18 THE ORIGIN OF MAN 



ing or swimming, they move about by means of 

 the so-called foot, a creeping sole situated under- 

 neath the belly. Within the mantle, which is a 

 thin membrane that secretes the shell when such 

 is present, is the mantle cavity in which are the 

 gills, organs setting up currents of water out of 

 which is extracted not only the oxygen for res- 

 piration but also food for sustenance. The mouth 

 of molluscs is furnished with a flexible chitinous 

 ribbon known as the radula, a sort of tongue, 

 usually provided with numerous minute, sharp 

 hard teeth and moved by special muscles for 

 breaking up the food. Most of the Mollusca in- 

 habit the shallow parts of seas and oceans but 

 some are found in the fresh waters of the land. 

 A word about the shells may be interesting. The 

 shells usually consist of three layers: an outer, 

 a very thin, brown, horny layer; a thick, middle, 

 limy layer referred to as the porcelanous layer 

 and an inner, generally thick prismatic layer. 

 Of the molluscs are the Lemellibranchia, the term 



