356 Miscellaneous. CHAP. xxv. 



of urging the body forwards not only ceases, but during its flexion on the 

 opposite side in the line a 0, it tends to draw the body backward in the 

 direction o e. Having reached the point o it is again rapidly extended in 

 the line o e, causing an impulse on the centre of gravity in c b parallel to 

 o e. If the two forces c h and c b acted simultaneously we should obtain the 

 resultant cf\ but as they do not, the point (c) will not move exactly in the 

 right line c/, but in a curved line which lies evenly between d cf and a line 

 drawn parallel to it through h. The fish being in motion while the tail 

 moves from side to side, according to Borelli, it describes an ellipse instead 

 of a circular arc, which would be the case if the body were stationary and 

 the tail only moving. The velocity with which fishes move, and the con- 

 tinuance of their movements, are enough to give us an idea of the great 

 strength of their muscles, especially when we reflect on the density of the 

 fluid which is opposed to their speed." 



This is doubtless the manner in which the tail is used when the 

 fish swims rapidly forward in a straight line. When it wishes to turn 

 abruptly to any side, successive strokes of the tail are made on one 

 side only, and the body curled round as much as possible, and the 

 pectoral fin on the inside of the curve is, I think, thrust out, and the one 

 on the outside of the curve worked. The pectoral fins are certainly 

 used for turning slowly, but the tail seems to be the great motive power 

 when turning rapidly, as, for instance, when passing a fleeing fish and 

 turning round so as to take it in head foremost. Every ray in the 

 tail, and in any other fin, is under as much separate command as each 

 toe in a duck's foot, and in drawing up the tail for a blow, the fish 

 can contract the tail as a duck does its webbed foot, expanding it 

 again for the blow ; or it can shape and use it like the cross-fanned 

 screw of a steam vessel. As the ventral fins enable fish to maintain 

 a horizontal position in the water so does the dorsal fin. I have 

 noticed a goldfish which had no dorsal fin, and though it had been 

 hatched thus it could not well command its position in the water, 

 but rolled slightly. Goldfish are unusually subject to deformity. The 

 pectoral fins are used to swim slowly backwards. The anal fin also 

 seems to be used in maintaining the horizontal. It and the dorsal 

 fin are much prolonged in the murral, which, in accordance with its 

 bottom-seeking habits, has not so much of the ordinary compressed 

 shape of a fish, but, being more than ordinarily depressed, seems to 

 need this additional provision. It is the same with the extensive anal 

 fin of the bottom-feeding Wallago attu (Freshwater Shark). 



