INTRODUCTION 



XIX 



which in turn communicates with the exterior by a compara- 

 tively narrow cleft, the external gill-cleft or external gill- 

 opening. There are, thus, from 5 to 7 external openings in 

 the sides of the " neck " The iointed gristly structures 



^Section tArou^A CD 



Fig. 6. — Respiratory organs in a Shark and a Bony Fish. 



known as the gill-arches lie between the gill-pouches, and 

 encircle the walls of the gullet like hoops to provide supporting 

 girders. The gill-plates are placed on the opposing walls of 

 each of the gill-pouches. In the Bony Fishes there is the same 

 series of internal openings in the walls of the gullet, but, 

 instead of opening directly to the exterior by a similar number 



