MOLLUSCA+ 139 
The general form of these animals is somewhat ovate. 
The tunic appears in some genera, as the Clio and Pneumo- 
dermon, to be double, the external one soft and thin, the 
internal exhibiting a fibrous structure, corresponding to the 
muscular web of the skin of the higher classes. In these 
animals, however, the two layers are unconnected through- 
out the greater part of their expansion. In some, as the 
Cymbullia, the tunic is cartilaginous, while in others it is 
strengthened by a shell. In these last, the shell in the Li- 
macina is a spiral univalve, covering the abdominal viscera, 
and in the Hyalea, where it serves the same purpose, it ap- 
proaches in character a bivalve shell. It is, however, des- 
titute of a hinge, the two valves being united together at 
their caudal margins, and there is no appearance of a trans- 
verse adductor muscle. 
The organs of motion in all the genera consist of two fins, 
or membranaceous expansions, one being seated on each 
side of the head. They have no foot wherewith to crawl, 
nor any suckers by which they can adhere to objects. They 
are, therefore, free animals moving about in the water by 
means of their fins, and probably possessing, at the same 
time, a power of varying their specific gravity, as they are 
capable of varying, to a certain extent, the form of their 
bodies, and of enlarging or reducing theirdimensions. There 
is nothing peculiar in their nervous system. 
The organs of digestion differ greatly from those of the 
Cephalopoda, which we have already considered. They are 
generally regarded as destitute of eyes and ears. Their 
tentacula are either seated on the head, forming two com- 
plicated branches of filaments, or spread along the margin 
of the tunic. There are no arms for seizing the food. The 
mouth, however, is furnished with lips; and in some there is 
