The branchice (fig. 768. e) exist as a large membranous band, arising from the dorsal sur face of the thoracic cavity, below the gangliform tubercle, and by its opposite extremity fixed to the space situated between the oesophageal ori fice and the termination of the intestine, thus separating the cloacal from the great pharyn geal or respiratory cavity ; only, in place of presenting on each side simple striae furnished with vibratile cilia, as in the Salpians, this kind of vertical body bears right and left a series of filiform appendages, directed hori zontally towards the ventral side of the respi ratory cavity, where they are fixed on each side of the middle sulcus, and, during their passage across, are united together by a number of other slender vertical filaments. From this disposition of the parts there results a kind of trellis-work, which fills up all the pharyngeal portion of the branchial chamber, permitting no communication between the latter and the cloaca, except through the meshes of its net work, which are bordered all around with vibratile cilia. This complicated branchial apparatus adheres also to the thoracic tunic by its two extremities. The dorsal column, which thus forms the base of the branchial organ, and which represents the simple bran chia of the Salpians, is considerably prominent in the interior of the respiratory cavity, and exhibits along its ventral margin a series of ten membranous languets, which are usually straight, and are apparently susceptible of a kind of erection. The interior of this mem branous band is occupied by a large vascular cavity, which M. Milne Edwards terms the bronchial or dorsal sinus. The transverse little fillets which spring therefrom are in number about twelve pairs, and the vertical filaments that unite the latter are nearly of the same size : these are about thirty in a rank, and there are thirteen ranks, viz. eleven situated between the two transverse fillets, and two which stretch from the first and second fillets to the wall of the thoracic chamber, and are fixed along the two yellow lines, placed as rings at the two extremities of that chamber. Some membranous processes appear also to stretch from different points of the surface of the branchial net-work to the walls of the cavity in which the latter is suspended ; but these are few, and do not hinder the eggs, deposited in the cloaca, from insinuating themselves frequently into that portion of the cavity situated on each side of the respiratory sac. The spaces circumscribed by the narrow elongated meshes, supported by the vertical filaments, and bordered with vibratile cilia, are not occupied by a membrane, but form openings, somewhat like a button-hole, called bronchial stigmata. These chinks or spiracles, through which the water passes from the branchial sac in the thoracic chamber to es cape outwards by the anal orifice, are conse quently disposed in the same manner as the vertical filaments that circumscribe them, that is, parallel to each other and in transverse ranks, thirteen in number. Lastly, each of these vertical filaments is perforated by a canal, opening by its two extremities in other similar but larger canals, that occupy the in terior of the transverse fillets. The latter vessels in their turn empty themselves by one of their extremities into the great dorsal or branchial sinus, and by the opposite extremity into the vertical fold of the ventral wall of the thoracic chambers, which is circumscribed by two parallel, vertical, yellow lines, and which places the respiratory organs in relation with the great thoracic sinus, lying between this chamber and the ventral portion of the internal tunic of the body. These two sinuses communicate also one with another by the vessels that surround the branchial orifice, and from which spring other vessels that de scend towards the abdomen.
The pharyngeal portion of the thoracic chamber, tapestried with the branchial net work, opens externally at its upper extremity, by the branchial aperture, which occupies nearly the whole of its diameter, and is furnished with a kind of radiating net-work of tentacular filaments. This cavity is cylindrical, and at its inferior extremity has a large trans verse slit, the opening of the cesophagus, oppo site to the external oral aperture. The ceso phagus is large, and descends vertically into the abdomen terminating in the stomach (fig. 7683), which is ovoid and swollen. The intestine(m) springs from the inferior extremity of the stomach, and at first running vertically down wards, and then bending forwards and upwards, so as to form a loop, it ascends towards the thorax by the side of the stomach and oesophagus, slightly covering them on the right side. Arrived at the superior part of the abdomen, it is again bent, passes by the side of the cesophagus, ascends rather behind the branchial sac, and terminates at the inferior part of the cloaca (n), at about the level of the antepenultimate row of the branchial stigmata. Throughout its length the intestine preserves nearly an equal calibre, but varies in its ap pearance, and is divisible into three portions.
The first part, the duodenum, succeeding to the stomach and forming the intestinal loop, is colourless and transparent. The next por
tion, placed on a level with the stomach, but on the ventral side of the body, is on the contrary of a dull-yellow colour, and the tissue of its walls has a glandular appearance. This M. Milne-Edwards considers to be the hepatic portion of the alimentary canal, and comparable to the organ known in insects as the chyliflc ventricle. Lastly, in its third portion, the intestine again becomes mem branous and colourless : here the fecal matter is collected into brownish pellets, and from its functions and position this portion of the tube is termed the great intestine, or rectum. To the right of the intestinal loop is placed a glandular mass, the chief part of which is formed by the ovary, recognised by its vesicles, and by the eggs of different degrees of development contained in it. Below the ovary is the testicle, a mass of whitish and ramifying fila ments, spread out in some degree on the intestine. From this glandular mass arises a milk-white filiform canal, which ascends be tween the stomach and the intestine, passes on the left side of the cesophagus, and opens into the cloaca, near the orifice of the rectum. This canal contains a silvery white fluid, crowded with spermatozoa, and must be con sidered as a was deferens. There being no other visible duct leading from the ovary, this canal perhaps serves also as an oviduct. The eggs are minute, circular, and of a greenish yellow colour.
The heart is situated at the inferior part of the abdomen, lying against the intestine and the ovary, to the right and in front of the former, at the posterior part of the cavity of the mantle, between the tunic and the intestine, and much resembles that of Ascidia intestinalis. It is enclosed in a membranous sac or pericardium, and is cylindrical in form ; its superior extremity is on a level with the centre of the stomach, and its lower ex tremity, turned up a little behind, passes some times a little beyond the intestinal loop.
The Perophora, first described by M. Lister in Phil. Transact. 1834, p. 378., as a " small Ascidia," is parasitic on Conferviw, Sze., and appears to the eye like minute lumps of pellucid jelly, with a spot of orange and grey. The group consists of several individuals, each having its own circulatory, respiratory, and digestive systems, but fixed on a pedicle that branches from a common creeping stem, and all are connected by a circulation that extends throughout. They are very trans parent, and their interior is easily seen. The two orifices are very short tubes ; the bran chial is at the summit, and the anal is a little lower down. The longest diameter from the peduncle to the space between the openings is about •085 inch (see.fig. 306. A and a, p. 623. Vol. I. Citict). The test is subcartilaginous and tough, more pliable near the orifices. It is lined internally with the soft mantle, in which a ramifying circulation is very distinct. A great part of the interior is occupied by the branchial sac, which is subcylindrical, flattened at the sides, and has its axis vertical. It is united to the mantle above and behind ; the juncture, beginning in front of the oral open ing, extends backwards on each side of it, and then downwards in two lines ; between these, along the middle of the back, is a vertical compound stripe. At the bottom, the branchial sac appears to be in contact with the mantle, but at its sides and front a vacant space is left between them, communicating with the anal orifice (see fig. 306.s, f, loo. cit.). The sac is more compressed towards its lower part ; and here are placed, externally to it, the heart on the left, and the stomach and the other viscera on the right side, the anus opening upwards into the cloaca. On its sides and front the sac is perforated by four rows of narrow, vertical, irregularly oval holes or spiracles, about sixteen in each row, placed at less than the diameter of one of them apart from each other. Through these the water, which flows constantly in at the mouth when its orifice is open, is apparently conveyed to the vacant space between the sac and the mantle, and it then escapes by the anal orifice. The branchial tissue is extremely thin between the stigmata, but their edges are thickened, and are lined with closely set cilia, which by their motion cause the current of water. When these are in full activity, the effect upon the eye is that of delicately toothed, oval wheels, revolving continually in a direction ascending on the right and descending on the left of each oval, as viewed from without ; but the cilia themselves are very much closer than the apparent teeth, and the illusion seems to be caused by a fanning motion made by them in regular and quick succession, producing the appearance of waves, and each wave an swering to the apparent tooth. The spaces between the rows of spiracles are of much more substance than the intervals between the spiracles in a row ; filamentous processes are stretched from them across the side cavi ties, attaching the branchial sac to the mantle : these spaces also support finger-like processes, about eight in a row, that project nearly at right angles into the central cavity.