FORM AND LOCOMOTION 27 



phorus), in which the dorsal fin is wanting, employs the long anal 

 fin in a similar way (Fig. iob). The File-fish or Leatherjacket 

 (Monacanthus) has both dorsal and anal fins placed a little 

 obliquely, and makes use of both simultaneously for forward 

 progression (Fig. loc). Other fishes, such as the Globe-fishes 

 or Puffers (Tetrodontidae) and Porcupine-fishes (Diodontidae) , 

 move by flapping the short dorsal and anal fins in a fan-like 

 manner. The Sea Horse (Hippocampus) , which characteristically 

 swims in an upright position, glides slowly through the water 

 by means of rapid wave-like movements of the dorsal fin, 

 which has the appearance of a tiny propeller revolving in the 

 middle of the fish's back (Fig. 5E). The related Pipe-fishes 

 {Syngnathidae) swim in a similar way, but their bodies being 

 elongate and more flexible they are able to make more rapid 

 progress at times by lashing themselves into curves (Fig. 32E). 

 Flat-fishes, when moving about on the sea floor, often make use of 

 the long dorsal and anal fins which fringe the upper and lower 

 edges of the body (Fig. 8b) to obtain a grip of the ground, and 

 by undulating these fins are able to progress at a fair speed. 



Turning to the paired fins, the pelvics may be dismissed at 

 once, as these merely assist the dorsal and anal fins in main- 

 taining stability, and rarely, if ever, serve as organs of pro- 

 pulsion. The pectorals, on the other hand, are often used 

 partly, or in some cases almost exclusively, for locomotor 

 purposes, particularly in those fishes of slow or moderate 

 speed. In slow-moving fishes these fins are generally spatulate 

 in shape, and may produce forward movements of the body 

 by a simple synchronised flapping, as in some of the Wrasses 

 (Labridae). In others, of which the File-fish (Monacanthus) and 

 the Porcupine-fish (Diodon) are good examples, wave-like 

 motion similar to that described in connection with the caudal 

 fin, is employed. This type of motion is particularly well 

 marked in the Rays (Raia) and their allies, in. which the pectoral 

 fins are very much enlarged and constitute practically the sole 

 organs of locomotion. It will be noticed, however, that the 

 waves travel in a vertical plane instead of a horizontal one — up 

 and down instead of from side to side (Fig. iod). In a few 

 species, notably among the Damsel-fishes (Pomacentridae) , the 

 pectoral fins seem to be operated after the manner of oars, 

 being brought forward almost edgewise and pulled back 

 broadside on. In fishes of high velocity the shape of the fins is 

 generally long and falcate (i.e. sickle-shaped), and these are 

 probably used mainly for changing course or for slowing down, 



