AQUATIC MAMMALS 



unknown, and can never be known with certainty. Thus in some sorts 

 of whales it seems that the entire body takes part in swimming move- 

 ments, and that the center of motion accordingly migrates forward to the 

 center of mass, while it has been observed that when some sorts of por- 

 poises are swimming at speed the oscillations of the tail are through such 

 a short arc and are so rapid that the animal appears to be making no 

 movement at all. Which type of action is the more efficient cannot be 

 stated. 



If one should hold a fish by the snout and allow it to wriggle in the 

 water from side to side its motions might in some respects be compared 

 to the principles of a lever of the third order. The snout held immov- 

 able may be called the fulcrum while the water to be moved at either 

 side of the tail is the weight (fig. 1) The entire posterior half, or even 

 more, of the fish is the lever, and the muscles concerned in pulling the 

 tail first to one side and then to the other constitute a double, compound 

 'power arm. When the fish is released and it darts away, the circum- 

 stances will have been altered as follows. The fulcrum shifts backward 

 and becomes either a center of mass or a pivot of motion, according to 

 conditions, and to the pair of caudal power arms is added a pair of an- 

 terior power arms (fig. 1), and consequently, a second lever. In fish of 

 the usual type the anterior lever will always be much shorter than the pos- 

 terior. The posterior one may also be so short that as a result the length of 

 the anterior lever will be negligible (as in fish swimming by vibration of 

 the tail tip), or the posterior may be of sufficient length so that it is 

 continuous with the anterior lever — which is another way of saying that 

 swimming will then be accomplished by throwing the entire body into 

 a continuous curve. 



For an interpretation of the motions followed by a fusiform fish 

 (Breder's carangiform, and Abel's "torpedo principle") I follow Breder 

 (1926), who has discussed the aquatic movements of this class with 

 great thoroughness, and those who are interested in following the de- 

 tails at greater length may refer to him. If we take a fish of the form 

 of a mackerel the swimming motions will theoretically be as follows: 

 The contraction of the muscles of one side will throw the body into a 

 curve, but because of the resistance which the flattened posterior part 

 offers to the water this results in the head being thrown farther to one 

 side than the tail. With the momentum thus acquired by the head and 

 consequent inertia of the more massive anterior part of the body, the fish 

 is enabled to swing the tail to the opposite side with considerable force 

 (fig. 2). The pressure of the curved tail against the water initiates 



[16} ^ . 



