SHELLS. 223 



Gasteropoda constitute a very numerous class, of 

 ■which the slug and the snail give a good general 

 idea. Some species of this class — few as compared 

 with the great body — are naked, but the majority 

 are protected by a shell, in some cases very thin, 

 brittle, and glassy, in others somewhat horny, but 

 more generally of a stony texture, and of great 

 solidity and hardness. 



The upper surface of the body of this class of 

 animal is covered with a fleshy cloak, the edges of 

 which usually project in a greater or less degree, 

 overlapping the foot-disk and other organs. This is 

 called the mantle, or cloak. 



These shells are secreted by the mantle, which in 

 one family — that of the Chitons — consists of several 

 pieces; but in general it is simple, and takes the 

 form of a hollow bone, produced in various degrees. 

 In the limpets, which we see adhering abundantly to 

 our sea-side rocks, the cone is low and nearly sym- 

 metrical ; but in the great majority of this class the 

 bone is greatly lengthened and twisted upon itself, so 

 as to form a spire. 



The species of Gasteropoda are very numerous 

 around Bermuda, among which we shall briefly 

 enumerate a few of the most interesting. We will 

 follow Cuvier in his division of the class into the 



