MOLLUSKS. Sie 
543. The Mollusks have no skeleton outside or inside. 
The shells which some of them have are mere coverings, 
or houses, as we may callthem. They do not serve, like 
the bones of the Vertebrates and the armor of the Ar- 
ticulates, to furnish attachment to the muscles so that 
they may act. Those Mollusks that lead the stillest life, 
that is, which use their muscles least, generally have the 
firmest and thickest shells. 
544, The shell is composed of carbonate of lime, with 
some animal matter, while in the bones of the Vertebrates 
the mineral portion is the phosphate of lime. In some 
the mineral part predominates, and the shell is very hard, 
like porcelain ; while in others, as the oyster, there are 
distinct layers of the mineral matter, with a membrane 
of animal substance between them. The shell is secreted 
from the thick skin of the animal, which is called the 
mantle. It is formed from the blood, and the materials 
for it are taken in with the food. 
545. Shells are of two kinds—those which are in one 
piece, and those which are in two pieces, with a hinge to 
keep them together. Mollusks that have the first kind 
of shell are termed wnivalve, and those which have the 
second are termed divalve. Clams and Oysters are fa- 
miliar examples of bivalves. Two varieties of univalves 
are represented in Fig. 249. 
HN Wein 
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Fig. 249. 
546. Shells undergo some changes in form as they grow 
