The organs which he employs for this end are the outer foot-jaws or
pedipalps, which are of unusual length, and are fringed with incurving
hairs. Watching a Flat-crab beneath a stone close to the side of my
tank, I noticed that his long antennæ were continually flirted about;
these are doubtless sensitive organs of touch, or some analogous sense,
which inform the animal of the presence, and perhaps of the nature,
of objects within reach. At the same time I remarked that the outer
foot-jaws were employed alternately in making casts; being thrown out
deliberately, but without intermission, and drawn in, exactly in the
manner of the fringed hand of a barnacle, of which both the organ and
the action strongly reminded me. I looked at this more closely with
the aid of a lens; each foot-jaw formed a perfect spoon of hairs,
which at every cast expanded, and partly closed. That this may be
better understood, I may say that the foot-jaw resembles a sickle in
form, being composed of five joints, of which the last four are curved
like the blade of that implement. Each of these joints is set along
its inner edge with a row of parallel bristles, of which those of the
last joint arch out in a semicircle, continuing the curve of the limb;
the rest of the bristles are curved parallel or concentrical with
these, but diminish in length as they recede downwards. It will be
seen, therefore, that when the joints of the foot-jaw are thrown out,
approaching to a straight line, the curved hairs are made to diverge;
but as the cast is made, they resume their parallelism, and sweep-in,
as with a net, the atoms of the embraced water. The microscope revealed
to me a still higher perfection in this admirable contrivance. I then
saw that every individual bristle is set on each side with a row of
short stiff hairs, projecting nearly at right angles to its length;
these hairs meeting point to point those of the next bristle, and so on
in succession, there is formed a most complete net of regular meshes,
which must enclose and capture every tiny insect or animalcule that
floats within its range; while, at each out-cast, it opens at every
mesh, and allows all refuse to be washed away or fall to the ground.
For we are not to suppose that the captures thus promiscuously made
are as indiscriminately swallowed. A multitude of atoms are gathered,
which would be quite unfit for food; and a power of selection resides
in the mouth, whether it be the sense of taste or touch; or any other
analogous but recondite perception, by which the useful only is
admitted, the worthless, or at least the injurious, being rejected.