PHYLUM MOLLUSCA 



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 means of the salivary ducts into 

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



The cesopliagus (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 j^ffix*!^ 



diastatic ferment which j^^^\^^ 



converts starchy matters Jsfii^ kU 



into glucose, and are Mc /0/u«!v 



comparable to the pan- j^P ' * 



creas in vertebrate ani- ^lio 



mals. This secretion ^<m 



enters the stomach and ^^^^ 



aids in digestion. Ab- /<$W$y 



sorption takes place /$$<^ 

 chiefly in the intestine, /»I^ 

 and the faeces pass out J||P 

 through the anus (Fig. „ „ , t , , , . „, 



v 6 Fig. 182. — Part of the radula of Physa 



180, A ', Fig. 181, y). jontinalis, with central tooth and two marginal 



CIRCULATION AND teeth highly magnified. (From the Cambridge 



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 (//) 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 capiU 



