TUNICATA. 



1203 



action of the cilia the currents are sent in the 

 direction of the oesophageal aperture. 



In Ascidia papillosa and Cynthia microcosmus, 

 a little soft tubercle is situated on the interior 

 of the branchial sac, not far from the orifice, 

 between the two branchial veins, not observed 

 in other Ascidians, but seen in all the Botryl- 

 lians. On the internal surface of the sac are 

 also seen several prominent lines, 12-15 in A. 

 mammillaris and A, papillaris, 5-6 in C. am- 

 pulla, more or less distant from one another 

 according to their length. These are folds 

 forming gutters converging towards the mouth, 

 and having also transverse channels leading 

 from one to another. Above they appear to 

 terminate in culs-de-sac. The vibratile cilia 

 covering them are very long and numerous. 



Van Beneden found that, on placing for a 

 short time a living Ascidian in water coloured 

 with carmine, the particles of the colouring 

 matter quickly filled these little ducts, making 

 them appear like injected vessels. This dis- 

 tinguished naturalist considered them to be 

 somehow connected with the digestive appa- 

 ratus, moulding, perhaps, the particles of food 

 into cord-like masses before entering the 

 stomach. 



In Chelyosoma (Jig. 111. i) there is a remark- 



Fig. 111. 



Anatomy of Chelyosoma Alacleayanum. (After 

 Eschricht.') 



The inner or under side of the superior plated 

 surface of the animal is shown, the branchial cavity 

 being split open, and the abdominal viscera re- 

 moved, a, branchial orifice, partly closed by a 

 membrane, and surrounded by its hexagonal 

 sphincter muscle and the accompanying six fan- 

 shaped muscles ; b, anal orifice, similarly provided 

 with muscles ; c, c, muscles bordering the carapace- 

 plates ; </, the central hexagonal plate ; e, e, e, e, the 

 surrounding plates; /, the nerve-ganglion and 

 nerve-fibres ; g, h, the auditory apparatus ( ?) . i, the 

 row of tentacles anterior to the oesophagus; j, 

 stomach; k, part of the intestine. 



ably large row of simple tentacles running 

 from the interior of the branchial orifice to 

 the entrance of the oesophagus, and closely 

 connected with the branchial tissue, as in 

 Cynthia microcosmus, &c. They appear to 

 surmount a longitudinal vessel or duct, pos- 

 sibly the branchial vein ; they are about 23 

 in number, and have a transverse direction 

 towards the left side. The posterior tenta- 

 cles are the largest, of about a similar size to 

 the tentacles of the branchial orifice ; the 

 others, towards the anterior extremity of the 

 row, become gradually smaller. Their use is 

 not very evident ; possibly they are connected 

 with the organs of digestion ; but, more pro- 

 bably, they are auxiliary respiratory organs, 

 like the circlet of tentacles within the branchial 

 tube. 



The respiratory cavity, in addition to 

 its external or oral orifice and its oesopha- 

 geal aperture, presents also, in some cases at 

 least, a lateral opening, first noticed by 

 Cams*, and since by Van Beneden. The 

 presence of this communication allows the 

 water, received into the branchial sac for 

 respiration and the conveyance of food, to 

 pass directly out by the anal aperture. Other- 

 wise, when no such communication exists, 

 the water must be ejected through the oral 

 aperture by muscular contraction, as in the 

 Acephala. 



In ClaveUinidfS and Botryllidce the open 

 meshes of the respiratory network f, or bran- 

 chial stigmata, also allow of the free passage 

 of water from the respiratory cavity to the 

 cloaca. In Pyrosoma, Pelonaia, and Salpa, 

 the disposition of parts admits of the free pas- 

 sage for the water from the one external 

 orifice to the other. 



Externally the branchial membrane presents 

 very similar appearances to its net-like interior 

 surface : where folded, however, of course the 

 folds and sulci are reversed. It is attached 

 more or less firmly to the inner surface of the 

 mantle on the one side, and on the other to the 

 intestines, stomach, and ovaries, by transverse, 

 short, delicate, perhaps vascular, threads, one 

 of which proceeds from each angle of the 

 meshes. 



" In the young Ascidian," says Carus, " the 

 respiratory sac can be distinctly recognised as 

 an integral part of the intestinal canal. As 

 the body increases, this originally crop-like 

 dilatation gradually attains a more consider- 

 able extent, and differs in structure from the 

 intestinal canal in having exceedingly delicate 

 and transparent parietes ; in a word, diverg- 

 ing more and more from the intestine." 



We may further observe, that the vertical 

 vessels entering into the composition of the 

 framework of this branchiferous pharyngeal 

 sac, and representing, according to Van Bene- 

 den, the tentacula of the Bryozoa, are not 

 only analogous to the latter in their respira- 

 tory function, but are subservient also to the 

 purposes of alimentation, like the tentacles 



* Meckel's Archiv f. Physiologic, B. ii. 1. 4. 

 | M. Coste (Comptes Rendus, vol. xiv. p. 182. 

 1844) denies the existence of open stigmata. 



4 H 2 



