SOME LOWER ANIMALS 253 



backwards and sideways as well as forward. Vast num- 

 bers of lobsters and crabs are caught and eaten on the sea- 

 coasts. 



Mollusks constitute another type of animal life. Clams, 

 oysters, and snails all are interesting creatures to study. 

 These animals as 

 a rule have a limy 

 shell which is se- 

 creted by the skin. 



Snails are to be 

 found in ponds 

 and streams, on 

 sticks and stones, 

 and on water 

 plants. Some 

 kinds are found 

 on land, in gar- 

 dens, on bark, and 

 old logs. Some 



Snails have a Shell FIG. 96. Land Snails. 



of a cornucopia shape, and others have a flat spiral. Some, 

 like the slugs, have no regular shell, but only a small, scaly 

 mantle on the back. The shell is increased at the free edge 

 by the addition of new layers. 



Many snails breathe air in the water by means of gills. 

 Among the aquatic snails the kinds that can close the opening 

 of the shell with a sort of flap or door are gill-breathers. 

 Other aquatic snails are lung-breathers, and must come to the 

 surface of the water periodically for more air. Land snails 

 also have simple lungs. 



When a snail is "out of its shell" there will be seen a long, 



