386 ESSENTIALS OF ZOOLOGY 



Just ventral to the moutli is the opening of the pedal gland which 

 deposits a highway of mucus over which the snail usually glides; 

 the gliding movements are scarcely perceptible. In some marine 

 snails the surface of the foot is covered with cilia, the latter facili- 

 tating movement. The visceral hump, which encloses the digestive, 

 circulatory, respiratory, excretory, and reproductive systems, is 

 protected by the shell which is lined with the mantle. A thick 

 collar is produced where the mantle joins the foot, and just beneath 

 this mantle-collar is the respiratory aperture; back of the latter 

 is the anal opening. 



Internal Morphology 



Digestion.— Just within the mouth of a snail is a rounded organ 

 known as the buccal mass. The latter is composed of a ribbon of 

 minute recurved teeth, the radula, supported and moved by con- 

 nective tissues and muscles. On the roof of the mouth is a horny 

 jaw which pulls food into the mouth cavity. It is then rasped by 

 the radula into fine particles and mixed with saliva which flows 

 into the buccal cavity from salivary glands that lie on each side of 

 the crop. The masticated food is then passed into the esophagus 

 which widens, forming the crop. Here the food may be mixed with 

 a brown liquid produced by the digestive gland which occupies 

 most of the visceral hump. Enzymes produced by this gland con- 

 vert starches into glucose, and, in the case of HeliXy the ferment is 

 powerful enough to dissolve the cellulose of plant cells, thus releas- 

 ing the protoplasm so that it may be utilized. From the crop, food 

 enters the stomach and is passed on into the intestine where absorp- 

 tion takes place. Feces are discharged to the outside through the 



anus. 



Respiration 



Land and most fresh-water pulmonate snails breathe by a fold of 

 the richly vascularized mantle which has been modified into a 

 primitive lung, whereas the branchiate snails breathe by true gills. 



In all probability pulmonate snails that inhabit the deep water 

 of lakes use the pulmonary sac as a gill and breathe like the bran- 

 chiates. When the water is cold, it is not necessary for aquatic pul- 

 monate snails to make periodic trips to the surface in order to re- 

 new their air supply, but when the water becomes sufficiently 

 warm, cutaneous respiration alone is inadequate and the snail must 

 come to the surface to get additional oxygen. The pulmonary sac 



