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Notice: ©1991 John Wiley & Sons, Inc. This manuscript is an author version with the final publication available at http://www.wiley.com/WileyCDA/ and may be cited as: Svane, I., & Young, C. M. (1991). Sensory structures in tadpole larvae of the ascidians Microcosmus exasperatus Heller and Herdmania momus (Savigny). Acta Zoologica, 72(3), 129-135. doi:10.1111/j.1463-6395.1991.tb00939.x

~<{ /( A cra Zoologica (Stockholm) . Vol. 72.No . 3. pp. 129- 135. 1991 000 1- 7272/9 1$3.00 + .00 Printed in G reat Britain Pergamon Press pic © 199\ T he Royal Swed ish Academy of Sciences

Sensory Structures in Tadpole Larvae of the Ascidians Microcosmus exasperatus Heller and Herdmania momus (Savigny)

Ib Svane ' and Craig M. Young" ' Kristineberg Mar ine Biological Station. Kristineberg 2130. S-450 34 Fiskeb acksk il,Swede n ' Harbo r Branch Oceanog raphic Institutio n. 5600 O ld Dixie Highway. Ft. Pierce . Florida. U.S.A . (Accepted fo r publication 14 A ugust 1990)

Abstract Svane, I. & Youl/g. C. M. 199 1. Sensor y str uctures in tad pole larvae of the ascidians Micro­ COS /l IlIS exasperatus Heller and Herdmania momus (Sa vigny) .-Acta Zoologica (Stockho lm) 72: 129-1 35.

In this paper we describe the larval mo rphology of two spec ies fro m the ascidian family Pyuridae , .. Microcosmus exasperatus and Herdm ania momus, with spe cial emp hasis o n co mpo nents of the ce rebral vesicle . Larvae have not previously been described fo r any spec ies in the large genus Microcosmus. Besides a difference in size (larvae of H. m omus are about 40'X. larger than those of M. cxasperatuss, larvae of the two species differ prima rily in the num ber and arra ngement of senso ry structures. Bot h spec ies possess a well-developed sta tocyte but o nly H. momu s has an oce llus. T he abse nce of an oce llus in M. exasperatus is uniq ue amo ng pyurid ascidians. An auxiliary vesicle was found situated o n the left side of the ce rebral vesicle in bot h species. However . un like the larvae of H. momu s and ot he r pyuri d spec ies. ther e is no appa rent communica tio n between the auxiliary and ce rebral vesicles of M. exasperatus . Ep ithelial cells in the auxiliary vesicles of bo th spec ies carry mod ified cilia abo ut 2 u rn in diameter ; auxiliary vesicles of H. /110mus also have simple cilia with axo ne mes in a 9 + 0 microtubu le configuration. In H. /l 1O/JIl IS the membran es of the epithe lial cells are highly convoluted and exte nd into the lumen of the auxiliary vesicle . Mo rp hological arrange me nts of auxiliary vesicles and globular cilia rep or ted so far in ascidia n tadpole s are contras ted and discussed .

Ib Svane, Kristineberg Marine Biological Station, Kristineberg 2130. .'1-450 34 Fiske biicks kil, Sweden.

Introduction parent materi al surrounding a sma ll gra nule of melanin (Grave 1944; Whittaker 1966). In the larvae of Styela Ascidian larval morphology has attracte d the interest of coriacea, Polycarpa spp., Dendrodoa spp., and severa l many workers because of possible infer ences relevant to species of Molgula, a statocyst is present, but no ocellus chordate struc ture , function, and ph ylogeny (BerriIl 1948; (Grave 1926; Berri1l1931; Millar 1971). Finally, members Dilly 1964; Barnes 1971; Cavey & Cloney 1976). Ind eed , of the styelid subfa mily Botryllinae and tadpoles of the the classificati on of tun icat es as chordates is based almost solita ry stye lid Cnemidocarpa jinma rkiensis possess com­ entirely on larval structures (Be rrill 1955; Barrington pound senso ry structures known as phot oliths, which are 1965). Ascidian larvae ofte n carry complex adult struc- presum abl y sensitive to both light and grav ity (Berrill tures which become funct iona l only during rnetamor- 1975; Vorontosova & Malakhov 1984; Torrence & Clo ney phosis; likewise . specialized lar val structures such as 1988). Several species of Pyuridae in the genera Boltenia, oce lli and statocysts are ultim ately lost , leaving no trace Pyura and Halocynthia possess an additio nal struc ture of in the adult (Clo ney 1982). unknown function , the auxiliary vesicle (Svane 1982) or Solitar y ascidians gene ra lly have simple tadpo le larvae 'ne uro hypo physea l cavity' (Clo ney 1959), situated in the with undifferentiated adult tissues and organs. T hose lar- left wall of the cereb ral vesicle. It has been sugges te d val senso ry structures loca ted in the cereb ral vesicle of that this structure may be hom ologou s with the verte brate the trunk dem on strate substa ntial variab ility amo ng Iarnil- sacc us vasculosus (Svane 1982). A ltho ugh the structure ies and genera (reviewe d by Torren ce & Cloney 1988; has not been reported in any othe r ascidia n family, struc­ Svane & Young 1989) . In its most commo n for m, the tures resembling the ciliary compo ne nts of the pyurid cerebra l vesicle includes an oce llus for light reception and auxiliary vesicle have been reported in the stye lid Cnemi­ a statocyst for perception of gravity. Outside the sensory docarpa finmarkien s is. and in seve ra l phlebobranchiate vesicle , the major receptor s are probabl y mechanorecep- species with simple larvae (Voro ntosova & Malakhov tors and che mo rece pto rs located in the three ante rior 1984). papillae , and ciliary senso ry struc tures, possibly proprio- With respect to adult structure, the ascidian family ceptors, assoc iated with the tail epide rmis (To rre nce & Pyuridae contains th e most highl y developed species of Cloney 1982). A typical oce llus consists of three lens solita ry ascidians (Be rrill 1950) . Th e larvae of 11 pyurids cells, a nu mber of sensory ret inal cells, and a unicellul ar have been described previou sly (Tabl e 1). However , no pigment cup. Most larvae in the genus Stye/a , however, larvae in the genus Microcosmus (one of the four lar gest have a red uced ocellus consisting of a thin layer of tran s- gene ra in terms of species number) have been described . 129 130 I. Svane and C. M. Young

Table 1. Egg diameter, larval length, sensory structures, and occurrence of an auxiliary vesicle in described larvae of ascidians in the family Pyuridae. Sizes are in um. Egg diameters include the chorion except where specified. Tadpo le lengths are for the cellular portion of the animal only (i.e, excluding the caudal fin) except where indicated in footnotes. (+) Presen t; (-) not present ; ('1) not investigated

Egg Larval Oce llus! A uxiliar y Specie s dia meter len gth statocyte vesicle Authorfs)

Pyura microcosmus 235 1035 +1+ '1 Millar ( 1954) P. praeputialis 230 1100 +1+ '1 A nderson et al. ( 1976) P. pachydermatina 250 1000 +1+ oJ A nde rso n et al. ( 1976) P. squam ulosa 175 930t +1+ '1 Millar ( 1951) P. stolonifera 28U-31Xl 1040 +1+ '? G riffiths ( 1976) P. tessellata 160 780 +1+ + Sva ne ( 1982) P. haustor 220 1075 +1+ + Young (pe rs. obs.) ; Clon ey ( 1959) Boltenia echinata 180-200 975 +1+ + Svan e (1982. pers. obs. ) B. villosa 200 870 +1+ + Clon ey (1959. 1961); Young (pers. obs .) Halocynthia roretzi 360 1530:j: +1+ '? Harai (1941) H. aurantium 368 1357 +1+ '1 Young (pers. o bs.) H. pyriform is 260 +1+ '? Berrill (1929. 1935) , H. igaboja +1+ + Cloney ( 1959) Herdman ia mom us 280 1650 +1+ + Present study H. pal/ida 190' 1150 +1+ '1 Seb astian (1953) Microcosmu s exasperatus 250 1000 - 1+ + Present study

' Measurement exclusive of cho rion. t Measurement includes cauda l fin. :j: Details of measur em ent not spec ified .

In the genus Herdmania (a small genus with circumtrop­ pigment can be distinguished (Fig. I). Th e larval trunk is ical species) the development of onl y on e species, Herd­ about 250 IJ.m long and 125 IJ.m wide and the tail is about mania pallida (Sebastian 1953), has previously been 750 IJ.m long excluding the tail tunic. The larva is colour­ reported. In thi s paper, we describe the larvae of a species less and quite transparent. As in other tadpole larvae, in each of these two genera, emphasizing senso ry struc­ the test layer includes a wide vertical fin which runs along tures in the cerebral vesicle . [The systematic position of the dorsal and ventral side s of the trunk and tail. The fin the genus Herdmania is not clear. Monniot (1965) does contracts and is partially lost during fixation (Figs 2B , not recognize the genus and includes the group in the D). The fin extends about 100 IJ.m above and below the genus Pyura, but subsequent workers (Lowenstam & tail, and the caudal portion ext ends about 200 IJ.m beyond Abbott 1975; Lambert & Lambert 1987) have not fol­ the tail epidermis. The anterior end of the trunk has lowed this scheme.J three conical adhesive papillae arranged in a triangular configuration, two dorsally and one ventrally. The ventral portion of the trunk contains undifferen­ tiated tissue which will ultimately form the pharyngeal structures of the adult. Dorsally, the right side of the Materials and Methods trunk is occupied by the cerebral vesicle . A statocyte containing a pigmented statolith projects from the floor Adult Microcomus exasperatus Heller and Herdman ia momus (Savigny) of the cerebral vesicle (Fig . 2B). No ocellus is present. wer e collected o n mangrove roots. concrete bridge pilings. and othe r However, on th e dorsal left side of the cerebral vesicle , hard substrata ncar Jupiter Inlet. in the Indian River Lagoon of Eastern in the po sition where the ocellus would be located in a Florida . Lar vae wer e ob tai ned by artificial fertilization. Rip e gonads were rem oved by dissection fro m two ad ults of eac h species and macer­ typical tadpole larva, there is a larg e cell containing darkly ated th rou gh a 253 IJ. m Nitex mesh into filte red seawa ter. After the first staining basophilic vesicles (Figs 2A, B) . These vesicles cleavage was observed . the sea wate r was cha nged several times to are similar to those found in the cells of the undifferen­ rem ove excess sperm. Larvae were fixed in 2.5% gluta ralde hyde in 0.2 M Millonig's phosphate buffe r (pH 7.6) and post- fixed in 2% OS04 tiated ph arynx and may contain yolk materi al. Neither as described by To rre nce & Clon ey ( 1982). then embedded in Epo n len s cells nor retinal cells are present anywhere in the resin. Seria l sect ions wer e cut at 2 IJ.m thickn ess and sta ined in tolu idin e cerebral vesicle. One cell located in the ventral wall of blue for light microscopy. Ultrathin sect ions were cut with a diamo nd kni fe . stai ned with uranyl ace tate and lead citrate. and exa mined with the cerebral vesicle contains a large yolk vesicle , though a Ze iss EM 9 S-2 electro n microscop e . thi s vesicle is sometimes subdivided into two or three smaller on es. An auxiliary vesicle, about 10 IJ.m in diameter, is located on the left side of the dorsal half of the trunk Results (Fig. 2D). Thi s vesicle is lined with a uniform epithelium of cuboidal cells 6-8 IJ.m high. Modified cilia projecting from the apices of the cells contain globular membrane Larval m orphology of Microcosmus exasperatus profiles similar to those described previously for Boltenia echinata and Pyura tessel/ata (Svane 1982) (Fig. 2C). We Ext ernally , the tadpole larva of M . exasperatus resembles hav e found no connection between the auxiliary vesicle that of a typical solitary ascidian except that no ocellar and the cerebral vesicle in this species . Sensory Structures in Ascidian Tadpole Lar vae 131

Discussion

In ascidian tadpole lar vae , transien t struc tures that disap­ pear at met am orphosis or sho rtly thereafter (the 'larval action system' sensu G rave 1944; Scott 1946; Katz 1983) are gene ra lly thou ght to function in dispersal and site selection (Be rrill 1955) and probably have an evo lutio na ry development inde pe nde nt of th at of the ad ults. Thus, larval struc tures such as senso ry orga ns probably develop ed in respon se to selective pressures in the lar val stage . Interspecific differences in sensory vesicle structure could reflect different habitat requirem ents o r simply dif­ ferent so lutio ns to a common probl em of locating an appropriate adult habitat. The 's imple' typ e of ascidian larva is widespread amongst solita ry ascidians, traversing all of the conv en ­ tionally acce pte d taxonomic boundari es; it occurs in the .. Fig. I . Light-micrograph of a livc larv a of MicroCOSIIlIIS exasperatus phl ebobranchiate famili es Cionidae, Ascidiidae and showing the sta tocy tc hut no occllular pigment . st statocytc . Scale har Co rellidae as well as all three sto lido branchiate families, 250 urn. Pyuridae , Molgulidae and Styelid ae . The greates t modi ­ ficati ons of the basic ascidian lar val form are found in the co lonial ascidians, including all aplouso bra nchiate famil­ ies and the stye lid subfamily Botryllinae . A ltho ugh man y The tail of Microcosmus exasperatus is simi lar to those extre mes of tadpole mod ification appear to reflect het er­ of ot he r pyurid larvae ; the central not ochord and dorsal oc hronies in the development of adult structure, mod ifi­ ner ve cord are bo unded laterally by three ba nds of stri­ cat ion s of the larval actio n syste m, particularly those ated muscles. struc tures associated with the cerebral vesicle , are also commo n.

Larval morpholology of Herdm ani a momus Senso ry structures of the larvae

T he lar val trunk of Herdm ania momus is abo ut 450 J..Lm The greates t diversity of senso ry co nfigura tions is found lon g and 200 J..Lm in dorso-ventral width and th e tail in the famil y Styelid ae. All known lar vae in th is famil y (excluding the ca uda l fin) is 1200 J..Lm long. Except for its dem on strate some modification of the basic ascidian plan large size , th e tadpole lar va resembles larv ae of othe r (statocyst plu s oce llus). Som e have a reduced oce llus, solita ry ascidians. others have no ocellus at all , and still othe rs have bo th As in Microcosmus exasperatus, the ventra l portion of orga ns combined in a single senso ry struc ture , the phot o­ the trunk is occ upied by undiffer entiated ph ar yngeal tis­ lith. On e spec ies , Cnemidocarpa finmarkiensis, has bo th sue. Both a well-de veloped ocellus and a sta to lith are a phot olith and an ocellus (Vorontosova & Malakhov present in the cerebral vesicle, which occ upies the right 1984). By co ntrast, all pyurid lar vae described unt il now , side of the trunk's dorsal half (Figs 3A, B). including those of Herdmania momus, have bo th an ocel­ An auxiliary vesicle, located in the upper left half of lus and a sta tocy st (Ta ble 1).The larvae of Microcosmus the trunk, co mm unica tes with the ce rebral vesicle throu gh exasperatus, described here, are the first pyurid larvae a narrow opening slightly ante rior to the position of th e known to have a modi fied ocellar structure. with the statocyte . The auxiliary vesicle is abo ut 20 J..Lm in diamet er exception of Herdmania pallida which has been reported an d lined with a uni form epithe lium co mposed of cuboi da l to possess two ocelli (Sebas tia n 1953). cells 18 J..Lm high . In additio n to the nucleu s, each of these The larvae of Microcosmus exasperatus have no oce llar cells contai ns a large qu antity of ro ugh endo plasma tic pigment, no lens cells. and no retinal ce lls. However, a reticulum and numerous mitochondria (Fig . 3D) . The single large , yolk-lade n ce ll positioned dorsally near the apical sur faces of the epithe lial cells are highly convo l­ place where an ocellus would normally be found (Fig. ute d , with membranes extendi ng into the lum en of th e 2A), may be the remnant of an oce llus. If so, it does not auxiliary vesicle (Fig s 3A , C, D). From these convo luted fun ction as a photoreceptor. Our obse rvatio ns on tadpo le mem br an es, two struc tures arise : simple cilia with axo n­ behaviour (Yo ung & Svan e in prep .) indicate th at larvae emes composed of a 9 + 0 arra nge me nt of microtubles, of this species are not sensitive to light. and modi fied cilia, globular in sha pe, which co nta in Torren ce & Cloney (1988) have described a possible ro unded membran e profiles similar to those see n in the seco nd ocellus in the lar vae of th e apleuso bra nch Diplo­ mod ified cilia of M. exasperatus (Figs 3D, E) . All modi ­ soma ma cdonaldi. This struc ture is composed of mod ified fied cilia we re contai ned in the auxiliary vesicle ; none cilia with exte nsively fold ed ciliary membran es that int er­ was found projecting fro m the posteri or wall of th e cer­ digitat e with microvilli. The lum en conta ining th ese stru c­ ebral vesicle o r embedded amo ng the surrounding cells tures is located ben eath th e epide rmis on the left side of in this region, as has been described for ascidians in othe r the trunk and is apparently not associated with the cere­ fami lies (Vo rontosova & Malakhov 1984). br al ves icle . The membran e arrangeme nt of these modi - 132 I. Svane and C. M . Young

-st B

•

D .. .,. aux

aux

Fig. 2. Sect ion s of the ce re hra l region of the larva of Microcosmus exaspcrutus.- 11. Large d rop let-ti lled ce ll in the dor sal wall (E M: sca le bar 2 f.Lm ).- B. Droplet -filled ce ll and the sta tocytc in cross-section (LM: sca le bar III f.Lm ).- C. Modified cilia (E M : sca le ba r 2 f.L 111) .- D . C~oss­ sec tion sho wing the auxiliary a nd ce reh ra l vesicle (LM: sca le bar 10 um). allX a uxiliary vesicle . ce cerebra l ves icle. dr lar ge d roplet-filled cell. me modi lied cilium. .1'1 stutocytc . fied cilia is similar to that present in the auxiliary vesicles salinity in the exte rna l enviro nme nt, thereb y indicatin g a of both species describ ed in this paper as well as the role in osmo regulation (vo n Mecklenburg 1973; Ema n­ globular cilia described by Dilly (1969) and Eakin & uelsson & von Mecklenburg 1974). Furthermor e , studies Kud a ( 1971). Torre nce & Cloney (1988) have not ed their of Em anu elsson & von Mecklenburg ( 1972) and Jansen similarity to the structure of the ciliary endings of the & Flight (1969) point towards a secretory function of the phot or eceptors in the ocellus of Diplosoma macdonaldi. saccus vasc ulosus and show that the sec retory product Al thou gh some autho rs (Dilly 1969; Reverberi 1979; contains a mucopolysaccharide. Torrence & Cloney 1988) have assumed th at these modi­ fied cilia are photor eceptors, othe rs have spec ulate d that they function as receptors of hydrostatic pressure (Eakin A uxiliary vesicles and modified cilia of the ascidian & Kud a 1971) or chemorece pto rs that detect molecul es larvae of low weight (O lsson 1975). Th e funct ion or functions of the modified cilia rem ain unknown . With the addition of the present dat a on Microcosmus In the rai nbo w trout, the saccus vasculosus, which con­ exaspe ratus and Herdmania momus , modified cilia are tains num erou s modified cilia similar to the on es known to be distributed in four different ways within the described in ascidian lar vae, is activated by an increased nervou s systems of ascidian larvae (Fig. 4). Th e larvae of Sensory Structures in Ascidian Tadp ole Larvae 133

r me I..

c

... \

~ ,: " . E

" st. \-

Fig. 3. Sections of the cerebra l region of the lar va of Here/mania m omus.s-i-A , Ce reb ral vesicle showing the sta tocy te and auxiliary vesicle with co nvo luted epithe lium (E M; sca le ba r 20 f.l m) .- B. Cross -sect ion showing the auxiliary and ce rebral vesicle (LM; sca le bar 20 f.lm ).- C. Modified cilium and convoluted epithe lium of the auxiliary vesicle (E M; sca le bar 2 f.l m).-D. Simple modi fied cilia and convoluted epithe lium of the auxiliary vesicle (E M; sca le bar 2 f.lm).-E. Single cilia (sc) with axonemes of 9 + 0 micro tubules in cross­ and longitudinal sec tions of the auxiliary vesicle (E M; sca le bar 2 urn ). Abb reviations as in Fig. 2.

cionids and ascidiids have modified cells ansmg from Cnemidoc arpa finmarkiensis (Vorontosova & Malakhov epithe lial cells on the poster ior wall of the cereb ral vesicle 1984), or associated with a vesicle that may or may not (Di lly 1969; Eakin & Kud a 1971; Reverberi 1979). In be connec ted by a channel to the sensory vesicle (Svane other ascidians, the mod ified cilia are located on the left 1982, present paper). In additio n to the modified cilia, side of the cereb ral vesicle, eithe r embe dded in tissue as in lar vae of H. momus have normal , yet appa rently non- 134 I. Svane and C. M . Young

o logy of development o f simple ascidians.- Philosophical Trans­ action s of the Royal Soc iety B218: 37-78. Berrill, N. J . 1931. Studies in tunicat e development II. Abbreviat ion of development in th e Molgulid ae .-Philosoph ical Transactions of the Royal Society B219: 28 1-3 46. Berrill , N. J . 1935. St ud ies in tun icat e develop ment. III. Differen tial retardati on and acceleratio n.- Philosop hical Transactions of the Royal So ciety B225: 255-379. Berrill, N. J . 1948. Th e nature of the asc idian tad po le . with refe ren ce to Boltenia echinata.s-Journal of Morph ology 82: 269-285. Berrill, N. J . 1950. The Tun icata. Ray Soc iety . Lo ndon . Berrill , N. J . 1955. The origin of the vertebrates. Clare ndon Press. Oxford. Berrill, N. J . 1975.C ho rdata: Tunicat a . In A . C. G iese & J . S. Pearse (eds): Reproduction of marine invertebrates , Vol. II. pp . 24 1-2R2. A cad em ic Press. London . Ca vey. M . J . & Cloncy , R . A . 1976. Ultras tructure and differen tiati on of ascidi an mu scle . I. Ca uda l mu scul ature o f th e lar va of Diplosoma ma cdona ldi.c-Cell and Tissue Research 174: 2R9-313. Cloney, R . A . 1959. Larv al morphology and met amorphosis o f Boltenia villosa and other ascid ians . Ph .D . diss ertati on. Unive rsity of Wash­ ington. D ~ Cloney, R . A . 1961. Observ ations o n the mech an ism o f ta il resorption in asc id ians.-Ame ricall Zoologist I : 67-R7 . Clone y, R . A. 1982. Ascidian la rvae and th e events o f met amorphosis.­ American Z oologist 22: 817-826. Dill y, P. N. 1964 . Studies o n th e receptors in th e ce re bra l ves icle o f the asc id ian tadpole . II. The oce llus .- Q IIlJrlerly Journul of Micro­ Fig. 4. Schem at ic illustra tion o f th e kn own morphology o f the auxiliary scopical Science 105: 13-20. vesicles a nd position s of modi fied cilia amo ng the d iffe re nt species of Dill y. P. N. 1969 . Studies on the receptor s in Ciona intestinalis. III. ascid ian lar vae.- A . Phl ebobr an chi at e families Cio nidae a nd A second type of ph ot oreceptor in the tadp ole larva of Ciona Ascidiidae .-B. Cnemido carpa finmarkie nsis (Styelida e) .- C. Micro­ intestinalis.e-Z eitschr i]t f ar Zellforschung W Il/ mik rosk opische Alla­ cosmus exaspcrutus s--D , Pyura tessellata and Boltcnia echinata.e-E, tomie 96 : 63-65. Herdmania momus. Eakin. R . M . & Kud a, A . 1971. Ultras truct ure o f se nsory recep tors in ascidian tadp olcs.-Z eilschrifl Fir Ze llfo rschung und mik rosko­ pische A natomic 112: 287-312. Em anuclsson , H . & Mecklenburg. C. vo n 1972. Mctabolic activity in motile cilia also in the auxi liary vesicle, and th e cell sacc us vasc ulosus of th e rain bow trout . Salmo gairdneri surfaces lining thi s typ e of vesicle are highl y convo luted . (Richardso n) .-Cel/ and Tissue Research 130: 35 1-361. Ern anuelsson , H . and Meckl enburg C. vo n 1974. Ex pe rime nta l and Our findings (Svane 1982, this paper) with larvae of struc tura l ana lysis of the function of sacc us vasc ulosus in ra inbow Pyur idae indicate that the morpho logy of sensory struc­ trout. Salmo gairdneri (Richa rdso n) .-Cel/ and Tissue Research 148: tures in th is family is less conservative than previo usly 27-44. Grave . C. 1926 . Molgula citrina (A lde r & Han cock ). Activities and suspected . Comparative wor k on be havioural differences structure o f the free-swimming larva.-lourtlal of Morph ology 42: amo ng species may shed additiona l light on the signifi­ 453-471. cance, if any, of the various configurations of senso ry Grave , C. 1944 . Th e la rva o f Styela (Cy nthia] partita, Structu re . activities and durati on o f life .- l ournal of Morph ology 75: 173- 191. structures in sto lidobranchia te ascidian larvae. Griffiths . R. J . 1976. 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