THE ORIGIN OF MAN 19 



coming from lamella, meaning leaf, and branchia 

 meaning gills and was chosen because in these 

 molluscs the gills are leaf-like. This group con- 

 sists of all the headless and degenerate Mollusca, 

 such as mussels, clams, oysters, cockles and scal- 

 lops. The body is compressed from side to side 

 and always covered with two shells or valves, one 

 on the right side, the other on the left side. Their 

 habitat is mostly marine, living in the shallow 

 waters near the shore where the food is most 

 abundant. Then the Gastropoda which means 

 stomach-footed and has reference to the fact that 

 the animals creep about by the contraction of the 

 sole of the foot which lies on the ventral side. 

 This class includes such shelled forms as the lim- 

 pets, drills, periwinkles, whelks, conchs and 

 snails. They are most abundant in the shallow 

 seas where they crawl over the bottom; they are 

 far less varied in fresh waters and are represented 

 on land by the vegetarian, air-breathing or pul- 

 monate snails. They are in habit, as a rule her- 



