24 A HISTORY OF FISHES 



locomotor methods normally used in swimming, whereas those 

 with ribbon-like bodies are unable to progress at all outside 

 the liquid medium. While considering the question of the 

 movements of fishes out of the water, it may be of interest to 

 see why a species with the type of body similar to the Mackerel 

 flops from side to side when taken from its native habitat. In 

 contracting the body muscles on one side in the normal manner, 

 the tail is brought smartly down with a sharp smack, and the 

 accompanying reaction throws the fish upwards. Experiments 

 conducted on living fishes have shown that they are quite 

 unable to direct these movements, progressing indefinitely in 

 any direction, and that only a favourable wind or a slope in 

 the right direction enables them to find their way back to the 

 water. 



If the locomotion of the Eel be regarded as one extreme 

 type of body movement, that of the Trunk-fish (Ostracion) 

 undoubtedly represents the opposite extreme, the Mackerel 

 and other generalised fishes being intermediate between the 

 two types. In the Trunk-fish (Figs. 5B; 42F), with its head and 

 body enclosed in a hard and inflexible bony case, from which 

 the fleshy tail with a large fan-like caudal fin at the extremity 

 projects freely backwards, undulations of the body are clearly 

 impossible. Normally, the dorsal and anal fins form the chief 

 propefling agents, and the tail acts as a rudder, but where 

 greater speed is required the fish lashes the tail vigorously from 

 side to side, the movements being brought about by the alter- 

 nate contraction of the muscles on either side of the fleshy 

 part of the tail. A Trunk-fish swimming in this way may be 

 likened to a small boat propelled by means of a single oar from 

 the stern. 



The three types of body movements here described, and 

 exemplified by the Eel, Mackerel, and Trunk-fish respectively, 

 are not to be looked upon as other than purely arbitrarily 

 chosen examples. Among fishes we find such a complete 

 gradation from one extreme to the other that it is not easy to 

 say where one begins and the other ends. The extremes are 

 methods employed by comparatively slow-swimming forms, 

 mostly living close to the shore, whereas those of the Mackerel 

 and its kind are of the highest efficiency and pre-eminently 

 suited for high speed. With the sole exception of fishes such 

 as the Sea Horses, in which the locomotor emphasis is placed 

 entirely on the fins, all existing forms fall somewhere within 

 the series described above. . 



