On The Gait Of Animals   

bending one, extend the other leg simultaneously, so as to incline

forward and make a stride and still remain above the perpendicular;

for the legs form an isosceles triangle, and the head sinks lower when

it is perpendicularly above the base on which it stands.

Of limbless animals, some progress by undulations (and this

happens in two ways, either they undulate on the ground, like

snakes, or up and down, like caterpillars), and undulation is a

flexion; others by a telescopic action, like what are called

earthworms and leeches. These go forward, first one part leading and

then drawing the whole of the rest of the body up to this, and so they

change from place to place. It is plain too that if the two curves

were not greater than the one line which subtends them undulating

animals could not move themselves; when the flexure is extended they

would not have moved forward at all if the flexure or arc were equal

to the chord subtended; as it is, it reaches further when it is

straightened out, and then this part stays still and it draws up

what is left behind.

In all the changes described that which moves now extends itself

in a straight line to progress, and now is hooped; it straightens

itself in its leading part, and is hooped in what follows behind. Even

jumping animals all make a flexion in the part of the body which is

underneath, and after this fashion make their leaps. So too flying and

swimming things progress, the one straightening and bending their

wings to fly, the other their fins to swim. Of the latter some have

four fins, others which are rather long, for example eels, have only

two. These swim by substituting a flexion of the rest of their body

for the (missing) pair of fins to complete the movement, as we have

said before. Flat fish use two fins, and the flat of their body as a

substitute for the absent pair of fins. Quite flat fish, like the Ray,

produce their swimming movement with the actual fins and with the

two extremes or semicircles of their body, bending and straightening

themselves alternately.