194 



GENERAL ZOOLOGY 



beak and a rasping-tongue. A shell-forming mantle either 

 incloses the shell, as in the squid, or lies beneath it, as in 

 Nautilus and Orthoceras. 



There are two gills in the mantle-cavity in the squid, and 

 four in Nautilus. A siphon is present in all the examples 

 described. The sexes are separate. 



Definition of Mollusca (Lat. molluscus, soft). The phylum 

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

 tary study the most important ones are the Pelecypoda, 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 

 segmented. 



FIG. 99. Living Oyster-Drill and Periwinkle, x f 



