XII LOCOMOTION 351 
forced through the water. It is obvious, however, that the 
extension or effective stroke must have a considerable surplus of 
power over the flexion or non-effective stroke, and how this result 
is achieved will now be briefly considered. Experiment, and the 
observation of Fishes like the Sturgeon, which habitually move 
with sufficient slowness to allow the phases of their swimming 
movements to be followed without much difficulty, show that in 
swimming a Fish throws its body into two opposite and comple- 
mentary curves, a cephalic curve formed 
by the anterior half of the body and a 
caudal curve by the tail. The double 
curve enables the Fish always to present 
a convex, less resisting or non-biting sur- 
face to the water during the flexion of 
the tail to the right or left as the case 
may be, and a concave or biting surface 
during extension, that is when the tail is 
straightening itself during the effective 
stroke. 
Fig. 204, which represents a Fish in 
two successive positions while swimming, 
will serve to illustrate these conclusions. 
A Fish in the position A has its body 
thrown into a cephalic concavity directed 
to the right and a caudal concave surface 
facing the left. The tail is bent to the 
right of the line a b, which corresponds ot NE a ae 
to the axis of the Fish when at rest and Sod e idee hich thes talon 
to the course pursued by the animal = SE eee ae 
when swimming, and is in the position text for the lettering. 
which it assumes during a flexion stroke, cere tale fron 
with its convex non-biting surface directed yo 
outwards and its concave biting surface inwards, The tail is 
now ready for an extension stroke, and while this is in 
progress it is clear that the concave biting surface of the tail 
will meet the water, while at the conclusion of the stroke 
the tail will be in a line with a b. At the same time the 
cephalic curve has so far diminished that the long axis of 
the body for a momentary period will also coincide with a }, 
and the Fish is free to advance without impediment. The tail, 
