CLASSIFICATION AND CREATION. 57 
Conchs, 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 
cach other by an endless variety of form and 
color, — from the flat simple shell of the Limpet 
Limpet, Patella, cut transversely: u, foot; b, gills; ¢, mantle; d, shell; 
e, heart ; f, 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* 
