122 



PRINCIPLES OF ANIMAL BIOLOGY 



arches throup;h which water taken in at the mouth flows out over the 

 gills. These arches and sUts, in the embryo, are diagrammatically shown 

 in Fig. 182. In fishes and some amphibians a covering, the operculum, 

 conceals and protects the gills, but in other amphibians, as Necturus 



and some other salamanders, 

 the gills are external and are not 

 covered. A diagram showing 

 the structure of external gills is 

 shown in Fig. 86. 



Lungs. Lungs are organs 

 developed in the embryo of 

 vertebrates as evaginations from 

 the anterior end of the alimentary 

 tract as described in Chapter X. 

 By means of accessory organs of 

 respiration air is brought into 

 the lungs where it comes into 

 contact with mucous membranes 

 richly supplied with blood vessels. Here exchange of gases takes place. 

 In case of either lungs or gills the successful operation of the respiratory 

 mechanism is absolutely dependent upon the continual changing of the 

 blood in contact with the moist mucous surfaces. A halt in the flow of 



Fig. 86. — External gills of the amphibian, 

 Epicrium glutinosum. {From Wiedersheim, after 

 Sarasin.) 



Fig. 87. — Diagrams of types of lungs, a', amphibian lung Avith plain surface; b, 

 amphibian lung with low folds making simple alveoli; c, amphibian lung with higher 

 folds which are themselves folded making more numerous alveoli; d, trachea and bronchial 

 tree of a mammal. 



blood stops respiration very promptly, and unless the flow of blood can 

 be quickly resumed death results. 



The lung in lower amphibians is a bag-like organ with a large central 

 cavity (Fig. 87); but in higher amphibians it becomes more complex 

 since its inner surface is thrown up into corrugations which with cross 



