Genus Polyclinunz, Savigny.—Common body gelatinous or cartilaginous, polymorphous, sessile or slightly pedunculate ; systems nu merous, convex, somewhat stellate, with cen tral cloacal cavities ; individuals (10 to 150) placed at very unequal distances from their common centre ; the cavity in the common tegument occupied by each animal is divided into three chambers one above another by contractions of its calibre ; abdomen much smaller than the thorax ; post-abdomen pe duncidate.
Genus Aplidiunz, Savigny ; synonym, Al eyoniunz, Auct. — Common body gelatinous or cartilaginous, sessile ; systems very nume roux, slightly prominent, annular or tical, without central cavities; the animals (3 to 25) are placed in a single row, at equal distances from their common centre ; branchial orifice six-rayed ; division of the thorax and abdomen not always distinctly marked.
Genus Sidnyum, Savigny. — Common body gelatinous, presenting a series of closely grouped cones, truncated and starred at the summit,rising from a common incrusting base ; each cone composed of a fascicle of indi viduals, varying in number from five or six to ten or twelve, and forming a margin around a depressed centre. The animals partake of the characters of those of Synoicum and Aplidium, resembling the former in the struc ture of their stomach, and the latter in their branchial sac. Each has an eight-toothed branchial orifice and a simple tubular vent folded against the thorax. The ovary is pe duncled, and very conspicuous at the ex tremity of the animal. (Forbes.) Genus Synoicum, Phipps.*—Common body semicartilaginous, cylindrical, peduncled, iso lated or attached to similar cylindrical bodies; system single, circular, comprising six to nine animals, terminal ; branchial orifice six rayed; anal orifice having six very unequal rays, the three largest forming the exterior margin of the central star ; post-abdomen sessile.
Genus Amaroucium, Milne-Edwards.—Com mon body fleshy or coriaceous, polymorphous, subpedunculate or sessile, and incrusting ; systems numerous ; individuals arranged irre gularly around common cloaca] apertures ; divisions of the thorax and abdomina faintly marked. This genus resembles Aplidium and Synoicum in the general form of the animal, and Polyclinum in the presence of a central common cavity to each system (fig. 782.).
The tribe Didemnina (les Didemniens, Milne Edwards) is formed of all the compound Ascidians whose body is distinctly divided into two parts, thorax and abdomen. These closely approach the Mod/ince, and are dis tinguished from the Polyclinina by the absence of the post-abdomen and by the position of the organs of generation and the heart, which ,are raised up by the side of the intestine. This tribe, like the Polyclinina, is divided into two groups, according to the presence or ab sence of marginal rays around the anal orifice. M. Milne-Edwards observes that the bistellate group (Distomus and Dianna) forms a con „ netting link between the Clavellinw and the Bohyllidcc.
Genus Distomus, Gaertner+ ; synonyms, Alcyonium, Auct., Polyzona, Com _ mon, body semi-cartilaginous, polymorphous, sessile ; systems numerous, usually circular ; individuals placed in one or two ranks at un equal distances from their common centre. Both orifices six-rayed.
Genus Dianna, Savigny. — Common body gelatinous, orbicular, sessile or subpedunculate ; system single, expanded into a disc, compara ble to that of a flower or of an Actinia; animals very prominent, arranged in con centric circles (fig. 770.) ; branchial and anal orifices six-rayed ; abdomen peduncled ; ovary enclosed in the intestinal loop.
Genus Didemnum, Savigny.— Common body coriaceous, polymorphous, sessile and incrust ing; systems numerous, compressed, without central cavities or distinct circumscription ; animals without any appreciable order of arrangement; abdomen peduncled ; ovary placed by the side of the intestinal loop, in creasing in length when the eggs are fully developed.
Genus Eucalium, Savigny. Common body gelatinous, sessile and incrusting ; systems numerous, without central cavities or distinct circumscription ; animals sometimes scat tered, sometimes arranged in a quincunx ; branchial orifice circular, without distinct rays ; anal orifice very minute and indistinct ; abdominal viscera pushed up by the side of the thorax.
Genus Leptoclinuni, Milne-Edwards.—Com mon body sometimes coriaceous, sometimes gelatinous, thin, sessile and incrusting ; sys tems few ; individuals arranged irregularly around common cloacal cavities ; abdomen peduncled, short, smaller than the thorax.