MOLLUSKS. 317 



543. The Mollusks have no skeleton outside or inside. 

 The shells which some of them have are mere coverings, 

 or houses, as we may call them. 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 

 tirmest 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 univalve, and those which have the 

 second are termed bivalve. Clams and Oysters are fa- 

 miliar examples of bivalves. Two varieties of univalves 

 are represented in Fig. 249. 



Fig. 249. 



546. Shells undergo some changes in form as they grow 



