44 FISHES. 



tlie Acantliopterygian affinities of a fish can nearly always be 

 determined. The numbers of the dorsal and anal rays gene- 

 rally correspond to the number of vertebrae in a certain 

 portion of the spine, and are therefore constant specific, 

 generic, or even family characters; but when their number 

 is very great, a proportionally wide margin must be allowed 

 for variation, and the taxinomic value of this character be- 

 comes uncertain. The numbers of the pectoral and caudal 

 rays are rarely of any account. 

 Function The fins are organs of motion ; but it is chiefly the tail 



Z.- ^ and the caudal fin by which the fish impels itself forward. 

 To execute energetic locomotion the tail and caudal fin are 

 strongly bent, with rapidity, alternately towards the right 

 and left ; whilst a gentle motion forwards is effected by a 

 simply undulating action of the caudal fin, the lobes of which 

 act like the blades of a screw. Eetrograde motions can be 

 made by fish in an imperfect manner only, by forward-strokes 

 of the pectoral fins. When the fish wants to turn towards the 

 left, he gives a stroke of the tail towards the right, the right 

 pectoral acting sunultaneously, whilst the left remains ad- 

 pressed to the body. Thus the pectoral fins assist in the 

 progressive motions of the fish, but rather directing its course 

 than acting as powerful propellers. The cliief function of the 

 paired fins is to maintain the balance of the fish in the water, 

 which is always the most unsteady where there is no weight 

 to sink it : when the pectoral of one side, or the pectoral and 

 ventral of the same side are removed, the fish loses its balance 

 and falls on the side opposite ; when both pectorals are re- 

 moved, the fish's head sinks ; on removal of the dorsal and 

 anal fins the motion of the fish assumes a zig-zag course. A 

 fish deprived of all fins, as well as a dead fish, floats with the 

 belly upwards, the back being the hea\der part of the body. 



In numerous groups of fishes which live in mud, or are 

 enabled to pass a longer or shorter time in soil periodi- 



