SWIMMING 



forward motion and this is maintained by alternations of the movements 

 from side to side. The important points to note are that the body and 

 tail do not oscillate about the snout, but that the anterior and posterior 

 parts of the animal oscillate from a point in the body anterior to the 

 middle. Whether this point is actually the center of gravity I do not 

 know, for an aquatic vertebrate of this sort really has no center of 

 gravity while it is in the water, so I prefer to employ the term "center 

 of equilibrium" to designate this point. If the animal .throw itself 



FIGURE 2. Diagrams illustrative of the swimming principles of an anguilli- 

 form or eel-like body above, and of a fusiform body below (redrawn from 

 Breder). 



into an uninterrupted curve while swimming, then the center of equi- 

 librium will constitute a pivot of motion, from which both ends are 

 curved. If only the tail proper be involved in swimming motions (as 

 in ostraciiform fish), then this curve of the caudal amphikinetic part 

 (as of Breder) will begin at a point considerably posterior to the center 

 of equilibrium, and as an accompaniment the curve of the cephalic 

 amphikinetic part (always present in theory although at times reduced 

 to an inappreciable amount) must begin at a point correspondingly an- 

 terior to the center of equilibrium. The difficulty lies in defining the 



[17] 



