Here are the familiar Limpets, too: let us look at _them_ awhile.[24]
They are not generally very attractive in appearance, the shell
being coarse and rubbed, especially in the larger specimens; and in
an aquarium they do not live long, and are so inert as to afford no
amusement even while they survive. Yet we occasionally find examples
prettily coloured; and there are facts in their economy which make them
worthy of a few moments’ notice.
If you look carefully over the rocks, especially when these are of a
somewhat soft nature, as the slates and shales, you will find oval
depressions, sometimes but just discernible, at other times sunk to the
depth of an eighth of an inch, corresponding in outline to the shell of
a Limpet; and in many instances you will actually see a Limpet imbedded
in such a pit, which it accurately fills. Strange as it may seem, it
has been ascertained that these cavities are formed by the animals,
which make them their ordinary resting-places, wandering away from them
nightly to feed, and returning to them to rest early in the morning.
The force with which a Limpet adheres to the rock is very great,
especially when it has had warning of assault, and has had time to put
out its muscular strength. Réaumur found that a weight of twenty-eight
or thirty pounds was required to overcome this adhesive force. His
experiments seem to prove, however, that its power is mainly owing, not
to muscular energy, nor to the production of a vacuum in the manner of
a sucker. If an adhering Limpet were cut quite through perpendicularly,
shell and animal, the two parts maintained their hold with unabated
force, although of course a vacuum, if there had been one, would have
been destroyed by the incision. The power is said to reside in a very
strong glue, a very viscid secretion, deposited at the will of the
animal. “If, having detached a _Patella_,” says Dr. Johnston, “the
finger be applied to the foot of the animal, or to the spot on which it
rested, the finger will be held there by a very sensible resistance,
although no glue is perceptible. And it is remarkable that if the spot
be now moistened with a little water, or if the base of the animal be
cut, and the water contained in it allowed to flow over the spot, no
further adhesion will occur on the application of the finger: the glue
has been dissolved. It is nature’s solvent, by which the animal loosens
its own connexion with the rock. When the storm rages, or when an enemy
is abroad, it glues itself firmly to its rest; but when the danger has
passed, to free itself from this forced constraint, a little water is
pressed from the foot, the cement is weakened, and it is at liberty
to raise itself and be at large. The fluid of cementation, as well as
the watery solvent, is secreted in an infinity of miliary glands with
which the foot is, as it were, shagreened; and as the Limpet cannot
supply the secretion as fast as this can be exhausted, you may destroy
the animal’s capacity of fixation by detaching it forcibly two or three
times in succession.”