186 THE WORLD OF THE SEA. 
Tf all this be true—for we must not forget that many observers 
have no small powers of imagination—but if this be true, we have 
a very striking example of a will independent of instinct. 
The woolly ascidia (Ascidia ampulla), contrary to the habits 
of its race, is free. Here the adult keeps the prerogative of the 
child. It is a species which is only dredged up with the sand 
from great depths. Its sac is round, and of a brown-red colour, 
the interior of the orifices being bright scarlet. We do not know 
whether this mollusk buries its lower extremity in the sand or 
not; when in captivity it simply lies horizontally, and makes no 
effort to descend lower or to change its position. 
The shelled mollusks are more numerous than the naked 
acephala. They are termed dzvalves, because their shell is in two 
parts, which are united by a hinge. Between this double carapace 
the creature is enclosed, as the leaves of a book between its covers. 
Although they have no head, yet they feed themselves; they 
appear to have certain feelings; and they can, of course, reproduce 
their kind. They have friendships and enmities, and perhaps even 
passions; these, probably, are not very lively, for the bivalve very 
rarely changes its position. Many of them are fixed to the rock 
on which they were hatched, and vivid sensations seem incom- 
patible with immobility. Bivalves are found in every sea, and, as 
might be expected, some of their species are peculiar to certain 
regions. The Pandoras, for instance, people the northern seas; 
the Chama is a native of the southern waters. The conditions 
of life under which they can exist are very varied; they can live 
in the sand, in the aquarium, upon rocks, or in the very midst 
of aquatic vegetation. They can support a great pressure of 
water, and have been found existing at great depths. Edwards 
brought up an oyster upon a sounding-lead from 1,400 fathoms! 
The shell of the bivalve is ovoid, globulous, trigonal, heart- 
shaped, elongated like a pea-pod, or flat like the leaves of a tree. 
In some cases, each valve is exactly alike, in others one is flat 
and the other convex. The two valves often possess accessory 
pieces. Such mollusks used to be called maltivalves. 
All the bivalves are not destitute of the power of locomotion ; 
for some can change their place by means of a fleshy extensible 
