THE ANIMAL BODY 147 



A third fundamental peculiarity is a series of perforations 

 or slitr, through the throat and body wall. In the lower 

 forms the gill slits provide an exit for the current of water 

 entering by the mouth and, being richly supplied with blood, 

 afford the chief means of respiratory interchange between the 

 animal and the surrounding medium. In the higher Verte- 

 brates the gill slits are present merely during a transient 

 phase in the development of the individual since the function 

 of aerating the blood is taken over by the lungs. (Figs. 74, 

 75.) 



