Tunicata

TUNICATA, a well-marked class of marine animals, normally distinguished by the possession of an external cuticle of cellulose (the tunic or test), and of two more or less prominent siphons, one inhalant, the other exhalant, leading into corresponding internal chambers (the branchial sac and atrium), which communicate with one another by a system of ciliated gill-slits or stigmata. These maintain through the body a circulation of sea-water subservient both to food-collection and respiration. The class was instituted and named in 1816 by Lamarck, who included the fixed Solitary Ascidians or Sea-squirts (the Tethya of Aristotle) and the Com pound Ascidians, which form colonies by budding (first dis tinguished from Alcyonarian polyps in the previous year by Savigny), together with various pelagic free-swimming forms (Thalia, Salpa, Pyrosoma), at that time very imperfectly known. The minute tailed Appendicularians, first described in 1819 by Chamisso, were added to the class thirty years later either as larval forms (Joh. Miiller, Krohn, Vogt, 1846-1854) or as a definite adult type (Huxley, 1851).

"Acephalous Mollusca..

Cuvier (1798, 1815) ranged the Ascidians alongside the bivalved Lamellibranchs as "Acephalous Mollusca," being led to this view by the similarity of their siphons, and by the interpretation of the combined branchial sac and atrium as a mantle-cavity containing a pair of "branchial mem branes," The opening of the oesophagus into the branchial sac was regarded as the true mouth. This view dominated zoology until 1871, when it became clear, as a result of Kowalevsky's observations on the development, that the so-called "branchial sac" was simply a dilated pharynx, and that the real affinities of Tunicata were with the Vertebrates, owing to the larval possession of notochord, dorsal nerve-tube and gill-slits. The group is now recognised as forming with Amp/iioxus (q.v.) the lowest section of the Chordata, the subphylum Protochordata of Balfour.

The feature which, above all others, distinguishes this sub phylum from the rest of the Chordata is the possession through out life of an endopharyngeal ciliary feeding mechanism of sub divided gill-slits, glandular endostyle, and ciliated peripharyngeal bands. This apparatus, beautifully adapted for straining off mi nute floating particles is still represented by vestiges in Vertebrate embryos (cf. development of thymus and thyroid glands, which are modified remnants of tongue-bars and endostyle). Thus the Protochordata have the unique interest of preserving some of the characters of the immediate ancestors of the Vertebrata.

Apart from this the class Tunicata presents a number of special features of considerable physiological interest, such as the pres ence in the test of "animal cellulose" (tunicin, the presence in the blood-cells of the rare element Vanadium, which probably acts as a catalyst, and, with its five oxides, accounts for the diversified colouring of these animals (Henze, 1911-1913 ; Hecht, 1918) ; the reversibility in the direction of the heart-beat; the prevalence of gemmation of different types, whether simple or complicated by polymorphism; and, in Appendicularians, the phenomena of cell-reduction and of partial neoteny.

Classification.

With the exception of a few imperfectly known forms, existing Tunicata fall readily into one or other of three sub-classes named by Herdman (1891) Ascidiacea, Tha liacea and Larvacea. The first includes the fixed Ascidians, both simple and compound, the second the pelagic, tailless, f ree swimming types reproducing by budding (inclusive of Pyrosoma, which Herdman referred to the Ascidiacea), and the third the minute, solitary, tailed, Appendicularians. The name Larvacea, however, has not received general recognition, and is here re placed by Gegenbaur's older (1878) and in other ways more acceptable term Copelata (from Kopelcites, oarsman, scullery.

The order in which these groups are here presented is believed to represent the natural, or phyletic, sequence; but the pelagic Thaliacea and Copelata, though far less rich in numbers and variety than the Ascidiacea, are treated in greater detail, both on account of their importance to students of evolution, and from the fact that our knowledge of them has been greatly extended by the oceanic expeditions of recent times, which renders more com plete treatment desirable. The classification adopted will per haps be more readily intelligible as a record of Tunicate evolu tion if it be added that the ground-plan of the Tunicate pharynx, like that of Amphioxus, consists of a median ciliated and glandu lar groove below (the endostyle), flanked by gill-slits of the Amphioxus pattern on either side, i.e., each U-shaped, completely bisected by a tongue-bar, and each half (or protostigma) further divided into a row of stigmata by outgrowth of processes (synap ticulae) from the tongue-bar. It should also be noted that the power of budding is assumed to have been possessed by the earliest Tunicata, since in some form or other many solitary Ascidians, even Amphioxus and Appendicularians, retain structures which are probably vestiges of the endodermal budding organ or epi cardium. (See below.) Indeed loss of budding has probably been no slight factor in the evolution of the later forms, both fixed and free, by releasing energy for increased size and longer life, or more continuous muscular activity.