CLASSIFICATION AND CKEATION. 57 



Conehs, Periwinkles, Whelks, Limpets, and the 

 like. Some of them have no solid covering ; but 

 the greater part are protected by a single shell, 

 and on this account they are called Univalves, 

 in contradistinction to the Acephala or Bivalves. 

 These shells, though always single, differ from 

 each other by an endless variety of form and 

 color, from the flat simple shell of the Limpet 



a 



Limpet, Patella, cut transversely : a, foot ; b, gills ; c, mantle ; d, shell ; 

 e, heart ; /, main cavity, with intestines. 



to the elaborate spiral and brilliant hues of the 

 Cones and Cowries. Different as is their ex- 

 ternal covering, however, if we examine the 

 internal structure of a Gasteropod, we find the 

 same general arrangement of parts that prevails 

 in the Acephala, showing that both belong to the 

 same great division of the Animal Kingdom. 

 The mantle envelops the animal, and lines its 

 single shell as it lined the double shell of the 

 Oyster ; the gills are placed on either side of it ; 

 the stomach, with the winding alimentary canal, 

 is in the centre of the body ; the heart and liver 

 are placed in the same relation to it as in the 

 Acephala ; and though the so-called foot would 

 seem to be a new feature, it is but a muscular 



3* 



