MOLLUSCA. 1 1 



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 bi/ssus, 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 

 is a cellular organ peculiarly destined for this purpose, which 

 may be regarded as in some measure analogous to the air- 



