THE GENERAL ANIMAL FUNCTIONS. 63 



by absorption through moist, thin-walled membranes. Such 

 surfaces, in connection with which blood vessels are usually 

 found, constitute the respiratory organs. Any exposed sur- 

 face meeting these requirements may serve as such. The 

 general surface of all animals is respiratory in some degree. 

 In the more complex animals, however, special additional 

 organs must be provided. This may be effected by thin out- 



FIG. 30. FIG. 31. 



b. c. ICl^tecBN ex. 



FIG. 30. Diagram illustrating gills or branchiae, b.c., cavity in which the body fluids 

 circulate; br., branchial filaments which are merely much thinned out-pocketings of the 

 body wall (w) ; ex, the external medium water in which the oxygen is dissolved. 



Question on the figure. What are the essential features of gills as 

 suggested by this figure? Why are they better suited to water than to air? 



FIG. 31. Diagram illustrating lungs or tracheae, b.c., the cavity in which the 

 body fluids circulate; /, the walls of the lung, which are much thinned inpocketings 

 of the body wall (w) ; ex., the external medium usually the atmosphere in which 

 the oxygen is found. 



Questions on the figure. What are the essential features of lungs as 

 suggested by the figure? Why are such organs better suited to aerial than 

 to aquatic life? In what respects are gills and lungs better than the mere 

 body-wall for the exchange of the gases? 



growths of the body surfaces, which are especially adapted 

 to water forms and are called gills or branchice (Fig. 30) ; or a 

 similar increase may be attained by pits or ingrowths of the 

 body surface, suited to get oxygen from the air. Such are 

 called lungs (or trachea) (Fig. 31). Carbon dioxid, a gas- 

 eous waste product resulting from the union of oxygen with 

 carbon which takes place in the tissues, is economically elimi- 



