MOLLUSCA. ll 
strengthened internally by hard parts, as in the arms of the 
cuttle-fish. The force with which some animals adhere is 
very considerable, and is strikingly displayed, for example, 
when we attempt to detach a limpet from the rock. 
The rest, which is maintained by cementation, in some 
cases depends on a glairy secretion, which glues the body 
of the animal to the substance to which it is disposed to be 
attached. By such an expedient, the shells of snails adhere 
to rocks, stones, and plants. It is probable that the bivalve 
shells of the genus Cyclas, which readily adhere to the side 
of a glass, secure their temporary attachment by means of 
their glutinous cuticle. In other animals threads are pro- 
duced, termed a byssus, from particular glands, and while 
one extremity is glued to the rock, the other remains in 
connection with the animal. But there is an attachment 
more durable than any of these, which takes place in some 
shells, they being cemented to rocks or stones by calcare- 
ous matter, and retained in the same position during the 
whole term of their existence. 
The locomotive powers of the mollusca are confined to 
creeping and swimming. The former action is performed 
by alternate contraction and relaxation of the foot, or mus- 
cular expansion, which serves as a sucker, and is analogous 
to the motion of serpents. The motion of swimming is 
executed either by the serpentine undulations of the foot 
and the body, or by the action of tentacula, or expanded 
portions of the integuments. Many species are aided in 
swimming, by being able to vary the specific gravity of their 
body at pleasure, and either rise or sink in the water as 
circumstances may require. In some, as the Janthina, there 
isa cellular organ peculiarly destined for this purpose, which 
may be regarded as in some measure analogous to the air- 
