LOCOMOTION IN WATER 223 



this alteration in the direction is entirely independent 

 of the action of the siphon. Even in molluscs of too 

 humble an organization for such co-ordinated move- 

 ments progression by means of a wave-like movement 

 may be observed. 



Fresh-water tortoises swim in various ways; generally 

 their mode of progression is something like that of 

 quadrupeds ; that is to say, with diagonally associated 

 movements of the limbs, noticeable, for instance, in 

 trotting. 



In exclusively marine species, the feet are shaped 

 something like fins, or, rather, like wings, and the 

 movement of the anterior appendages is much like 

 that of a bird. The result is a kind of flight through 

 the water, something similar to that of a penguin. 

 This kind of locomotion, which we have as yet had 

 no opportunity of studying by means of chrono- 

 photography, is a functional link between chelonians 

 and birds — animals which are closely allied in morpho- 

 logical characteristics. 



Slow Movements of Star-fish. — The slow movements 

 of certain aquatic animals are easily studied by means 

 of a series of photographs, and they form an interesting 

 subject of investigation. Xothing is more fascinating 

 than to watch the evolutions of a star-fish, which has 

 turned on to its back, in its attempts to regain its 

 normal position. It finally succeeds by extraordinary 

 feats of equilibration. It can be seen (Fig. 157) 

 gradually insinuating one of its rays beneath its body, 

 while it raises two others until its centre of gravity 

 is outside the base of support. Then, all of a sudden 

 losing its balance, it falls forwards on to its ventral 

 surface. There is now nothing left to be done except 

 to extend its rays and gradually assume its normal 

 position. It then moves along the bottom of the 

 aquarium with a crawling motion peculiar to star-fish. 



