CLASS VI. TUNICATA.
The calcareous shell entirely disappears in the animals of this Class, their organs being enclosed in a case of leathery substance, more or less thick and tough, usually called the tunic or test. In general, they bear a close resemblance to the Conchifera, if the latter be deprived of their protecting valves, as has already been noticed in speaking of the Myadæ. The ordinary form which they assume is that of a leathery, usually semi-pellucid sac, with two openings, the one for the entrance, the other for the exit, of the nourishing water. The gills never appear in the form of free leaves, but constitute a kind of bag, the surface of which is covered with oblong cells or depressions, lined with cilia. Their circulatory system has this remarkable peculiarity, that it ebbs and flows: the blood, being driven from the heart through the vessels in one direction for a certain time, suddenly stops, and after a few moments pursues a retrograde course for a like period.
All the Tunicate Mollusca are marine; many of them are permanently fixed to other substances, others are permanently free; all, however, have the power of locomotion during the infancy of their existence. Some of the genera are single and isolated; others are social, always living aggregated in groups; and others are united into masses,