MOLLUSCA 249 



siphon of the same animal is regarded as a modification of a part 

 of the foot and takes part in backward locomotion. The ten- 

 tacles about the mouth are also looked upon as arising from the 

 anterior part of the foot, hence the name Cephalopoda, which 

 means "head-footed." 



290. Digestive Organs. — Mouth and anus both occur, and 

 are usually widely separated. In the coiled forms (as the snail), 

 however, by the looping of the digestive tract they are brought 

 close together. In all except the group of headless moUusks 

 (lamellibranchs) the mouth is suppHed with a radula, or tooth- 

 bearing tongue. This lies in the floor of the mouth and, as it is 

 worn away in front, is renewed from behind in the radula sac 

 (Fig. III). It rasps small particles from solids and conveys 



Fig. III. 



Fig. III. Diagram of mouth of snail, showing the lingual ribbon (radula). br, brain; c, buccal 

 cavity; Co., ccelom; g, gullet; 'j, jaw, against which the radula works; m, mouth; r., radula; r.5., 

 radula sac, in which the radula is renewed as it is worn away in front. 



Questions on the figure. — What parts go to make up the "odontophore?" 

 How do the parts act in biting? 



them backward into the esophagus. In the gasteropods there 

 is a plate in the upper jaw against which this organ works. In 

 the cephalopods beak-like jaws occur suited to their carnivorous 

 habit. The mouth is followed by a gullet, which may com- 

 municate at once with the stomach (lamellibranchs), or may 

 expand into a crop (gasteropods and cephalopods) . The stomach 

 is well marked and opens into the intestine which is usually long 

 enough to make one or more coils in the body mass. It may 

 open externally (gasteropods) or in the mantle chamber (cepha- 

 olpods and lamellibranchs). Salivary glands pour their secre- 



