -''' a o Xi C3 WHITE SHRIMP FROM THE GULF OF MEXICO 105 The posterior joints permit free extension and for the laterotergal plates has become embedded flexion, but probably resist lateral motion. We ineradicably in the literature of crustacean sys- have, then, 4 kinds of intersegmental hinges func tematics is therefore most regrettable. tioning in 2 different ways. No reason for this Beyond what has been brought out about the multiplicity of structure in the hinges is readily general structure and articulation of the abdom apparent. inal segments, little need be said further except for a brief mention of the modified posterior end SKELETAL ELEMENTS of the sixth abdominal segment. Unlike the five The abdominal segment of arthropods is com anterior abdominal segments from which light appendages project, the sixth abdominal segment prised of a ring of cuticle. This ring, however, must be strong enough to bear the tail fan. For may be subdivided in parts. The abdominal this reason the posterior end of the sixth segment somite in the Crustacea has usually been said to is heavily sclerotized. In addition, the contents consist of an arched dorsal tergum and a flat, con of the sixth segment are largely devoted to me cave, or convex ventral sternum connected to the diating the flexions of the tail fan which project tergum by two lateral pleura, the latter often pro caudad from the segment rather than ventrad as duced ventrad in a fold. The lateral pleuron has is the case with the preceding abdominal segments. been variously interpreted. Snodgrass (1935) The result is the visible difference in shape of the describes the pleural areas of arthropods as "typ sixth segment compared to that of the typical ab ically membranous" to permit the movement of dominal segment. More will be said about the appendages arising from the pleuron. Pleural sixth abdominal segment when the tail fan is sclerites, when present, represent a contribution of considered. the proximal parts of the appendages, according to this worker. He sets forth the fact also that the MUSCLE ELEMENTS appendages articulate with the ventral margins of the pleural sclerites and with the lateral edges of The characteristic of the abdominal muscles, the sternal sclerites. That this typical situation apart from their great mass, is the unusual man may not always obtain is shown by Hart (1952) ner in which they are laid out. These muscle who found that the lirst pleopod in Canibarus bodies are extraordinarily heavy and so inter longulus longulus Girard, 1852, is connected to the twined with one another that their separation for abdominal venter entirely by sternal components. study is difficult. In accordance with the system Snodgrass (1935) apparently was of the opinion of Daniel (1931c), the abdominal muscles are that the arthropod pleuron represented a dis divided into the following four functional muscle tinct region of the segment. In a study of the groups: (1) The superficial ventral muscles, (2) Crustacea made at a later date, Snodgrass (1952) the lateral muscles, (3) the dorsal muscles, and describes in Anaspides tasmaniae Thomson, 1892, (4) the main ventral muscles. They will be a generalized malacostracan, a clearly demarked treated in that order. pleural sclerite with which the limb basis articu SUPERFICIAL VENTRAL ABDOMINAL MUSCLES lates. In the same study he refers to the pleural sclerite as a "laterotergal pleural plate" and states FIGURES 61, 63 that the pleuron of the crustacean belongs to the The superficial ventral abdominal muscles of tergum. Support of the more recent opinion of Penaeus are attached between yokes of thin cuticle Snodgrass (1952) that the pleuron of crustaceans is tergal in origin may be found by a consideration (fig. 63) lying transversely across the posterior of the abdominal segment of Penaeus setiferus. portion of each abdominal segment beneath the The sternum and tergum are distinct regions ventral nerve cord. These yokes are associated in the white shrimp, but no line or suture can be with the folds of articular cuticle on the ventral seen which distinguishes a pleural component of surface, and they are so arranged that the super the abdominal segment (fig. 59). From this we ficial ventral muscles run from the posterior re may conclude that the pleura are not needed in a gion of one abdominal segment to the posterior morphological construct of the abdominal seg region of the next. The superficial ventral ab ment of the crustacean. 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