The Tunicata or Urochordata are hermaphrodite marine Chordate animals, which show in their development the essential Vertebrate characters, but in which the notochord is restricted to the posterior part of the body, and is in most cases present only during the free-swimming larval stages. The adult animals are usually sessile and degenerate, and may be either solitary or colonial, fixed or free. The nervous system is, in the larva, of the elongated, tubular, dorsal, Vertebrate type, but in most cases it degenerates in the adult to form a small ganglion placed above the pharynx. The body is completely covered with a thick cuticular test ("tunic") which contains a substance similar to cellulose. The alimentary canal has a greatly enlarged respiratory pharynx or branchial sac, which is perforated by two or many more or less modified gill-slits opening into a peribranchial or atrial cavity, which communicates with the exterior by a single dorsal exhalent aperture (rarely {64}two ventral apertures). The ventral heart is simple and tubular, and periodically reverses the direction of the blood-current. [Illustration: FIG. 27.—Sketch of the chief kinds of Tunicata found in the sea.] This Class is divided into three Orders:—The Appendicularians, the Ascidians, and the Salpians (see Fig. 27). ORDER I. LARVACEA (APPENDICULARIANS). Free-swimming pelagic forms, in which the posterior part of the body takes the form of a large locomotory appendage, the {65}"tail," in which there is a skeletal axis, the urochord. A relatively large cuticular test, the "house," may be formed with great rapidity (in an hour or so) as a secretion from a part of the ectoderm; it is, however, merely a temporary structure which is soon cast off and replaced by another. The branchial sac is simply an enlarged pharynx with two ventral ciliated openings (stigmata) leading to the exterior. These may be regarded as the representatives of the primary gill-slits (undivided) of the Ascidian. There are thus a single pair. There is no separate peribranchial, atrial, or cloacal cavity. The nervous system consists of a large dorsally placed ganglion and a long nerve-cord, which stretches backwards over the alimentary canal to reach the tail, along which it runs on the left side (morphological dorsal edge) of the urochord. The anus opens ventrally on the surface of the body, usually in front of the stigmata. No reproduction by gemmation or metamorphosis is known in the life-history.