XIII LOCOMOTION 353 



Many Fishes can jump out of the water, either in pursuit of 

 insect food, like the Trout, or to enable them to escape the pur- 

 suit of their foes, like the Flying- Fish (Uxocoetus), by means of 

 a single forcible stroke of the tail, when the Fish is in a nearly 

 vertical position close to the surface of the water. It is thus 

 that the Salmon executes its remarkable leaps over weirs or up 

 salmon-ladders when ascending rivers for spawning. 



The tail is also used for steering. If kept bent to one side when 

 the Fish is moving the tail acts like a rudder, and the course of 

 the Fish is deflected to that side ; or the direction may be altered 

 by single strokes of the tail to the right or left, according to the 

 course which the Fish desires to pursue. 



In the majority of Fishes the paired fins are probably of 

 little use for propulsion, and their action in this as in other 

 functions is not always clear. In the Sharks and Dog-Fishes as 

 well as in some Teleosts their planes are nearly horizontal when the 

 fins are extended from the body ; in others they are more oblique, 

 so that the surfaces of the fins look upwards and backwards, and 

 downwards and forwards ; and in others again their surfaces are 

 so nearly vertical that their strokes will be backwards and for- 

 wards. The pectoral fins also vary in their position on the sides 

 of the body, being much more dorsal in some Fishes than in 

 others. The paired fins may act as lateral keels in steadying the 

 course of the Fish especially when the fins are extended and 

 their planes are horizontal. They certainly seem to q,ct as 

 balancers in keeping the Fish on an even keel, and in counter- 

 acting the tendency of the Fish to turn belly upwards — a result 

 which is attained by a slight upward and downward movement 

 of the fins, and particularly of the pectoral fins. A Fish deprived 

 of its pectoral members sinks downwards at the head and assumes 

 an oblique position in the water. Removal of both the pectoral 

 and pelvic fins of one side causes the Fish to roll over to that 

 side ; and if the fins are removed from both sides the animal 

 turns belly upwards like a dead Fish. The pectoral fins may 

 also be used for steering : a backward stroke of one fin while the 

 other is kept folded back against the body will wheel the Fish 

 round to the opposite side. From the ventral position of its 

 mouth a Shark is forced to turn over to one side in order to 

 seize its prey, and this movement of rotation is probably pro- 

 duced by the down strokes of the pectoral fin of one side. - In 



VOL. VII 2 A 



