220 



GENERAL ZOOLOGY 



Definition of Mollusca (Lat. molluscus, "soft"). The phy- 

 lum Mollus'ca includes five classes. For the purpose of ele- 

 mentary study the most important of these are the Acephala, 

 the Gasteropoda, and the Cephalopoda. 



On the basis of the facts presented in this and the pre- 

 ceding chapter, we may formulate the following statement 

 concerning the phylum. Mollusca are soft-bodied animals 

 provided usually with a hard, calcareous, partly chitinous 

 shell, which in some forms occurs on the exterior, and in 

 others is partially or completely inclosed by a sheet of shell- 

 forming muscular tissue, the mantle. The body proper is 

 never divided into somites, and the appendages are never 

 jointed. 



Fig. 119. Living oyster drill and periwinkle, (x §) 



