202 ZOOLOGY 



part of the food-canal called the crop. On the crop lie the 

 salivary glands (Fig. 19.5, 10), and the first stages of digestion 

 occur here. Beyond the crop is the stomach {14) into which 

 opens the great digestive gland (11). Here is done the main 

 work of rendering fluid the usable parts of the food ; the di- 

 gested portion enters into the blood, the unusable portion 

 passes through the intestine to the anus (is) which 1)}' a loop 

 is brought not far from the head, so that it may he exposed 

 outside of the shell when the animal is expanded (Fig. 193). 

 Tims we see that the striking peculiarities of the food-canal 

 are its radula, its huge digestive gland, and its U-shape. 



The digestive organs of the lamellibranchs lack the radula 

 and crop, but tliere is an enormous liver in the midst of which 

 the food-canal seems to lie. The anus is situated at the oppo- 

 site end of the animal from the mouth. In the case of cepha- 

 lopods there are jaws and a radula; also the large digestive 

 gland occurs and the anus lies well forward. In their diges- 

 tive apparatus the cephalopods closely approach the gastero- 

 pods. 



Organ of Respiration. — In most aquatic gasteropods 

 there is at least one gill (occasionall}' two) lying close to the 

 inner waU of the mantle cavity (Fig. 196). The gills consist 

 of numerous finger-like processes in which the blood-vessels 

 run. Water that is brought into the mantle cavity l)athes 

 the gills which extract oxygen from it. In ceph.alopods are 

 one to two jaairs of plume-like gills lying in the mantle ca\^t}^ 

 In common snails on the other hand, owing to their terrestrial 

 life, the gill is wholly wanting and the whole mantle ca\dty 

 becomes a lung richly provided with lilood-vessels. As stated 

 elsewhere, air-breathing moUusks can respire through the whole 

 surface of the laodj'. 



