ELEMENTS OF COMPARATIVE ZOOLOGY. 



26) we frequently find two parts, one chiefly glandular 

 while the other (gizzard) is extremely muscular. In the 

 ruminants (p. 118) the specialization is carried farther, and 

 we find four divisions to the organ. 



The intestine is the absorptive portion of the alimentary 

 canal. In some it is short and straight, in others long and 

 convoluted, there being usually a correlation between length 

 of intestine and the character of the food, this region being 

 longer in the vegetable feeders. Increased absorptive sur- 

 face is obtained in several ways, in addition to lengthening 

 of the intestine. In the lower Ichthyopsida this is accom- 

 plished by the development of an extensive internal fold 

 (spiral valve). In others there are numerous small longi- 

 tudinal folds, while in the highest vertebrates transverse 

 folds occur on which are minute finger-like outgrowths 

 (villi). In the lower vertebrates the hinder part of the 

 intestine receives the ducts of the excretory and reproduc- 

 tive organs, and at such times is called a cloaca. In the 

 mammals, the monotremes excepted, no cloaca is formed. 

 The vent is on the lower surface, in the median line. 



There are several accessory structures connected with the 

 alimentary canal. Thus frequently salivary glands are 

 present, emptying into the mouth. Behind the stomach 

 the ducts of the liver and pancreas pour in their secretions, 

 while in many fishes well-developed pyloric caeca occur, 

 just behind the stomach, which have a digestive function. 



The digestive organs are supported in the body-cavity by 

 a thin membrane (mesentery) which bears blood-vessels, 

 etc., and which is attached to the dorsal wall of the body- 

 cavity. This mesentery in reality is but the continuation 

 of the lining (peritonaeum) of the body-cavity. 



Vertebrates respire in three ways: by gills, by lungs, and 

 by the skin. Gills arise first as outpushings or pouches in 



