. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. 1242 TUNICATA. 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 Blaiuville, regarded them as acephalous Mol- luscs, and sufficiently distinct to form a sepa- rate class. In many of the Acepha
. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. 1242 TUNICATA. 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 Blaiuville, 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 Ascidia;, and the mantle closed throughout its length, the walls of its cavity lined with the branchiae, 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 suboesophageal or posterior ganglions are lost, and there remain only the buccal or supra-oesophageal ganglions. The differences between the heart and auricles of the Asci- dians and the other Molluscs are dependent on the modification of the branchiae ; the latter, in Ascidians, never being arranged in the four lamellae 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 hea
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