FORM AND LOCOMOTION 



21 



expelled from the gill-openings during the process of respira- 

 tion. The first method is the most common and of the greatest 

 importance, the others being, for the most part, auxiliary to it. 

 It must be borne in mind, however, that in the majority of 

 fishes the three are inter-related, and may all be used at different 

 times, or even at the same time, for the common end of driving 

 the fish forward according to its requirements. Locomotion by 

 means of fin movements, for example, may be employed when 



Fig. 9. — BODY MOVEMENTS OF FISHES USED IN SWIMMING. 



Shark (above) and Eel. (After Marey.) 



slow progress only is wanted, but, should danger threaten or 

 prey appear in sight, body movements quickly come into play, 

 and at the same time the increased rate of breathing due to the 

 emotion of fright or greed assists in the general acceleration of 

 the forward thrust. 



The Mackerel {Scomber), which depends almost entirely on 

 body movements for forward progression, will serve as an 

 excellent example of the first of these methods. We have 



