12 MOLLUSCA. 



bladder of fishes. In all these exertions the progress of mol- 

 luscous animals 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 

 perfectly exhibited in the river mussels. In a few instan- 

 ces, especially among the slugs, a thread is formed of the 

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

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

 a spider. 



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 branchiae, is so great, that the protect- 

 ing 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 the organs are contracted or withdrawn into the body.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 moments 

 of danger, 



The characters furnished by the muscular system, are of 

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

 struction of genera and higher divisions. They are inti- 

 mately connected with the habits of the animal, and merit 

 the attentive examination of the philosophical naturalist. 



In the molluscous animals the NERVOUS SYSTEM is less 

 complicated in its structure than in the higher classes, and 

 the brain is not restricted in its position to the head. The 

 whole nervous system appears in the form of ganglia and 

 filaments. The principal ganglion, or the one to which the 

 term brain is usually applied, is seated above the gullet or 

 entrance to the stomach. It sends out nerves to the parts 

 about the mouth, the tentacula, and the eyes. It may be 

 considered as analogous to the cerebrum of the vertebral 



