280 



SOFT-BODIED ANIMALS— THE MOLLUSKS 



reason why one should have an aversion to eating snails if he eats 

 clams, since the organs of their bodies are basically similar. 



Many of the aquatic snails are good scavengers. Such species are 

 a great aid in balancing a fish bowl or aquarium since they eat remnants 

 of food passed up by the fish and also eat the algae which otherwise 

 might cover the glass. This effectively keeps down the growth of both 

 bacteria and algae which often cause an aquarium to go foul. 



Photos by Winchester 



Fig. 19.9. A slug. In the dorsal view on the left note the resemblance to the snail 



except for the absence of an external shell. There is an internal rudiment of a shell, 



however. The long, creeping foot shows clearly in the ventral view of the animal 



crawling on a piece of glass at the right. 



Some persons think the snails can crawl out of their shells and re- 

 turn to them because they sometimes see animals that look like snails 

 without shells. These are the slugs which do not have noticeable shells 

 as adults. However, a study of the embryonic development of the slug 

 reveals that a shell is formed in the embryo just as it is in the snail, but 

 fails to continue its development to a functional size. 



