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 eanal, 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, 
