IX. I 



FUNCTION OF FINS 



245 



In the more specialized teleosteans, however, the pectorals are 

 placed high up on the body and are used as brakes (Fig. 148). The 

 plane of the fins' expansion is vertical and they thus produce a large 

 drag force and a small lift force. This lift, of course, tends to make the 

 fish rise in the water when stopping, and there is also a pitching 



Fig. 148. Use of the paired fins for braking. A. Forces produced by the fins of 

 Lepomis during deceleration. The pectoral and pelvic fin planes are represented by 

 the heavy lines. P and V, the resultant forces on the pectoral and pelvic fin respectively. 

 Dotted line and force P', condition during action of pectoral fins only, pelvic fins being 

 held in 'neutral' position. G, position of centre of gravity. B. Sun-fish stopping by 

 extending pectorals. Pelvic fins amputated. Although body remains horizontal, the fish 

 rises during the stop. c. Front view of sun-fish producing a rolling moment by the action 

 of one pelvic fin. // and /, horizontal and lateral forces. (From Harris,^, exp. Biol. 15.) 



moment, depending on the position of the fin in relation to the centre 

 of gravity, usually positive. That the fish does not rise in the water, or 

 pitch, when it stops is apparently due to the anterior position of the 

 pelvic fins, so characteristic of higher Actinopterygii, which has 

 puzzled many morphologists. Experiments on the sun-fish (Lepomis) 

 have shown that after amputation of the pelvic fins the fish rises in the 

 water when stopping and raises its head (positive pitch). In fact the 

 pelvic fins are able to produce a downward moment and they tilt 

 the nose downwards. By alterations in their position they can be used 

 to control the rising or diving movements and turning one of them 

 outwards produces rolling (Fig. 148). It has been suggested that the 



