CHOIUDATA 337 



of the glands, and the pancreas one of the most important in 

 digestion. The intestine may open directly on the exterior 

 (most mammals), or into the ectodermal pocket (cloaca) which 

 also receives the excretory and genital products (reptiles and 

 birds). 



353. Exercises for Field and Library. 



I.. What difiference 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 special adaptation of 

 the parts of the digestive tract to the peculiar food and habits of the animal pos- 

 sessing 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? 



354. 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 dioxide 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 usually 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 vertebrates. 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 between the 

 slits may be supported by cartilages or bones {visceral or gill- 

 arches). The gill-slits ordinarily 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. 32, 33.) 



