Usually, ascidians are described as sessile organisms with sac-like bodies firmly attached by the posterior end to the substratum. In reality, the shape of different ascidian species may be very diverse, and sometimes it is even difficult to recognize them as belonging to Ascidiacea. Although they grow side by side, is hard to imagine that the large, upright, cylindrical, solitary Ciona species belong to the same order as the Didemnum, which look like thin, encrusting stones and resemble a sponge colony.
The body is always covered with the test, a protective layer of cellulose-like material secreted by the epithelium. The test may be clear and often brightly colored, as in the spectacular Pacific species Halocynthia aurantium, or it may be covered by various kinds of spines, as in the northern Boltenia echinata; it even may contain calcareous spicules, as in snow-white Ba-thypera ovoida. Among species living on a sandy or muddy bottom, the test often has long, thin outgrows, or test hairs, and sometimes is covered by a dense layer of sand grains, making the specimen cryptic, like many species of the family Mol-gulidae.
The body muscles typically are arranged in transverse and longitudinal bands. Sometimes they are numerous, thick, and strong, forming a solid muscular wall; in many species, however, the muscles are represented by only a few thin fibers. There are two openings on the body: an oral or branchial opening and an atrial opening. These openings may be sessile or set on the ends of siphons of various lengths, the atrial siphon always dorsal to the branchial siphon. The oral siphon leads to the voluminous pharynx, called in ascidians the "branchial sac," The branchial sac occupies most the space in the body of solitary ascidians. The space inside the branchial sac is the branchial cavity, and the space between the branchial sac and the body wall is the atrial cavity. The atrial cavity opens to the exterior through the atrial siphon, and the branchial and atrial cavities are filled with seawater. The sea-water, with food particles and oxygen, is drawn into the branchial sac through the branchial siphon and a circle of oral tentacles and passes from the branchial to the atrial cavity through numerous perforations in the wall of the branchial sac, where food items are sieved; then filtered water moves out from the body through the atrial siphon.
Perforations of the wall of the branchial sac generally are small and have ciliated margins called stigmata, The shape of the stigmata is an important taxonomic character: they may be straight and arranged in transverse or sometimes longitu dinal rows, or they may be spiral figures, sometimes forming high funnels protruding into the branchial cavity. The wall of the branchial sac has transverse and longitudinal branchial vessels crossing each other and forming rectangular meshes. In some deepwater ascidians true stigmata are absent, the wall of the branchial sac is reduced, and the branchial sac is represented only by wide rectangular meshes formed by crossing longitudinal and transverse branchial vessels. The branchial sac is bilaterally symmetrical, with its mid-dorsal line marked by a fold termed the "dorsal lamina." The mid-ventral line has an endostyle, a groove lined with ciliated glandular epithelium secreting mucus. The mucus constantly moves from the endostyle to the dorsal lamina and then, with the filtered food particles, to the esophagus, to which the bottom of the branchial sac opens. The esophagus typically is short and always is much narrower than the branchial sac. It leads to the stomach and then to the intestine. The intestine makes a loop and opens into the atrial cavity.
The position of the gut loop in relation to the branchial sac is an important taxonomic character of the suborder and family levels: the gut loop may be on the left or, in one family, on the right side of the branchial sac; between the branchial sac and the body wall; or, mostly in colonial ascidians, under the branchial sac. Gonads are hermaphroditic and situated in the gut loop, under the gut, or on the body wall on the sides of the branchial sac. Gonoducts always open into the atrial cavity, and only in one highly specialized genus do they penetrate the test and open directly to the exterior. The nervous system, as in all sessile animals, is simple and represented by an elongated ganglion situated on the dorsal side between the siphons and a few nerves running from it. The heart is a thin-walled tubular organ on the ventral side of the body or, in colonial ascidians, in the bottom of elongated zooids.
In the case of colonial ascidians, the test forms the so-called common test, a mass in which individuals, called "zooids," are completely or partly embedded. The general plan of the structure of the zooids is the same as in solitary ascidians, but details may differ significantly. The body may be undivided or divided into two (thorax and abdomen) or three (thorax, abdomen, and post-abdomen) regions. The thorax contains the branchial sac, the gut loop is in the abdomen, and the gonads are either in the abdomen or the post-abdomen. Branchial siphons of all zooids in a colony open directly to the exterior, but atrial apertures in many species open into the cloacal cavity within the common test. The colony may contain single or several isolated cloacal cavities, each exposed to the exterior through one or several openings. Zooids connected with one cloacal cavity form a system. The shape of the systems varies; zooids may be arranged in circular systems around the single cloacal opening in the center, or they may form long double rows along cloacal canals. The form of the systems usually can be recognized easily on living colonies; the systems make a characteristic pattern on the surface of the colony, which, in certain cases, helps to identify the species.
The size of most ascidians varies from 0.04 to 0.4 in (1-10 mm), rarely 0.6 in (15 cm). Certain species, however, are much larger; in favorable conditions some solitary species may reach
19.7 in (50 cm) in height, and thin, encrusting colonies of certain species of didemnids grow to 9.8 sq ft (3 sq m). The largest species is the colonial Antarctic Distaplia cylindrical its long, sausage-shaped colonies may be up to 23 ft (7 m) in length and 3 in (8 cm) in diameter. The smallest species is the deepwater Minipera pedunculata, whose diameter is only 0.02 in (0.5 mm).
Many ascidians are brightly colored, usually red, brown, and yellow or, rarely, blue. The coloring of colonial ascidians is especially diverse: that of the zooids and the common test may be different, resulting in complex, often very beautiful patterns on the colony surface. Some species, such as Clavelina, have a transparent body, often marked with variously colored spots and lines.
Many ascidians have a peculiar feature: they are able to accumulate vanadium. More primitive species tend to have higher levels of vanadium in their tissues.
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