SWIMMING 



Terrestrial vertebrates may swim with much lost motion, thrashing 

 about and straining with most of the muscles of the body. As soon 

 as one of these has become even slightly aquatic, however, movement 

 through the water is accomplished more easily and effectively, as it 

 must be in order not to exhaust the swimmer. 



By such of the vertebrates as spend much of their time in the water, 

 aquatic propulsion would seem to be accomplished by three methods: 



(1) Rhythmic undulatory or oscillating movements of the body 

 proper. 



(2)' Rhythmic movements of the appendages. 

 (3) Expulsion of jets of water. 



As far as I am aware, no vertebrate swims exclusively by expelling 

 jets of water, but Breder (1926) and others have shown that many fish 

 habitually employ the force of the water expelled through their gills 

 as a material though secondary aid to some other primary means of 

 propulsion. For our present purpose this method of swimming may 

 be dismissed with no more than this brief mention. 



Swimming by means of rhythmic movements of the body include 

 oscillations either in a vertical or horizontal plane of the body proper 

 and these movements are in consequence always transmitted to the tail, 

 which, if the latter be of sufficient size, then acts as a primary means 

 of locomotion. This includes all vertebrates which swim chiefly by 

 means of the tail except such fishes as hold the body rigid while rapidly 

 oscillating the tail tip. 



Swimming by means of rhythmic movements of the appendages is 

 employed by all vertebrates which propel themselves through the water 

 by movements of their limbs, even though their bodies be wriggled dur- 

 ing the process, by undulations of long dorsal or ventral fins, or of a 

 vibratory tail tip. 



In the case of such forms as are but slightly modified for an aquatic 

 life it is frequently difficult to decide which is the primary means of 

 propulsion through the water, as body, tail and limbs may all be used 

 to some extent, but such doubtful cases are always mentioned in the 

 text. Another difficulty in properly classifying swimming methods is 

 introduced by the fact that some particular vertebrate may have experi- 

 enced a sequence in its propulsive mechanism during its evolutionary 

 modification, a more inefficient method being temporarily employed 

 pending development of the final primary propulsive organ. 



[11] 



