AFFINITIES OF THE TUNICATA. - Many of the early naturalists (as we have noticed in the first part of the article) noticed the analogies existing between the Ascidia and the Ostrea, Mytilus, and other Molluscs!. The compound Ascidians, however, long remained grouped with Alcyonium, until Sa vigny, Lesueur, and Desmarest pointed out their alliances with the simple Ascidians and with Molluscs. Lesueur also demonstrated the ascidian character of Pyrosoma, and re moved it from the Radiata, amongst which it had been grouped ; and Cuvier pointed out the alliance of Salpa with Ascidia and Mollusca. Soon after the publication of the important zoological conclusions thus arrived at by his distinguished countrymen, Lamarck formed these animals into a new group, under the ap pellation of Tuniciers* (Tunicata), provision ally placing them between the Radiata and Venues, and expressed strong doubt of their general alliance with the Mollusca.
John Hunter, who perceived the relations subsisting between Ascidia and Salpa, and knew the true analogy of their exterior co vering, proposed to distinguish them as a dis tinct group of Molluscs, under the term " soft shells," which more truly accords with their real nature than "shell-less" (Acephales sans coquilles), as they have been subsequently de signated by Cuvier*, who also, together with Blainville, regarded them as acephalous Mol luscs, and sufficiently distinct to form a sepa rate class.
In many of the Acephala we find two tubes for the ingress and egress water, like those of the Aseiduz, and the mantle closed throughout its length, the walls of its cavity lined with the branchim, and the mouth at the base of this cavity. The Ascidia being immovable, has neither adductor nor foot-muscles; for, being destitute of a locomotive organ and of valves, the muscles disappear, with the excep tion of some sphincter-like bands. Together with the muscles, the pedal nerve-ganglions, or rather subcesophageal or posterior ganglions are lost, and there remain only the buccal or supra-cesophageal ganglions. The differences between the heart and auricles of the Asci diens and the other Molluscs are dependent on the modification of the branchire ; the latter, in Ascidians, never being arranged in the four lamellm usually found in Molluscs. The digestive tube and the generative organs are essentially the same in Tunicata and Ace phala, and in the latter, as in the former, the ovary and testicle are enclosed in the intes tinal loops.
Yet however much resemblance we may find between the Acephala and the Tunicata, there is still more between the latter and the Bryozoa. Van Beneden observes, that if a Bryozoon was confined to its cell, and still to carry on its respiratory function by its un protruded tentacles, and if anastomosing com munications were established between the tentacles and a heart at their base, we should have the complete idea of an Ascidian. There would be two external orifices, a single mouth situated at the base of the branchial sac, vibratile cilia throughout the extent of the anastomosing tentacles, a folded intestinal canal, a supra-oesophageal ganglion, some muscular bands, and organs of generation developed around the digestive tube. There would be only wanting to complete the animal, a cloaca to receive the excrements, the sperm, and the ova, and a direct passage of commu nication between the respiratory and the cloacal cavities. The gemmiparous mode of reproduction observed in the Clavellince, and Botryllidec closely resembles that which ob tains in the Bryozoa. With regard to the de velopment of the Acephala, compared with that of the Tunicata and Bryozoa, Van Beneden points out the following analogies : — " There exist common characters between the Bryozoa and the Anodonta in this re spect ; their larvae respectively swim about by means of vibratile cilia. They have a dif ferent form in the young and the adult states, and they undergo true metamorphoses. The same phenomena are seen in the Ascidians. These also have metamorphoses, perhaps, even more complete, but instead of vibratile cilia, the embryo is provided with a caudal appendage sufficiently long to serve for the purposes of locomotion ; which tail disappears, just as the vibratile cilia of the former, as soon as the animal becomes definitely fixed." The embryogenic characters appear to en tirely accord with the anatomical dispositions of these groups ; and altogether we are, per haps, justified in considering, that they ap proach on one side to the Acephala, and, through the Pedicellinat, they are, on the other hand, allied to the Bryozoa, forming a closely uniting link between these two impor tant groups.