THE CLASSIFICATION OF ANIMALS 51 



The scientific name Pelecypoda (Gr. pelekus, an axe) is derived 

 from this circumstance, whilst the popular name bivalve has 

 reference to the shell. In another group the mantle is a 

 circular flap surrounding the body and the shell is at first a 

 cap, but as the animal grows both mantle edge and shell 

 become lengthened and deepened, and this process goes on 

 unevenly, so that the cap becomes twisted into a spiral. The 

 foot has a flat surface, by means of which the animal crawls 

 over the rocks and stones on which it lives. Here again the 

 scientific name Gastropoda (Ut. belly-foot) is derived from 

 the shape of the foot, whilst the popular name univalve 

 refers to the shape of the shell. Lastly, in the Cuttle-fish the 



FIG. 22. Side view of a cuttie-fish swimming as an ex- 

 ample of the division of Mollusca called Cephalopoda ; 

 ff, fore-foot produced into arms bearing suckers ; hf, 

 hind-foot or funnel the dotted line shows upper 

 edge of foot ; m, mantle ; s, concealed shell. 



mantle is also a circular flap surrounding the body, but the 

 shell, primarily, as in Gastropoda, a cap, is usually concealed 

 in the mantle which grows up at its sides and closes over it. 

 The foot is in two divisions, the fore -foot and the hind-foot. 

 The former has grown up so as to surround the front part of 

 the body like a collar, and the edges of this collar are drawn 

 out into strap-like muscular arms beset with suckers, which 

 are most formidable weapons for attack and defence. With 

 these arms it seizes and securely holds its prey ; and its mouth 

 is armed with a strong beak with which it tears the prey to 

 pieces. The hind-foot consists of a vibrating muscular tube 

 termed the funnel through which a strong stream of water is 

 driven backwards, by the recoil of which the animal is driven 

 forwards. The scientific name Cephalopod (head-foot) which 



