VERTEBRATES. 307 



by the development of an extensive internal fold (spiral 

 valve). In others there are numerous small longitudinal 

 folds, while in the highest vertebrates transverse folds 

 occur on which are minute finger-like outgrowths (wlli). 

 In the lower vertebrates the hinder part of the intestine 

 receives the ducts of the excretory and reproductive 

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

 mammals, the monotremes excepted, no cloaca is form- 

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

 line. 

 There are several accessorv structures connected with 



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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 

 cceca occur, just behind the stomach, which have a diges- 

 tive 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 



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continuation of the lining (peritonceum) 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 the sides of the pharynx, and then these break through 

 to the exterior, giving rise to gill-slits or clefts, through 

 which water taken in at the mouth can pass out. On the 

 sides of these clefts the gills proper are developed. These 

 are thin-walled leaves or filaments with a rich blood- 

 supply, and through these thin walls there is an exchange 

 of dissolved gases (oxygen and carbon dioxide) between 

 the water and the blood. 



