FORM AND LOCOMOTION 19 



evolution which has been ah'eady mentioned [cf. p. 14). 

 The so-called Symbranchoid Eels (Fig. 2ib), for example, are 

 in no way closely related to the true Eels, and the same type 

 of body in the two groups has been evolved in response to the 

 needs of similar environments, or as the result of the adoption 

 of the same mode of life. 



In the Sea Horses {Hippocampus) the form of the body is 

 unique, the head being bent at right angles to the trunk in a 

 manner suggestive of that of a horse, and the trunk itself is 

 definitely curved (Fig. 5E). The possession of a distinct neck 

 is not the only remarkable feature, and the tail is also unique 

 in that it is prehensile and can be used by the fish to anchor it 

 to moving or fixed objects. The body is protected by a series 

 of bony, ring-like plates, and the spines or membranous pro- 

 cesses with which these are ornamented serve to break up the 

 outline and so render the fish inconspicuous when swaying to 

 and fro among aquatic vegetation. The Sea Horses are defence- 

 less creatures, and depend largely on this mimetic resemblance 

 to escape from predatory fishes. 



The locomotion of fishes provides the biologist and physicist 

 with a number of interesting problems, and has also attracted 

 the attention of the marine engineer, some of whose mechanical 

 inventions owe their inception, at least in part, to observations 

 made upon living animals. Owing to the difficulty of studying 

 fishes in their natural habitat, and to the fact that when trans- 

 ferred to the unnatural surroundings of an aquarium they tend 

 to behave in a manner somewhat diflferent from the normal, 

 our knowledge of the different methods of locomotion is still 

 incomplete. Much progress has been made within recent years, 

 however, and the researches of an American investigator, Mr. 

 Breder, have proved very illuminating. Good results may be 

 expected from the use of the cinematograph, and with the 

 improvement of under-water photography it should be possible 

 to take good films of swimming fishes, and to analyse the 

 movements in detail by the use of the slow-motion picture. 

 Although actual swimming and its associated movements forms 

 the main subject for consideration in this chapter, it must be 

 remembered that this is by no means the only locomotor 

 method in use, and walking or creeping over the sea floor, 

 skipping about on sand or mud, burrowing, wriggling on dry 

 land, leaping, flying, and so on, are also indulged in by some 

 fishes for purposes of progression. These are rather in the 

 nature of specialised developments, however — secondary 



