The angles of the meshes of the branchial tissue bear papillae, more or less prominent, in some genera (Ascidia, Chelyosoma, &c.) ; but in others they are not papillated (Cynthia, &c.). These papillm, or minute pouches, are, according to Savigny, analogous to the fila ments that border the branchial vein, in most of the simple and compound Ascidians, indi cating the junction of the transverse vessels with that vein. The appearance of the reticu lation is large and coarse in Cynthia, minute in Ascidia, and indistinct in Cystingia.
Each mesh of this respiratory network is thickly fringed with vibratile cilia, as in the rest of the Tunicates and Acephalans ; and Van Beneden points out that this ciliated ex terior of the branchial vessels is analogous to the ciliated tentacles of the Bryozoa. By the action of the cilia the currents are sent in the direction of the cesophageal aperture.
In papillosa and Cynthia mierocosnzus, 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. maninzillaris 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 C helyosoma (fig. 777. i) there is a remark 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 microcosms, &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 Carus*, 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 Clavellinidce and Botryllidce the open meshes of the respiratory network t, 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 netlike 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 Bone 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 referred to, by producing, and being traversed by, the ciliary currents that bring the food to the oesophageal aperture ; and that the ten tacula of Bryozoa are essentially members of the alimentary apparatus is shown by the fact, that animalcules, &c. are frequently caught and detained by the action of the tentacles, one or more of which, and sometimes even the whole, bend suddenly inwards, and secure such particles as come within their reach, thus taking the character of prehensile labial or oral appendages.* The cavity of the branchial sac is often the habitation of parasitical Entomostraca.