CHAPTEE IX. 



SNAILS AND LIMPETS. 



NYONE who kept an aquarhun or aquaria just 



simply for tlie sake of the fresh-water molluscs 



would be amply rewarded for his trouble by the 



pleasure and instruction with which they would provide him. 



The Mollusoa {mollis, "soft") form one of the sub-divisions 



of the Animal Kingdom whose members are characterised -by 



having soft skins and fleshy 

 bodies, but are without bones 

 or joints. Molluscs are either 

 naked or are covered with a 

 shell of one or two valves, but 

 with some of the latter only 

 we have to do in this chapter. 

 The shell is composed of 

 carbonate of lime and animal 

 matter, which is secreted by 

 the skin or mantle of the 

 creature. 

 The MoUusca are divided into the Uncephalous (possessing 

 a head) and the Acephalous (without a head). In the former 

 division are placed the Gasteropoda (beUy-footed), or Univalves, 

 &c., and in the latter the Conchifera (shell-bearing), or Bivalv«L 

 The fresh-water Univalves are commonly called Snails, and 

 the fresh-water Bivalves, Mussels. Both the snails and mussels 



Fig. 111. Planokbis corneus. 



