Tunicata

Tunicata a class of animals forming a subphyllum of the Chordata (q.v.), and thus allied to the Vertebrates. It includes the Sea-squirts, such as the Common Ascidian (q.v.). The animals are degenerate members of the Chordata, in which the notochord is confined to the tail. This appendage only occurs in the larva in most forms. The nervous system is usually reduced to a single ganglion. The animals are simple or colonial, fixed or free, and are hermaphrodite. They are enclosed in a test; the mouth opens to a respiratory pharynx, from which the water escapes by the mouth or an additional aperture. They are all marine. The Tunicates were originally grouped with the Bryozoa (q.v.) and Brachiopods (Lamp-shells) to form the phylum Molluscoidea, and supposed to be closely related to the Mollusca. This has now been abandoned from a study of embryology, and the Tunicates are allied to the Vertebrates. The main connection with these is due to the relative positions of the nervous system and the notochord. This is best seen in the order Larvacea, where it is a rod of cartilaginous material lying along the tail. Above it is the nervous cord, which gives off branches to the masses of muscle (or myomeres) on either side of the tail. The same arrangement can be made out in the embryoes of the other two orders, but during development the tail is reduced, the notochord lost, and the nervous system concentrated into a single ganglion. The structure of a typical Tunicate is described under ASCIDIAN. The classification of the class is as follows:-

I. LARVACEA: Simple free-swimming forms, with a tail provided with a notochord, e.g. Appendicularia (q.v.).

II. ASCIDIACEA: Well-developed test; large pharynx.
  (1) Ascidiae simplices: Simple or in compound colonies, but each individual has its own test, e.g. the common Ascidian (q.v.).
  (2) Ascidiae compositae: Fixed and colonial, with a common test, e.g. Botryllus (Sea Grapes).
  (3) Ascidiae salpaeformes: Free-swimming colonies; the only genus is the phosphorescent Pyrosoma.

III. THALIACEA: Free-swimming barrel-shaped forms with thin test, e.g. Salpa Doliolum.