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, hke the Flying-Fish (Zxocoetus), 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 act 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 shght 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 2A 
