• • LOCOMOTION. XU1 



great assistance, flying as in the flying-fish, floating .as .in the globe-fish 

 aided by the air sac, creeping under nets as the bass, springing over them 

 as in the grey mullet, retrograde wriggling as in congers or in some of the 

 pipe-fishes and which is cai-ried out solely by means of the muscular 

 system. 



Animals having such diverse forms of bodies and such variously con- 

 structed fins as exist in fishes have as a consequence their powers of locomotion 

 modified. In such as are enclosed in bony cases as the box-fish or Ostracion 

 voluntary progress can only be made by means of the fins, whereas in almost 

 finless forms, as in some' eels, movements of the body itself must be the 

 motive power. During rapid progression the paired fins are pressed closely 

 against the body, as may be observed in a mullet when pursued, or a 

 gurnard, but which latter widely expands its pectorals when swimming 

 leisurely along. Swimming forwards is principally effected in fishes by means 

 of lateral or right and left strokes of the caudal portion of the body aided 

 by the caudal fin, while gentle progression in the same direction may be 

 effected by slight undulations of the caudal fin-rays. Flat fishes, as* soles, 

 produce a forward motion by vertical or up-and-down strokes of the caudal 

 portion of the body. The flexible bodies of sharks may bend in more than 

 one curve when moving rapidly, which is mainly effected by powerful 

 lateral strokes. Among rays and skates the broad pectoral fins by means 

 of an undulating movement effect this ; while , the angel-fish (plate clxiii), 

 has a sort of mixed motion. In some forms, as the horse-fishes or hippo- 

 campus, the dorsal fin rapidly undulated from end to end after the manner of 

 an archimedean screw, is the chief motive power. In eels, wherein fins 

 are very deficient, forward motion is effected by snake-like curves of the 

 body. 



The pectoral fins are largely employed in keeping the body properly 

 balanced, or raising the head, and when one of these' fins is removed, the 

 body- falls over to that side, or should both be thus treated, the head sinks : 

 it being largely developed in most sharks and rays, often compensating for 

 the absence of an air-bladder. It can assist in a forward movement by rapid 

 strokes towards its body, and likewise in a backward movement by means of 

 reversed or forward strokes. Also, as already observed, it may be enormously 

 developed as in the flying-fish and form a parachute enabling it to pass 

 through the air jn order to escape from its enemies. The vertical fins likewise 

 assist the pectorals in maintaining the proper balance of the body, and incline 

 it to one side when only one fin acts, or they are able to ' depress it by their 

 joint action. 



The dorsal and anal fins are mostly restricted in their functions to increase 

 or diminish the extent of the lateral surface of the fish by means of their 

 being elevated or depressed, and so prevent violent lateral oscillations, or the 



