THE METABOLIC MACHINERY OF ANIMALS 



283 



cured by increasing the length or diameter of the aUmentary tract, or 

 by the formation of pockets, or caeca, of different sizes and shapes. A 

 carnivore such as a cat has an ahmentary tract which is only three to 

 five times the body length, whereas a cow, being herbivorous, supports 

 one over twenty times the length of its body. Man, who is interme- 

 diate as well as an "omnivorous beast," has one about ten times longer 



small intestine. 



esopVia^us 



man °PP^-^^^ rabbit 



appendix dixode«UTn 



Cctt 



rabbit- 



^3) 



Comparison of digestive tracts of a 

 carnivore and herbivore. How can the 

 differences in the size of the caeca and 

 length of the gut be explained P (After 

 WeUs, Huxley, and WeUs). 



£..r<3ctcx\ glandi 



Alimentary canal of the dogfish. 

 State the function of the vahular 

 intestine. What are the principal dif- 

 ferences between this digestive tract 

 and that of the rabbit P 



than the body. A modification quite characteristic of some groups is 

 the caecum, which is noticeably large in rodents. Other types of caeca, 

 like the pyloric caeca, sometimes occur near the juncture of the 

 stomach and the intestine among fishes, and there should be men- 

 tioned here the longitudinal fold, or typhlosole, of the earthworm. 

 When such a longitudinal fold is twisted spirally, there results a 

 structure known as a spiral valve, v/hich is characteristic of sharks. 



