56 CLASSIFICATION AND CREATION. 



an animal of this class shows very distinctly the 

 bilateral arrangement of the parts. In such a 

 section we see the edge of the two shells on either 

 side ; within these the edge of the mantle ; then 

 the double rows of gills ; and in the middle the 

 alimentary canal, the heart, and the bloodvessels 

 branching right and left. Some of these animals 

 have eye-specks on the edge of the mantle ; but 

 this is not a constant feature. This class of 

 Acephala includes all the Oysters, Clams, Mus- 

 sels, and the like. When named with reference 

 to their double shells, they are called Bivalves ; 



Common Hen-Clam, Mactra, in motion. 



and with them are associated a host of less con- 

 spicuous animals, known as Ascidians, Brachio- 

 pods, and Bryozoa. 



The second class in this type is that of Gas- 

 teropoda, so named from the fleshy muscular 

 expansion on which they move, and which is 

 therefore called a foot: a very inappropriate 

 name ; since it has no relation or resemblance to 

 a foot, though it is used as a locomotive organ. 

 This class includes all the Snails, Slugs, Cockles, 



