334 ZOOLOGY. 



also receives the excretory and genital products (reptiles and 

 birds). 



345. Exercises for Field and Library. 



1. What differences have you observed in the number, position, and 

 kinds of teeth in the various vertebrates of your acquaintance? 



2. Can you cite from your observation any evidences of adaptation of the 

 digestive tract to the peculiar food and habits of the animal possessing it? 

 Supplement by library references. 



3. To what extent is food prepared in the mouth, i. e.. antecedent to 

 swallowing, in the various vertebrates whose habits you have observed? 



346. Respiration. As in all higher animals there are two 

 things to be considered in the respiration of vertebrates : ( I )' 

 the exchange, between the blood and the external medium, 

 air or water, of carbon dioxid for oxygen, which may be 

 called external respiration, and (2) the exchange by which 

 the blood gives the cells of the body oxygen and receives their 

 carbon dioxid, or internal respiration. - The former is usu- 

 ally meant when the simple term respiration is used, though 

 the latter is really the vital process. A certain amount of 

 respiration takes place through the skin in almost all verte- 

 brates. Beside this, special devices both gills and lungs are 

 developed by which the blood and the medium containing the 

 oxygen are brought into closer relation. In fishes and larval 

 amphibians gills are present ; in most adult amphibians and in 

 reptiles, birds, and mammals, only lungs occur. 



Gills are thin-walled external folds or groups of filaments 

 bounded by a mucous membrane, in which the blood circulates 

 freely. In vertebrates they are found on the wall of passages 

 leading from the pharynx to the outside (gill-slits). The 

 water passes into the mouth and out over the gills, through the 

 thin walls of which the gases are exchanged. The walls be- 

 tween the slits may be supported by cartilages or bones 

 (visceral or gill-arches}. The gill-slits vary from four to 

 eight in number. In the higher, air-breathing vertebrates 

 traces of the gill-slits appear in embryonic development, but 

 they never bear gills. (See Figs. 30, 31.) 



