And, indeed, since the commencement of the present century, the organisation of this group has been studied with great care, rewarding the labours of naturalists with discoveries of the highest interest. " It was in the animal of the Scapa," says Van Beneden, "that Van Hasselt discovered a heart of such extraordinary character, changing incessantly its auricle to ventricle and its ventricle to auricle, its art-, teries to veins and its veins to arteries. The Ascidians, too, furnish the first examples of complete metamorphosis in the lower ranks of the animal kingdom. The honour of this discovery is due to MM. Audouin and Milne-Edwards. From the late discoveries of M. Sars, these metamorphoses increase in interest, and appear to he still more remarkable. And, lastly, the Ascidians have contributed very considerably to our knowledge of the cir culatory apparatus of the Mollusca generally." The term Tunicata was first used by La marck ; its synonyms are Tethya, Auct.; Soft shelled Molluscs, Hunter ; les Acephales sans coquilles, Cuvier ; Actphalophora heterobran chiata, Blainville ; Tunicaries, Kirby ; Gynina cephala, Bronn. The CLASS of animals to which it is applied may be zoologically de fined as consisting of acephalous Molluscs ; with a soft shell or test, organised, coriaceous or gelatinous, frequently destitute of mineral constituents, having a large proportion of cellulose in its composition : animals single or aggregate ; the test of each animal provided with two apertures, one bran chial, the other anal ; the mantle forming an interior coat ; the branchim attached wholly or in part to the internal surface of the mantle ; the mouth, without labial ten tacles, placed below the branchial apparatus ; animals hermaphrodite, undergoing metamor phosis in their young state.
The cavity, whether of single or compound Tunicates, is occupied by a more or less muscular sac, provided, like the external tunic, with two orifices. This sac, identical with the" mantle" of the Acephalans, is attached to the inner surface of the test, generally only at the orifices, and contains the viscera. The di gestive, reproductive, and circulatory organs are disposed at the base of the sac, and its upper and larger portion, lined with, or traversed by, the branchim, forms the branchial cavity. This is placed at the commencement of the alimen tary canal, of which it forms as it were the antechamber. The branchim have generally the form of ridges, more or less complicated, and seldom symmetrical. The alimentary
canal is simple, and barely distinguishable into gullet, stomach, and intestine. It is always convoluted or folded once on itself. The liver adheres to the stomach, and in many species is divided into distinct lobes. The heart consists of a slightly bent, contractile tube, and is situated near to or within the in testinal loop. The reproductive organs, con sisting of ovary and testicle, are often lodged in the fold of the intestine.
The animal of these "soft-shelled" Mol luscs has very close affinities with that of the other Acephalans, especially the lamellibran chiates. And "were the test of an Ascidian converted into a hard shell, symmetrically divided into two plates, connected together dorsally by cartilage, and capable of separation so as to expose the mantle along a ventral mesial line, whilst the orifices protruded at one extremity, it would present the closest similarity with many bivalve Molluscs."* (Forbes.) All the Tunicata are free during the earlier periods of their existence ; some remain per manently free, floating in the water (Salpa, &c.), but the generality (Botrylli,Ascidice,&c.) become fixed to shells, seaweeds, and other marine bodies. Some exist as distinct indivi duals (Ascidia, Cynthia, &c.), whilst various degrees of combination are affected by others (Botryllus, Clavellina, Pyrosoma, &c.) ; and some are simple in one generation and com bined in the next (Salpa).
From the above-mentioned various con ditions of individualism or aggregation under which these animals exist, the family is divisible into two groups — the simple and the aggregate. Such forms of the latter group as were known to the earlier natu ralism were, from a general similarity of appearance, classed by them with Alcyonia. In the former group have been placed, until a late date, the numerous species of the Sal pidce, which now however, like the above mentioned groups, have been separately treated of and illustrated in, memoirs and monographs devoted to them. Indeed, although the ex pressions, simple and aggregate, as will be seen hereafter, are very convenient in describing the physiology of the Tunicata, yet late re searches have shown that the conditions to which they refer have but a subordinate value in reference to the natural grouping of the class.
As may naturally be inferred from what is stated above, the Tunicata have been sub divided in various manners. The arrangement we are inclined to adopt is as follows :—