MUSSELS AND SNAILS. 165 
can have all the food he wishes brought to him by 
currents of water there is no use for head, or eyes, or 
sensitive feelers. On the other hand, an animal which 
must forage for his food can do it more advantageously 
if he learns to walk head foremost. In order to make 
great progress in this way he must have some means 
of discovering obstacles in his pathway. Hence the 
necessity of feelers and eyes. In the struggle for food 
among animals the headward growth and specialization 
is a great advantage. 
Active exercise in earning a living in competition 
with other animals which desire the same food not only 
Fic. 129.—A Snail. 
sharpens the wits but actually increases the physical 
development of those organs on whose activity intelli- 
gence depends, just as vigorous muscular exercise 
develops the parts trained. 
Mussels and snails belong to the sub-kingdom Mol- 
lusca. The mussels, clams, oysters, and similar 
bivalves are members of the class Pelecypoda. These 
animals are sometimes called Lamellibranchiata be- 
cause they have four leaf-like gills, and sometimes 
Acephala because they have no heads. 
Univalves like the snails and the limpets, and slugs 
which have no shells, are classed as Gastropoda. 
