106 GENERAL BIOLOGY 



other animals the tube is open at each end and the 

 food passes slowly from mouth to vent. A few forms, 

 such as the snails, have both openings close together. 

 Some animals, such as the pigeon, or the deer, are 

 compelled to seek their food in danger of attack and 

 have developed the habit of taking in a quantity of 

 food to be stored in a crop or similar sac to be di- 

 gested at leisure in safer surroundings. Probably 

 the stomach, which is found in one form or another 

 in most animals, functions primarily as a storage 

 chamber. 



The differentiation of the alimentary canal is 

 usually specially related to the habits of the animals. 

 Thus, meat eaters require far less bulk to obtain the 

 same amount of food substance than do vegetable 

 eaters, and we find the alimentary canal in the latter 

 much larger and longer. In the cat the alimentary 

 canal is but three times the body-length, whereas in 

 a sheep it may be as much as twenty-eight times as 

 long. In order to increase the absorptive surface 

 and hasten the taking up of large quantities of 

 digested food the inner surface of the alimentary 

 canal is often thrown into folds and wrinkles which 

 greatly increase the superficial area without increas- 

 ing the size of the tube. Thus in the earthworm 

 there is developed the so-called typhlosole, which is a 

 fold of the dorsal wall of the intestine that hangs 

 pendent within the cavity of- the tube. In many 

 fishes the same end is attained through the djevelop- 

 ment of the spiral valve, which is a fold of the inner 

 lining of the intestine developed in the form of a 



