12 MOLLUSCA. 
bladder of fishes. In all these exertions the progress of mol- 
Juscous 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 branchie, 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the organs are contracted or withdrawn into the body.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 SysvTem 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 
