PHYLUM MOLLUSC A 255 



being rasped into fine particles by a band of teeth termed the 

 radula (Fig. 182). The radula and the cartilages and muscles 

 that move it backward and forward constitute the buccal mass. 

 The salivary glands (Fig. 181, j) which lie one on either side of 

 the crop pour their secretion by rheans of the salivary ducts into 

 the buccal cavity, where it is mixed with the food. 



The cesophagus (2) leads to the crop, and from here the food 

 enters the stomach (4). The two digestive glands (5) occupy a 

 large part of the visceral 

 hump. They secrete a 

 diastatic ferment which 

 converts starchy matters 

 into glucose, and are 

 comparable to the pan- 

 creas in vertebrate ani- 

 mals. This secretion 

 enters the stomach and 

 aids in digestion. Ab- 

 sorption takes place 

 chiefly in the intestine, 

 an^ the faeces pass out 

 through the anus (Fig. " FIG l82 ._ Part of the radula of Physa 



I So, A. j Fig. iSl, Y). fontinalis, with central tooth and two marginal 

 P teeth highly magnified. (From the Cambridge 



3 Natural History.) 



RESPIRATION. The 



blood of the snail consists of a colorless plasma containing 

 corpuscles, and serves to transport nutriment, oxygen, and 

 waste products from one part of the body to another. The 

 heart lies in the pericardial cavity (Fig. 181, 14). The muscular 

 ventricle (n) forces the blood through the blood-vessels by 

 rhythmical pulsations. One large aorta arises at the apex of the 

 ventricle; this gives rise at once to a posterior branch, which 

 supplies chiefly the digestive gland, stomach, and ovotestis, and 

 an anterior branch which carries blood to the head and foot. 

 The blood passes from the arterial capillaries into venous capil- 



