OF FISHES IN GENERAL* § 



ward, fifhes can, bj extending thefe, flop their motion, 

 and produce mora ;and, wlicn fwimming in a (Ireight di- 

 region, they can, by folding either, while the other- 

 continues to ply, direfb their motion to that fide. The 

 iize of thefe fins, is, in general, proportioned to that of 

 the head of the lifli to wliich they belong ; and it is 

 probably one ufe of them, to prevent the fsili from be- 

 ing overbalanced by the weight of its head, and pre- 

 cipitated to the bottom. In fome fillies, the perioral fins 

 ferve the fame purpofe as the wings of birds ; for, by 

 their means, they are enabled to rife from their watery 

 element, and to fly for a confiderable fpace, till their fins 

 be fo dried by the air, that, in fpitc of every exertion, 

 they again fink into the Avater. 



A filh completely equipped for fwimming, has fcvea 

 fins, two pairs, and three fingle ones ; and of the latter, 

 two are above, and one below : But, fince thofe w^hich 

 have the greateil number cf fins, are not the fwifteft, it 

 has been concluded, that the tall is the principal caufe o£ 

 the fwiftnefs of a fifii's motion. When in purfuit of its 

 prey, or avoiding an enemy, all the fmaller fins are laid 

 clofe to its body ; then, by the impulfe of the tail alone, 

 itfkims through the water with the incredible velocity of 

 a dart or an arrow * The mufcles, by which the ta:i 

 3S moved, are, by far, the thickeil and fiirongeO: of the 

 whole body ; and to give direclion to the great impetus 

 which they communicate to the fifli, feems to be the chief 

 purpofe of all the fmaller fins. 



The motion of fifnes, is fuppofed to be affified by the 

 fwimming bladder, which, by contra6lion and dilatation, 

 fcrves to raife or fink them in the watery element at 

 pleafure ; and, as that element is of very difierent degrees 



Vol. hi. B cf 



* Wllloubii Ich:hyo]. p. 4. 



