Mr. Couch, in his Cornish Fauna, notices the unusual length of the
antennæ. “These organs,” he says, “are of some use beyond their common
office of feelers; perhaps, as in some other crustaceans, they assist
in the process of excavation; and, when soiled by labour, I have seen
the Crab effect their cleaning by alternately bending the joints of
their stalks, which stand conveniently angular for this purpose.
Each of the long antennæ is thus drawn along the brush that fringes
the internal face of the other, until both are cleared of every
particle that adhered to them.” This suggested use of the antennæ
does not seem to me to be a very felicitous guess of the excellent
Cornish naturalist: I should fancy them to be somewhat inefficient
instruments in excavation: perhaps I can help him to a better. I have
observed that, when these Crabs are kept in an aquarium, they are
fond of sitting bolt upright, the antennæ placed close together, and
also pointing straight upward from the head. This is, doubtless, the
attitude in which the animal sits in its burrow, for the tips of the
antennæ may often be seen just projecting from the sand. When the
chosen seat has happened to be so close to the glass side of the tank
as to bring the antennæ within the range of a pocket lens, I have
minutely investigated these organs, without disturbing the old warrior
in his meditation. I immediately saw, on each occasion, that a strong
current of water was continuously pouring up from the points of the
approximate antennæ. Tracing this to its origin, it became evident that
it was produced by the rapid vibration of the foot-jaws, drawing in
the surrounding water, and pouring it off upwards _between the united
antennæ_, as through a long tube. Then, on examining these organs, I
perceived that the form and arrangement of their bristles did indeed
constitute each antenna a semi-tube, so that when the pair were brought
face to face the tube was complete. It is difficult to make this
arrangement intelligible by mere words; but I may say that if either of
the antennæ were broken off in the middle and viewed vertically, the
bristles would be seen to project from each side of the inner face,
in a curved form, each making about a fourth of a circle, so that the
two corresponding bristles enclose, with the body of the antenna,
a semicircle. Of course, those of the opposite antenna make another
semicircle, and, when placed face to face, the points of the bristles
just cross each other, and a circle is enclosed. Now, the whole length
of the antennæ (about an inch and a half) is closely beset with these
bristles, and thus a long row of rings is formed with very narrow
interspaces between them; and these rings do in effect constitute a
tube quite sufficient to retain the stream of water that is poured
through it.
I think then that we may, with an approach to certainty, conclude
that the long antennæ are intended to keep a passage open through the
sand, from the bottom of the burrow to the superincumbent water, for
the purpose of pouring off the waste water, rendered effete by having
bathed the gills: and it is one of those exquisite contrivances and
appropriations of structure to habit which are so constantly exciting
our admiration in the handiwork of the ever blessed God, which cannot
be predicated by the _à priori_ reasoner, however astute, but are ever
rewarding the research of the patient observer.
Our walk along the sands with steady downcast poring gaze suddenly
ends, and we find ourselves among low ledges of black rock (ruddy,
however, in its recent fractures, for it is the old red sandstone),
clad with sweltering weed, and intersected by little sparkling pools
and basins, in which the tiny fishes and entangled prawns shoot
hither and thither at our approach. It is the low-lying ridge in the
midst of the broad sandy bight that I have already spoken of. Well,
_n’importe_; there is plenty of game to be obtained here, and all is
fish that comes to our net. What have we here, creeping over the broad
brown leathery leaf of this _Laminaria_? Is it a little scrap torn
from an old newspaper? It looks like it at the first glance, only that
it moves steadily onward with a smooth gliding motion, which shows
that it possesses a life of its own. Examine it closely: it is exactly
like a bit of white paper, about as large as a rose leaf, and cut into
that shape, only with an even edge, its clear white surface marked all
over with black parallel lines, some thinner, some thicker, running
lengthwise, and as clear as if drawn with a pen. What answers to the
base of the leaf is the head of the creature, the pointed end being the
tail, where the two most strongly marked black lines meet; from the
head end arise two curious ear-like leaflets, which are studded with
crowded black dots, and are thrown back upon the general surface. With
a lens we may discern on the surface of the body, just between these
ear-like tentacles, a group of black specks. These are ascertained
to be veritable eyes, notwithstanding their number, for they have
a cornea, a light-refracting body surrounded with pigment, and a
nerve-bulb.