312 TUNICATA. 
CLASS VI. TUNICATA. 
(Ascidian Mollusks.) 
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 pro- 
tecting valves, as has already been noticed in 
speaking of the Myade. 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 witlr 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 aggre- 
gated in groups; and others are united into masses, 
