1242 



TUNICATA. 



cloacal cavities. The gemmiparous mode of 

 reproduction observed in the ClavellincE, and 

 ButryllidcB closely resembles that which ob- 

 tains in the Bryozoa. With regard to the de- 

 velopment of the Acephala, compared with 



Fig. 790. 



B, 



Ideal figure of 

 Bryozoon. 



a, entrance of the 

 sheath ; b, tentacles, hol- 

 low throughout their 

 length, equivalent to the 

 straight branchial ves- 

 sels; d, the sheath; c, 

 mouth, leading to the di- 

 gestive cavity ; g, anus ; 

 i, integuments ; k, peri- 

 intestinal cavity. 



subsisting between Ascidia and Salpa, and 

 knew the true analogy of their exterior co- 

 vering, proposed to distinguish them as a dis- 

 tinct group of Molluscs, under the term " soft- 

 shells," which more truly accords with their 

 real nature than "shell-less" (Acephales sans 

 coquilles), as they have been subsequently de- 

 signated by Cuvier*, who also, together with 

 Blainville, regarded them as acephalous Mol- 

 luscs, and sufficiently distinct to form a sepa- 

 rate class. 



In many of the Acephala we find two tubes 

 for the ingress and egress water, like those of 

 the Ascidice, and the mantle closed throughout 

 its length, the walls of its cavity lined with 

 the branchiae, and the mouth at the base of 

 this cavity. The Ascidia being immovable, 

 has neither adductor nor foot-muscles; for, 

 being destitute of a locomotive organ and of 

 valves, the muscles disappear, with the excep- 

 tion of some sphincter-like bands. Together 

 with the muscles, the pedal nerve-ganglions, 



or rather suboesophageal or posterior ganglions 



are lost, and there remain only the buccal or 



supra-cesophageal ganglions. The differences 



between the heart and auricles of the Asci- 

 dians and the other Molluscs are dependent 



on the modification of the branchiae ; the 



latter, in Ascidians, never being arranged in 



the four lamellae usually found in Molluscs. 



The digestive tube and the generative organs 



are essentially the same in Tunicata and Ace- 

 phala, and in the latter, as in the former, the 



ovary and testicle are enclosed in the intes- 

 tinal loops. 



Yet however much resemblance we may 



find between the Acephala and the Tunicata, 



there is still more between the latter and 



the Bryozoa. Van Beneden observes, that if 



a Bryozoon was confined to its cell, and still to 



carry on its respiratory function by its un- 



protruded tentacles, and if anastomosing com- 

 munications were established between the 



tentacles and a heart at their base, we should 



have the complete idea of an Ascidian. There 



would be two external orifices, a single 



mouth situated at the base of the branchial 



sac, vibratile cilia throughout the extent of 



the anastomosing tentacles, a folded intestinal 



canal, a supra-cesophageal ganglion, some 



muscular bands, and organs of generation 



developed around the digestive tube. There 



would be only wanting to complete the animal, 



a cloaca to receive the excrements, the sperm, 



and the ova, and a direct passage of commu- 

 nication between the respiratory and the 



* Catal. Hunterian Museum, vol. i. p. 260. note. 



f For Van Beneden's researches on the development of the Pedicellina, &c. see art. POLYPIFERA. 

 t The following tabular arrangement, given by Prof. Van Beneden in his clever exposition of the 

 alliances of the simple Ascidians (toe. cit. p. 58.) is a valuable illustration of the subject. 

 Hypocotyledones. 



A, Ideal figure of an 

 Asddia. (After Van 

 Beneden). 



a, a, the external ori- 

 fices, branchial and anal ; 

 6, straight branchial ves- 

 sels; c, transverse and 

 anastomosing branchial 

 vessels ; d, respiratory 

 cavity ; e, oesophagus ; /, 

 digestive cavity ; g, rec- 

 tum ; h, cloaca, commu- 

 nicating with branchial 

 cavity, rectum, and ge- 

 nerative organs ; i, inte- 

 gument; k, peri-intesti- 

 nal cavity. 



that of the Tunicata and Bryozoa, Van 

 Beneden points out the following analogies : 

 " There exist common characters between 

 the Bryozoa and the Anodonta in this' re- 

 spect ; their larvae respectively swim about 

 by means of vibratile cilia. They have a dif- 

 ferent form in the young and the adult states, 

 and they undergo true metamorphoses. The 

 same phenomena are seen in the Ascidians. 

 These also have metamorphoses, perhaps, 

 even more complete, but instead of vibratile 

 cilia, the embryo is provided with a caudal 

 appendage sufficiently long to serve for the 

 purposes of locomotion ; which tail disappears, 

 just as the vibratile cilia of the former, as soon 

 as the animal becomes definitely fixed." 



The embryogenic characters appear to en- 

 tirely accord with the anatomical dispositions 

 of these groups ; and altogether we are, per- 

 haps, justified in considering, that they ap- 

 proach on one side to the Acephala, and, 

 through the Pedicellina \, they are, on the 

 other hand, allied to the Bryozoa, forming a 

 closely uniting link between these two impor- 

 tant groups. J 



ANIMALS - - 



Epicotyledones. 



Allocotyle'done's 



("Mollusques - - 



.Polypes - 



Echinodermes. 



/'Cephalopodes. 

 J Gasteropodes. 

 j Acephales. 

 ^Tuniciers 

 rBryozoaires. 

 J Medusaires. 

 j Anthozoaires. 

 (^Alyconaires. 



fSalpiens. 

 I Pyrosomiens. 

 I Ascidiens. 

 I Polyascidiens. 

 [_Pe'rophoriens. 



