AQUATIC RESPIRATION OF FISHES 



23 



Water enters the oro-branchial cavity when its volume is 

 increased by a lateral expansion and lowering of the floor, which 

 mainly result from the elastic recoil of the head skeleton. The 

 hypobranchial musculature only comes into action during 

 hyperventilation and when the branchial region expands 

 before biting. The valves of the gill slits are closed and water 

 enters through the mouth and spiracle. During the subse- 

 quent decrease in volume produced by the action of the con- 



Oro-branchial cavity 



Parabranchial 

 cavities 



Fig. 7. 



Diagram of a dogfish in side view (anteriorly) and horizontal 

 section (posteriorly) to show the path of the respiratory current. The 

 flow is unilateral. The horizontal sections pass through the external 

 gill slits and illustrate the changes in volume of the parabranchial 

 and oro-branchial cavities. Pressures in these cavities are indicated 

 with respect to zero pressure outside the fish. Full line arrows show 

 the movements of the mouth and branchial regions, with their thick- 

 ness indicating the relative strength of contraction. 



