244 ZOOLOGY. 



the radula sac (Fig. 109). It rasps small particles from solids 

 and conveys them backward into the oesophagus. 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 



FIG. 109. 



FIG. 109. Diagram of mouth of snail, showing the lingual ribbon (radula). br, 

 brain; c, buccal cavity; co., coelom; g, gullet; /, jaw, against which the radula works; 

 m, mouth; r., radula; r.s., 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 " odonto- 

 phore"? How do the parts act in biting? 



a gullet, which may communicate 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 (cephalopods and lamellibranchs). 

 Salivary glands pour their secretion into the mouth cavity or 

 into the gullet, and the so-called liver connects with the 

 stomach or intestine. 



285. Respiration. The oxygen may be derived from the 

 water (lamellibranchs, cephalopods, and some gasteropods) 

 or from the air (pulmonate gasteropods). In the latter a 

 pulmonary chamber is formed by the mantle. Blood is richly 

 supplied to the walls of this, sac and is there aerated after the 

 manner of lungs. In the water-breathing forms the gills are 

 variously constructed. Lamellibranchs possess a pair of " gill- 



