MOLLUSCA. 407 



by calcareous matter, and retained in the same position dur- 

 ing 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 cont^'action 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 por- 

 tions of the integuments. Many species are aided in swim- 

 ming, 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 pur- 

 pose, which may be regarded as in some measure analogous 

 to the air-bladder of fishes. In all these exertions their 

 progress is proverbially slow. Some bivalve shells have the 

 power of leaping or shifting their position by a sudden 

 jerk, produced by shutting the valves rapidly. This is 

 strikingly displayed in the common Scallop, and is less per- 

 fectly exhibited in the river mussels. In a few instances, 

 especially among the slugs, a thread is formed of the vis- 

 cous secretion of the skin, by which the animal is enabled 

 to suspend itself in the air from the branches of trees. 



Although the progressive motions of molluscous animals 

 are comparatively slow, the other muscular actions are exe« 

 cuted with ordinary rapidity. The irritability of some 

 parts, as the tentacula and branchise, is so great, that the 

 protecting movements are executed almost instantaneously, 

 and the organs are contracted or withdrawn into the body. 

 But these rapid exertions are only called forth in the mo- 

 ments of danger. 



The characters furnished by the uuiscular system, are of 

 gTcat value in the discrimination of species, and in the con- 



