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CHAR LER OVI 
THE MOLLUSKS. 
THE great sub-kingdom of the J/ol/lusca stretches its limits from 
the fish on the one hand to the polypes on the other, and at each 
extremity glides almost imperceptibly into the adjoining classes. 
It is a difficult, indeed, an impossible matter, to find a definition 
which will include the mollusks and exclude the members of other 
tribes. As their name imports, the mollusks (wzo//7s, soft) possess 
soft bodies—they have no internal skeleton. This body is enve- 
loped in a soft elastic skin, not always fitting closely, but which 
often hangs in folds; and this peculiarity has probably suggested 
its name—the mantle. 
The mollusks possess in general a complicated digestive and 
circulating apparatus. The great bulk of their bodies is made up 
of the stomach, the liver, and other members of the alimentary 
system ; but the organs of sensation and motion are comparatively 
undeveloped. Indeed, most of the mollusks are condemned to a 
sedentary life ; very few can move at all, and these but sluggishly. 
Hence the whole class is endowed with the power of sustaining 
long fasts; for not being able to go in search of their food, they 
are dependent upon the capricious bounty of the waves and 
currents. In the great majority of the mollusks, the mantle is 
capable of secreting first a horny covering, and from this exudes 
calcareous matter which forms the shell. This shell is thick and 
massive in those species which are incapable of motion, but light 
and fragile, and often altogether absent in those which lead a 
more active life. 
As the shells of the mollusks are the parts of their structure 
by which they are most easily described, and most readily recog- 
nised, it is but natural that the first attempts at their classification 
should have been founded upon the difference of their shells. 
Linnzus divided the race into wnivalves, bivalves, and multivalves, 
M 
