ASCID IA, a Linnwan genus of marine mollusca, now much restricted as a genus, but the type of a family called aseidiadce. The name ascidians is also commonly employed to designate all those tuuicated mollusea which form the order sambranchiata of Owen, or in which respiration is carried on by means of gill-sacs (branellial sacs); and these a 04 divided into compound and solitary ascidians (aggregata and solitaria). The ascidians, along with the other tunicata, are acephalous, or destitute of a head, and are inclosed, not in a shell, hut in an elastic tunic with two orifices, composed of a substance appar ently identical with the cellulose of plants, consisting only of carbon and hydrogen. Within the external tunic is a muscular membrane, regarded as corresponding to the mantle of other mollusca. and the openings of which agree with those of the tunic. The greater part of the cavity of the mantle forms a brunt:Ida] sac, the lining of which, folded in various ways, constitutes tla, gills (branclii6c); and into it currents of sea-water are continually brought by the respiratory movements, passing out through the vent or anal orifice. Multitudinous cilia in the mouth and bronchial sac, cause by their action this continual flow of water. The motion of the cilia is apparently quite involuntary. By this flow of water, the particles of food requisite for the animal are brought in, so that the aeration of the blood and the supply of the stomach are carried on together and by the same means. The cesophagus or gullet opens from the bronchial sac, is indeed regarded as probably an expansion of the upper part of it—a dilated pharynx. Under the bronchial sac is the stomach; and the alimentary canal, which is more or less tortuous, finally returns upon itself, so that the two orifices are not far separate. The liver consists of follicles produced into tubes, and communicating with the stomach by a single opening. There is a heart and a circulation of blood, with the remarkable_ peculiarity of alternations in its course, the circulation every now and then pausing and being reversed. The transparency of many of Alm ascidians permits these and other internal I ..ovements to be easily observed. the nervous system is very simple, consisting of a single ganglion, situated between the mouth and the anal orifice, and which sends out filaments to both of them, and other branches over the surface of the mantle. The mantle is capable of contracting suddenly to eject a jet of water, and along with it any body the presence of which is disagreeable. It also contracts and ejects water, if the animal is touched, and this appears to be the only means of defense which these crea tures possess. There is no trace of eyes or of other organs of special sense.
The ascidians are found in all seas, and often constitute an important part of the focrd of fishes. Some of them are occasionally used as human food, as cynthia microcosmu3 on the shores of the Mediterranean. Many of them are very small, but some attain a size of 5 or C in. in diameter, and when touched, eject water to a considerable height, the largest of them to about 3 ft. They are all fixed by the base, in their
state, to some solid substance, as a rock or seaweed; sometimes by the ititervent,ion of a stalk or peduncle. In some kinds (social ascidians), the pecLuieles of a number of indi viduals are connected by a tubular stem, and to some extent they have a common circulation of blond, although each has its own heart, respiratory apparatus, stud digestivo system; and if a ligature is drawn around the peduncle of one so as to cat it off from the co/ninon circulation, circulation takes place in it as in a solitary aseidian. In other kinds (more strictly called compound designation, however, is by some authors applied to those just described, whilst these are called aggregate ascidians), the tunics of many are united into a mass, and they form systems like zoophytes. The compound system sometimes bears a general resemblance to an actinia. Very frequently it forms n slimy crust upon alga;, shells, etc., or projects iu globular or conical masses, " more like a lump of inanimate matter than a being endowed with vitality"—" a mil-ions and interesting internal organization, veiled by the coarsest exterior." The individuals are sometimes connected by a gelatinous flesh, which consists of cellulose, and there is sometimes a calcareous deposition in this connecting substance as in the compound polypes. The individuals in these systems have always sprung by generation from one, and both the and solitary compound ascidians propagate by eggs. The young have the power of active locomotion, resemble tadpoles in form, and swim by means of a Vibra tile tail, which disappears when they settle, being usually detached by contracticat at the. base. The sexes are supposed to be distinct only in some of the ascidians. The ovaries are usually large, and the ova are carried away by the stream which passes thrcugh the. animal. It is in the solitary ascidians that the highest organization is to be okserved, and in which alone a distinction of sexes appears. In them, a muscular ring surrounds the mouth, and can be closed to exclude what is unfit to enter. Within this aperture thetas is also a fringe of tentacula, short and simple, or longer and minutely divided. In the compound ascidians, gemination does not begin till the single animal has been fully developed; thereafter, bud after bud is produced, according to the plan upon which the eompound system is constructed, and "the procreative force of the germ-mass finally exhausts itself in the formation of male and female organs, in which that force is again mysteriously renewed under its two forms of the spermatozoon and the germinal by the combination of which the reproductive cycle again begins its course." The name ASCIDIAN ZOOPHYTES (zoopl:yta ascidioida) has been used to, designate. those zoophytes or polypes which form the class polyzoa of Thompson, ltryozna of Ehrenberg, and which in certain features of their organization resemble the A., although: iu other respects they widely differ from them The akyonidium and alcyonel.a, already noticed in the article alcyonium, belong to this class. See Poronn and ZoorurrE.