244 TEXTBOOK OF ZOOLOGY 



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 Helix, 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 oxj^gen. The pulmonary sac 

 of aquatic pulmonates not only serves in the capacity of a gill or 

 lung but also may serve as a hydrostatic organ, thus enabling snails 

 to ascend to the surface by flotation. Such movements are prob- 

 ably made possible through contraction of the mantle walls, thus 

 decreasing or increasing the volume of air. Most of the marine 

 species are gill breathers, and some, such as the sea slugs, have 

 external feather-like gills. 



Circulation 



The blood of the snail consists of a plasma which is usually color- 

 less, but in Helisoma, hemoglobin is dissolved in the plasma, thus 

 giving it a red color, and in Lymnaea and some species of Helix the 

 blood has a bluish tinge due to the presence of a copper-containing 

 pigment, hemocyanin. In the plasma, float the colorless corpuscles. 

 The blood serves as a transporting medium whereby digested food, 

 excretions, secretions, and gasses may be carried from one part of 

 the body to another. The heart, which consists of an auricle and a 

 ventricle, lies in the pericardial cavity. Blood is pumped from the 



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