TUNICATA 



193 



Buds formed on stolons which are vascular outgrowths from the pos- 

 terior end of the body, containing prolongations from the ectoderm, 

 mesoderm, and endoderm of the ascidiozooid. Branchial sac not 

 folded; internal longitudinal bars usually absent; stigmata straight; 

 tentacles simple. This family contains three genera : Ecteinascidia 

 (Herdman), with internal longitudinal bars in branchial sac ; Clavel- 

 ina (Savigny), with intestine extending behind branchial sac ; 

 and Perqphora (Wiegmann), with intestine alongside branchial sac. 



Family II. — AsciDiir^. Solitary fixed Ascidians with gelatinous 

 test ; branchial aperture usually eight-lobed, atrial aperture usually 

 six-lobed. Branchial sac not folded; internal longitudinal bars 

 usually present ; stigmata straight or curved ; tentacles simple. 

 This family is divided into three sections : — 



Sub-family 1. — Hypobythin^. Branchial sac with no internal 

 longitudinal bars. One genus, Eypobythius (Moseley). 



Sub-family 2.— AsoiDiNiE. Stigmata straight. Many genera, of 

 which the following are the more important : — Oiona (Fleming), 

 dorsal languets present ; Asddia (Linnseus, =PhaUiisia, Savigny), 

 dorsal lamina present (see figs. 1 to 10) ; Ehodosoma (Ehrenberg), 

 anterior part of test modified to form operculum ; Abyssasddia 

 (Herdman), intestine on right side of branchial sac. 



Sub-family 3. — Corbllin^. Stigmata curved. Three genera : — 

 Corella (Alder and Hancock), test gelatinous, body sessile ; Coryn- 

 ascidia (Herdman), test gelatinous, body pedunculated; Chelyo- 

 soma (Brod. and Sow.), test modified into homy plates. 



Family III. — Cynthiid^. Solitary fixed Ascidians, usually with 

 leathery test ; branchial and atrial apertures usually both four-lobed. 

 Branchial sac longitudinally folded ; stigmata straight ; tentacles 

 simple or compound. This family is divided into three sections : — 



Sub-family 1. — Styblinje, not more than four folds on each side 

 of branchial sac ; tentacles simple. The more important genera are 

 — Styela (Macleay), stigmata normal, and Bathyoneus (Herdman), 

 stigmata absent or modified. 



Sub-family 2. — Cynthin^, more than eight folds in branchial 

 sac; tentacles compound; 



body sessile. The chief " — — - _ ''' 

 genus is Cynthia (Sa- 

 vigny), with a large 

 number of species. 



Sub -family 3.— BoL- 

 TENlNiE, more than eight 

 folds in branchial sac ; 

 tentacles compound ; 

 body pedunculated (fig. 

 18, A). The chief genera 

 are — BoUenia (Savigny), 

 branchial aperture four- 

 lobed, stigmata normal ; 

 and Culeolus (Herd- Fia. 

 man), branchial aper- 

 ture with less than four 

 lobes, stigmata absent or 

 modified (fig. 18, B). 

 This last is a deep-sea 

 genus discovered by the "Challenger" expedition (see it). 



Family IV. — MoLGULlDa;. Solitary Ascidians, sometimes not 

 fixed ; branchial aperture six-lobed, atrial four-lobed. Test usually 

 incrusted with sand. Branchial sac longitudinally folded ; stigmata 

 more or less curved, usually arranged in spirals; tentacles compound. 

 The chief genera are — Molgula (Forbes), with distinct folds in the 

 branchial sac, and Sugyra (Aid. and Hanc), with no distinct folds, 

 but merely broad internal longitudinal bars in the branchial sac. 

 In some of the Molgulidse (genus Anurella, Lacaze-Duthiers, 2o) 

 the embryo does not become converted into a tailed larva, the 

 development being direct, without metamorphosis. The embryo 

 ' when hatched assumes gradually the adult structure, and never 



shows the features characteristic of larval Ascidians, such as the 

 urochord and the median sense-organs. 



Sub-order 2. — Ascidise Compositse. 



Com- Fixed Ascidians which reproduce by gemmation, so as to form 



pound colonies in which the asoidiozooids are buriediin a common invest- 



Asoid- ing mass and have no separate tests. This is probably a somewhat 



ians. artificial assemblage formed of two or three groups of Ascidians 



which produce colonies in which the asoidiozooids are so intimately 



united that they possess a common test or investing mass. This 



is the only character which distinguishes them from the Clavelmidie, 



but the property of reproducing by gemmation separates them 



from the rest of the Ascidix SimpUces. The Asddiss Composite 



may be divided into the following families :— 



Family I. DiSTOMiDa:. Asoidiozooids divided into two regions, 



thorax and' abdomen ; testes numerous ; vas deferens not spirally 

 coiled. The chief genera axR—Distoma (Gaertner) ; Distaplia (Delia 

 Valle) ; CdldU (Herdman), forming a pedunculated colony (see fig. 

 19 A) in which the asoidiozooids develop incubatory pouches, 

 connected ^vith the peribranchial cavity, in which the embryos 

 undergo their development {ij) ; and Chondrostachys (Macdonald). 



hyf i I 

 18. — Culeolus willemcesl. A. Entire body, 

 natural size. B. Part of brancliial sac mag- 

 nified, at, atrial aperture ; &r, branchial aper- 

 ture ; ped, peduncle ; br f, slight fold of branch- 

 ial sac ; i I, internal longitudinal bar ; mft, mesh ; 

 sp, calcareous spicules in vessels ; tr, transverse 

 vessels. (After Herdman, Challenger iJepori.) 



Family II. — CffiLOCOKMiDiB. Colony not fixed, having a large axial 

 cavity with a terminal aperture. Branchial apertures five-lobed. 

 This includes one species, Oosloeormus ImxUyi (Herdman), which is a 

 transition form between the ordinary Compound Ascidians [e.g., 

 Distomidas) and the Ascidiss Salpiformes [Pyrosoma). 



Family III. — DiDEMNiDiE. Colony usually thin and incrusting 

 Test containing stel- 

 late calcareous spi- i^iiiMiik f' \ _ /^ii,^ 

 culefs. Testis single, 

 large ; vas deferens 

 spirally coiled. The 

 chief genera are — Di- 

 demnum (Savigny), 

 in which the colony 

 is thick and fleshy -^^ ^ _ 



and there are only j^io. 19.— Colonies of Ascidiee Composita; (natural size), 

 three rows of stig- A. Colella quoyi. B. Leptoclinum'n^glectvm. C PTia- 

 mata on each side of ryngodiciym mirabile. D. Botryllus, showing ar- 

 +110 T^T-oTinTiiol c-r^/i . rangement of asoidiozooids in circular systems each 

 tne orancniai sac, of which has a central common cloaca. (After Herd- 

 and Leptoehnum man. Challenger Beport.) 

 (Milne-Edwards), in 



which the colony is thin and incrusting (fig. 19, B) and there are 

 four rows of stigmata on each side of the branchial sac. 



Family IV.^DiPLOSOMiDiB. Test reduced in amount, rarely con- 

 taining spicules. Vas deferens not spirally coiled. In Diplosoma 

 (Macdonald), the most important genus, the larva is gemmiparous. 



Family V. — PoLYCLiNiDiE. Asoidiozooids divided into three 

 regions, — thorax, abdomen, and post-abdomen. Testes numerous ; 

 vas deferens not spirally coiled. The chief genera are — Pharyngo- 

 dictyon (Herdman), with stigmata absent or modified, containing 

 one species, Ph. mirabile (fig. 19, C), the only Compound Ascidian 

 known from a depth of 1000 fathoms ; PolycUnum (Savigny), with 

 a smooth-walled stomach ; Aplidium (Savigny), with the stomach 

 wall longitudinally folded ; and Amaroucium (Milne-Edwards), in 

 which the ascidiozooid has a long post-abdomen and a large atrial 

 languet. 



Family VI. — BoTEYLLlDiE. Ascidiozooids having the intestine 

 and reproductive organs alongside the branchial sac. Dorsal lamina 

 present ; internal longitudinal bars present in branchial sac. The 

 chief genera are — Botryllus (Gaertn. and Pall. ), with simple stellate 

 systems (fig. 19, D), and Botrylloides (Milne - Edwards), with 

 elongated or ramified systems. 



Family VII. — PoLYSTYELiDiE. Ascidiozooids not grouped in 

 systems. Branchial and atrial apertures four-lobed. Branchial 

 sac may be folded ; internal longitudinal bars present. The chief 

 genera are — Thylacium (Carus), with ascidiozooids projecting above 

 general surface of colony ; Goodsiria (Cun- 

 ningham), with ascidiozooids completely 

 imbedded in investing mass ; and Chorizo- 

 cormiis (Herdman), with ascidiozooids 

 united in little groups which are connected 

 by stolons. The last genus contains one 

 species, Ch. reticulatns, a transition form 

 between the other Polystyelidse and the 

 StyeUnsB amongst Simple Ascidians. 



The methods of reproduction by gemma- 

 tion difi'er in their details in the various 

 groups of Compound Ascidians ; but in all 

 cases the process is essentially a giving off 

 from the parent body of groups of cells re- 

 presenting the ectoderm, the mesoderm, 

 and the endodei-m, which develop into the 

 corresponding layers of the bud. The first 

 ascidiozooid of the colony produced by the 

 tailed larva does not form sexual repro- 

 ductive organs, but reproduces by gemma- 

 tion so as to make a colony. Thus there 

 is alternation of generations in the life- 

 history. In the most completely formed 

 colonies (e.g., Botryllus) the ascidiozooids 

 are arranged in groups (systems or cceno- 

 bii), and in each system are placed with 

 their atrial apertures towards one another, 

 and all communicating with a common 

 cloacal cavity which opens to the exterior 

 in the centre of the system (fig. 19 D). 



Sub-order 3. — ABcidise Salpiformes. 



Free-swimming pelagic colonies having ^'^p^tl^W'C ^ Ascidia 



the form of a hollow cylinder closed at one ^' 1" Salpi- 



end. The ascidiozooids forming the colony j.,a. 20.-Pyrosoma elegans,fo^">^- 

 are imbedded in the common test in such a natural size. A. Side view 

 manner that the branchial apertures open of entire colony. B. Bnd 

 on the outer surface and theatrial apertures (Sinal.)"^™ extremity, 

 on the inner surface next to the central 



cavity of the colony. The ascidiozooids are produced by gemmation 

 from a rudimentary larva (the cyathozooid) developed sexually. 



2B 



Repro- 

 duction 

 by gem- 

 mation 



