ON THE STRUCTURE AND MECHANISM OF THE GASTRIC MILL IN DECAPODA.

III. Structure of the Gastric Mill in Anomura.

By S. S. PATWARDHAN, M.SC., Department of Zoology, College of Science, Nagpur.

Received January 8, 1935. (Communicated by Prof. M. A. Moahe, M.A., M.Sc., F.Z,s.) 1. Introduction. IN my previous communication (1935), a comparative account of the gastric mill of a number of representative Brachyura was given and an attempt was made to collect data on the correlation of the presence of the gastric mill and simple mandibles with the habits and habitats of the animals. In the present coinni.unication, it is proposed to give a similar comparative account of the gastric mill of some of the representative Anomura. 2. Material and Method. The seven types of Anomura examined by me were obtained from Ennur, Plymouth and Naples Biological Supplies Stations and represent all the three tribes of Anomura and four families. Tribe. Family. Galatheidea Galatheidae Galathea strigosa* I abr. Munida rugosa fi Leach. Hippidea Hippidae Hippa asiatica.* Albunidee Albunea symnista4 Paguridea Paguridee Diogenes diogenes^ Herbst. Eupagurus angulatus* Hell. Paguristes macuclatus* Hell. Two or three specimens of each type were dissected. The foregut was kept in a weak solution of Caustic Potash overnight and the softened muscular tissue was carefully scraped away with a needle.. A brief account of the cardiac and the pyloric stomach and a comparative account of the principal ossicles of the gastric mill is given below.

* From Naples. t From Plymouth. $ From Ennur.

405 Bl F 406 S. S. Patwardhan 3. The Cardiac Stomach. In all the Anomura examined by me, the body is less flattened dorsoventrally than in Brachyura and the thoracic portion is longer than broad. The cardiac stomach follows similar modifications and consists of an elongated sac broader at its posterior end and slightly depressed above downwards. The floor of the cardiac stomach is limited on either side by a deep ventral groove having the same disposition and function as in Brachyura. The characteristic covering of comb-like linear rows of elongated setae over the groove is always present. The lateral accessory teeth are not always present. They are wanting in Diogenes, Eupagurus, Paguvistes and Albunea. In Munida (Fig. 2, l.a.t.) and Galathea each tooth consists of a small triangular or oval plate bordered with rosethorn- shaped spines. It is interesting to note that similar lateral accessory teeth are present in the tribe Thalassinidea which is included in Anomura by some authors. In Hippa a distinct lateral accessory tooth is absent but on the anterior edge of the zygocardiac ossicle there is a small finger-shaped process (Fig. 4, X.) with a thickly chitinised tip and covered over with set. This process probably represents the lateral accessory tooth. The most interesting feature of the cardiac stomach is the cardiac pyloric valve. In all the types examined by me, it consists of a pair of pear-shaped chitinous thickenings meeting obliquely at the posterior end. These thickenings are covered over with short transverse rows of blunt setee or thin ridges which give them a corrugated appearance (Figs. 1 to 3, c.p.v.). In Hibpa (Fig. 4, c.p.v.) there is a single row of thick spines, each having a broad base. Similar condition of the cardiac pyloric valve is also met with in Thalassinidea (to be described in a subsequent communication) . The valve is always pigmented deep brown or yellow and thus indicates that in addition to its function of closing the cardiac pyloric opening, it acts as a fourth median ventral tooth, getting rubbed over by the movements of the dorsal median tooth. The cardiac pyloric valve is not generally a toothed valve in the Brachyura, except in Cardisomca and Paratelphusa, and also in Macs'ura. 4. The Gastric Mill. The gastric mill is composed of typical ten ossicles, viz., the median mesocardiac and the lateral pterocardiac ossicles forming the anterior arch, the pyloric, the paired exopyloric and the zygocardiac ossicles forming the posterior arch and the anterior urocardiac and posterior prepyloric ossicles connecting the two arches. A comparative account of each ossicle is given below. Gastric Mill in Decapocia 407

(a) Ossicles of the Anterior Arch.—The mesocardiac ossicle is peculiar in the fact that it forms the largest portion of the anterior arch. It may be semicircular in shape with the convexity pointing forwards, e.g.,

Munida, Paguristes (Figs. 2 and 3, m.c.) and Albunea, or triangular with the apex pointing forward, e.g., Eu^agurus (Fig. 1, m.c.) and Galathea. In Diogenes it is more or less oblong with the largest side across the 408 S. S. Patwardhan transverse plane. In Hippy (Fig. 4, m.c.) the triangular plate is folded downwards and slightly inwards. In all cases, the posterior portion of the ossicle, where it is articulated with the urocardiac ossicle, is very thick and densely calcified, while the anterior portion is thin and membranous. On either side of the base of the ossicle there is a small chink where it articulates with the pterocardiac ossicle of the same side. Correlated with the large size of the mesocardiac ossicle are the small pterocardiac ossicles. Each is a triangular curved plate articulating with the mesocardiac ossicle along the broad base and running obliquely downwards, its apex is attached to the anterior process of the zygocardiac ossicle. In the shape, size and disposition of the ossicles of the anterior arch the Anomura seem to resemble the Macrurous type of the gastric mill. (b) Ossicles of the Posterior Arch.—The pyloric ossicle is always a curved plate, convex on its upper surface, and forms the roof of the anterior portion of the pyloric stomach. In Eupagurus, Hippa (Figs. 1 and 4, p.) and Albunea it is strongly calcified at its sides, while the median portion is thin and less calcified and gives the ossicle a paired appearance. In other forms, e.g., Munida (Fig. 2, p.), Diogenes and Galathea, however, it is more or less uniformly thick and calcified. The posterior border of the pyloric ossicle is usually a circular curve, e.g., Eupagurus (Fig. 1, p.), Galathea and Albunea but in Paguristes, Hiipa (Figs. 3 and 4, p.) and Diogenes, the posterior border is drawn backwards into a more or less angular process. In Diogenes the pyloric ossicle presents a pentangular form and in Munida (Fig. 2, p.) it is heart-shaped in outline. The exopyloric ossicles are small and are not clearly differentiated from the pyloric. Each ossicle is either irregularly four-sided, e.g., Munida, Paguristes (Figs. 2 and 3, ex..) and Diogenes or more or less triangular, e.g., Eupagurus, Hippy (Figs. 1 and 4; ex..), Galathea and Albunea. Its basal portion is always thick, while towards its apex it gets thinner and less calcified. The zygocardiac ossicles resemble in shape and disposition a typical one in Brachyura. Anteriorly each ossicle is produced into a long process articulating with the outer end of the pterocardiac ossicle of its side and posteriorly into a short vertical process articulating with the exopyloric ossicle. Its median edge is modified into a lateral tooth. In Munida (Fig. 2, l.t.) and Albunea the lateral tooth consists of a single large anterior denticle followed by a row of thin vertical ridges. In the remaining forms examined by me, e.g., Eupagurus, Hippa (Figs. 1 and 4, ii.) and Diogenes, there are two large anterior denticles followed by rather thick ridges varying between four and twelve in number. The denticles and the ridges form a curved row with the convexity facing downwards and are, as usual, covered with a thick brown or yellow pigment. Gastric Mill in Decaioda 409

(c) Ossicles connecting the two arches.—The anterior median urocardiac ossicle is always flattened and T-shaped and runs obliquely backwards and downwards. The transverse part of the T-shaped plate is usually straight, e.g., Munida, Paguristes, Hippa (Figs. 2, 3 and 4, u.c.), Albunea, Diogenes and Galathea but in Eupagurus (rig. 1, u_c.) it is curved with the convexity backwards. It is firmly ankylosed with the base of the mesocardiac ossicle. The edges of the median longitudinal part of the T-shaped plate are more or less curved and posteriorly they bulge out and give the ossicle a flask- shaped appearance. On the posterior half of its inferior surface the urocardiac ossicle bears a median tooth which shows interesting modifica- tions. In Eupagurus, Paguristes (Figs. I and 3, m.t.), Diogenes and Galathea, it consists of a single ossicle which is U-shaped in outline, while, in Munida and Hippa (Figs. 2 and 4, m.t.) it is V-shaped. In Albunea it consists of two circular transversely ridged denticles placed side by side. In Paguristes (Fig. 3, m.t.) the sides of the U-shaped tooth are carved into a transverse row of thin ridges. In Hippa (rig. 4, m.t.) there is a pair of large, spindle-shaped, corrugated denticles situated in front of the V-shaped terminal tooth. The denticles and the ridges constituting the median tooth are impregnated with a deep brown or yellow pigment. The reyyloric ossicle is also a T-shaped, strongly calcified plate. The transverse part of the plate is held between the anterior processes of the pyloric ossicle, while the lower end of the vertical part of the T-shaped ossicle is usually bifid and firmly ankylosed to the lower and posterior end of the urocardiac ossicle. The ossicle may be bordered with more or less straight edges as in Eubagurus (Fig. 1, pr.p.), Diogenes and Albunea or may be bulged out and strongly curved to give the ossicle a flask-shaped appearance, e.g., Munida and Hippa (Figs. 2 and 4, pr..). Distinction between the Anomurous and Brachyurous types of Gastric Mill.— The gastric mills of a number of Brachyura and Anomura examined by me show that the gastric mill in Brachyura is characterised by the presence of a small mesocardiac and elongated pterocardiac ossicles; the posterior border of the pyloric ossicle is round and the cardiac pyloric valve is usually simple and covered with setae. On the other hand, in Anolnura the mesocardiac ossicle is invariably large and the pterocardiac ossicles are very small and curved; the posterior border of the pyloric ossicle is usually drawn backwards and the cardiac pyloric valve is modified into a grooved median ventral tooth. In Brachyura the lateral accessory teeth are usually present and consist of a group of stout spines. In Anomura they are more usually absent and in the forms in which they are present, e.g., Munida and Galathea, each tooth consists of a triangular or oval plate bordered with short spines. 410 S. S. Patwardhan

5. The Pyloric Stomach. The structure of the pyloric stomach is essentially similar to one found in Brachyura. The inter-ampullary ridge (Patwardhan, 1934, p. 184, Fig. 2, i.a.r.) is very thick and pad-like and effectively separates the dorsal from the ventral chamber of the pyloric stomach. The opening between the pyloric stomach and the midgut is guarded typically by six valves: a dorsal median from the middle of the roof of the pyloric stomach, a pair of 'dorsolateral and inferolateral from the sides of the dorsal chamber and one median ventral which is a backward prolongation of the inter-ampullary ridge. These valves are elongated, project backward and are covered with long set. Such a typical arrangement is found in Diogenes, Hip a, Paguristes and Eupagurus. In other forms examined by me the dorsal median valve is split into two and in Albunea the dorsolateral valves are absent. 6. The Midgut and the Hepatopancreas. Unlike Brachyura, the midgut is perhaps the largest portion of the intestine in Anomura, e.g., Eupagurus (Jackson, 1913) and Paguristes (Yonge, 1932). The extent of the midgut is indicated externally by the presence of three intestinal ceecee : a pair of elongated and coiled pyloric c ecae near the commencement of the midgut and a similar median dorsal one near the commencement of the hindgut (Jackson, 1913). The structure of the hepatopancreas, as described by Jackson, seems to he essentially similar to one found in other Decapods. 7. Masticatory Mechanism of Anomura. In all the Anomura examined by me the mandibles are remarkably similar. The head portion is simple and consists of (a) a median cutting plate, the incisor process, which may have a sharp and straight edge as in Diogenes, Galathea, Munida and Paguristes (Fig. 5, A, B, D and ), or indentations as in Eupagurus and Albunea (C and F) ; (b) an elliptical or spindle-shaped prominence on the inner or superior surface of the head region, the so-called " molar process ", distinguished from the functional molar process by the absence of deeply pigmented chitinous covering ; and (c) a two. jointed palp plying on the inner or superior surface of the cutting plate. In Hippa (Fig. 5, G) the mandibles are extremely reduced and are repre- sented by a short incisor process and a small two-jointed palp. The masticatory appendages of Anomura, therefore, subserve the function of tearing the food and thrusting the . morsels into the mouth, while the function of mastication, i.e., of reducing the food to a pulp, is left for the gastric mill. The simple mandibles, the complex gastric mill, the Gastric Mill in Decapoda 411

pyloric filter and the midgut and hepatopancreas are, as in Brachyura, correlated adaptations for proper alimentation of food. 8. Discussion. In the concluding part of my previous paper (1935), an attempt was made to explain why in Brachyura the function of mastication of food is wholly limited to the gastric mill and not shared by the mandibles. Similar considerations are also applicable to Anomura. Anomura are included in the sub-order Reptantia (Boas, 1880; Borradaile, 1907), a group characterised by the presence of gastric mill. It is believed that hermit-crabs are derived from primitive lobsters which discovered the advantages of portable shelter of gastropod shells (Calman, 1911). It may therefore he inferred that, as in Brachyura, the gastric mill is a legacy from the lobster-like ancestors. A study of the habits and habitats of the hermit-crabs also leads us to similar conclusions. Like Brachyura, the Anomura are also characterised by an almost universal presence of a variety of protective and sheltering adaptations. The Paguridze are the hermit-crabs dwelling in empty gastropod shells. An interesting form, Pylocheles rniersii (Alcock, 1906), inhabits pieces of bamboo. Other forms have abandoned this primitive portable shelter and have taken recourse to different means, e.g., holes in 412 S. S. Patwardhan corals, in the canals of sponges or beneath stones and in crevices of rocks, e.g., Galathea, Porcellana and Munida. Mole-crabs, e.g., Hip a and Albunea, have flattened legs to burrow holes in sand. Shore-crabs (Lithodidce), land- forms (Conobitida,) and the robber-crab (Birgus) are secondarily modified for free and independent life. The diverse modes of protective adaptations, coupled with the nervous anxiety of a " bather whose clothes have been stolen" (Taylor's quotation in Jackson, 1913) exhibited by hermit-crabs when the old shell is abandoned and a new one is to be secured after every moulting and at the occasional fights between fellow crabs for appropri- ating the precious suitably sized , shell, and the occurrence of hypnosis in Paguristes and Porcella•a (Flattely and Walton, .1922) lead us to the conclusion that the majority of Anomura lead a very anxious and nervous life which results in a hurried swallowing of morsels of food thrust into the mouth. Their thorough mastication to facilitate digestion necessitates the presence of a gastric mill in the cardiac stomach.

9. Summary. The seven types of Anomura examined contained a complex gastric mill. A brief account of the cardiac and the pyloric stomach and a comparative account of the principal ossicles of the gastric mill is given. In possessing a large mesocardiac ossicle and small pterocardiac ossicles, the gastric mill of Anomura resembles a Macrurous type. The chitinised tooth-like cardiac pyloric valve is a characteristic feature of Anomura occurring only in a few Brachyura and in the tribe Thalassinidea. The lateral accessory teeth, when present, resemble those found in Thalassinidea. The mandibles are remarkably simple and uniform in structure (excepting in Hi where they are reduced), and each consists of a median cutting incisor process, a molar prominence and a two-jointed palp. An attempt is made to explain the presence of simple mandibles and complex gastric mill with reference to the fact that (a) the Anomura are grouped in the sub-order Reptantia characterised by the presence of a gastric mill; (b) the hermit-crabs derived from lobster-like ancestors which possess a gastric mill, and (c) the usual occurrence of protective adaptations in Anomura and the nervous and anxious life of hermit-crabs necessitates a hurried swallowing of morsels of food without proper mastication, in the buccal cavity. Gastric Mill in Decapoda 413

EXPLANATION OF FIGURES. The ossicles of the gastric mill viewed on the same plane. Inner view. Fig..1. Eupagurus angulatus Hell. Fig. 2. Munida rugosa Leach. Fig. 3. Paguristes maculatus Hell. Fig. 4. Hippa asiatica. Fig. 5. Inner view of the "head" of the mandible: A. Diogenes diogenes Herbst. B. Galathea strigosa Fabr. C. Eupagurus angulatus Hell. D. Munida rugosa Leach. E. Fag uristes maculatus Hell. F. Albunea symnista. G. H'ippa asiatica.

REFERENCE LETTERS. c.p.v. .. Cardiac pyloric valve. ex.p. .. I+EMopyloric ossicle. l.a.t. .. Lateral accessory tooth. l.t. .. Lateral tooth. m.c. .. Mesocardiac ossicle. m.t. .. Median tooth. p. .. Pyloric ossicle. pr.p. .. Prepyloric ossicle. pt.c. .. Pterocardiac ossicle. u.e. .. Urocardiac ossicle. z.c. .. Zygocardiac ossicle.

REFERENCES Alcock, A. .. .. .. A Catalogue of the Decapoda Crustacea in Indian Museum, Part II, "Anornura," 1906. Boas, J. E. V. .. .. Kgl. Danslee Vidensk. Selsk. SI rifier, 1880, (6) 1 (2). Borradaile, L. A. .. .. Ann. Mag. Nat. fist., 1907, 19, See. 7. Calman, W. T. .. .. The Life of Crustacca, 1911. Flattely, F. W., and Walton, C. L. The Biology of the Sea Shore, 1922. Jackson, H. G. .. .. L. 111. B. C. Memoirs, 21, Eupaqurus. Patwardhan, S. S. .. .. Proc. Ind. Acad. ,Sci., Bangalore, 1934, 1, 5. Patwardhan, S. S. .. .. Proc. Ind. Acad. Sci., Bangalore, 1935, 1, 7. Sedgwick, A. .. .. A Students' Text-book of Zoology, 1909, Vol. III.

Smith, Woods and others .. Cambridge Natural History, 4, " Crustacea and Arachnida". Yonge, C. M. .. .. Proc. Roy. Soc. London, 111 B, 1932,