340 



PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



been called jaws, and a ribbon of chitin, called radula, 

 thickly set with sharp teeth, by which they rasp their 

 vegetable food. In carnivorous conchs the radula is 

 used for boring round holes through the shells of other 



species for the purpose 

 of sucking their juices 

 (Figs. 224 and 225). The 

 intestine, as in acephala, 

 winds through the liver, 

 and in shelled forms turns 

 forward to discharge at 

 the opening of the shell. 

 Here, in fact, in the snail 

 we find four tubes open- 

 ing: (1) Of course, the 

 mouth ; (2) the intestinal 

 opening (anus) ; (3) the 

 genital opening ; 

 and (4) the respir- 

 atory opening. 



Cephalopoda. 

 — For this class 

 we take the squid 

 [Sepia). Squids 

 take their food 

 by means of the 

 powerful muscu- 

 lar arms surrounding the mouth (Fig. 226) and divide 

 it with their powerful parrotlike beak (Fig. 227), which 

 moves up and down like the jaws of vertebrates, not 

 laterally, like those of arthropods. The digestive tube 

 is very simple, the long oesophagus passing into the 

 large stomach and the intestines coming thencefor- 

 ward to discharge in front, so that the dibris is carried 

 away by the water currents of respiration. These ani- 



Fig. 225. — Different forms of the teeth of 

 carnivorous gastropods. 



