CHAPTER XIV 

 RESPIRATORY SYSTEM 



Respiration is the process of taking in oxygen for metabolic 

 processes, and giving off the excess carbon dioxide from the 

 blood. In all vertebrates respiration is divided into (1) external 

 respiration, the mechanism for getting oxygen from the surround- 

 ing medium; and (2) internal respiration, the passage of oxygen 

 from the blood across the membrane of each cell of the body. 

 Anatomically, respiration is usually limited to the organs which 

 take the oxygen from the animal's environment. 



Any moist, vascular membrane may serve as an organ of 

 respiration. In the vertebrates there are a number of such res- 

 piratory membranes in isolated groups, but the most widely dis- 

 tributed organs are gills in the water-living groups, and lungs in 

 the land vertebrates. Some amphibia, however, lack both lung^ 

 and gills, and other groups have various organs modified for 

 respiration. Gills are the most primitive respiratory organs of 

 vertebrates. 



A. Gills 



1. Origin of the Gill Slits. An outline of the development of 

 gill slits was given on page 4. The embryonic pharynx is rela- 

 tively much larger than that of the adult, the difference being 

 much more marked in the higher groups of vertebrates. In Am- 

 phioxus there is a great number of the endodermal pharyngeal 

 pockets, each of which becomes secondarily divided into two. In 

 the cyclostomes the number is reduced to fourteen for the primi- 

 tive group, and seven for the lampreys. The latter number of gill 

 slits was carried over to the primitive sharks, and in several 

 living genera seven or six are found. In the dogfish the number 

 is reduced to the definitive vertebrate number, five, with the 



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