If we remove one of these Limpets from his selected area of
rock,--which we may readily do, notwithstanding the strength of his
cement, if we take him at unawares, and give him a smart sudden
horizontal rap with a piece of wood, or a moderated blow with a
hammer,--we shall obtain a view of a structure well worth looking at.
The animal is essentially like a Trochus or a Purple inhabiting a
conical shell; only in this case the cone is low and simple, whereas
in the others it is tall and slender, and rolled into a spire. One of
the most curious peculiarities in the Limpet is its gill or breathing
organ. This, we perceive, completely encircles the animal, forming a
ring interrupted only at one point. It lies in the fold between the
mantle and the foot, commencing on the left side of the neck, and
passing quite round the body, parallel with the edge of the shell,
in front of the head, till it terminates close to the point where
it began. It is a long cord closely beset with tiny leaflets, and
thus forming a continual plume. Each leaflet, conical in outline, is
permeated with blood-vessels, and clothed with minute _cilia_, whose
constant vibrations cause the circumambient water ever to play over the
surface of these organs in ceaseless currents, bringing fresh supplies
of oxygen to be respired; and this is absorbed by the blood through the
thin membrane by which they are protected.
There is a very pretty little shell, not uncommon in deep water off
these coasts, but rarely found by the shore collector, though it does
occasionally venture to peep at daylight at the verge of extreme
low-tide. It is the Slit Limpet,[25] which by the older naturalists was
placed in close alliance with the Limpets proper, as if a member of
the same family. They were, however, deceived by paying too exclusive
attention to the form of the shell, which is a cone, somewhat rounded,
and nearly simple, the summit being slightly turned over in a backward
direction. The margin of the shell is delicately notched, the points
being the extremities of the radiating ridges; for the entire surface
is covered with reticulations, one series of alternate furrows and
ridges proceeding from the summit to the margin, and another series
crossing these at right angles, running round the shell parallel with
the margin. The animal has its sides ornamented with short fleshy
processes, and possesses two symmetrical gill-plumes, one on each side.
It is rather attractive in appearance, but I cannot tell you anything
of its manners; for though I have kept specimens in the aquarium,
they are so habitually sluggish, and so reluctant to allow one a peep
beneath the edge of the jealous shell, that I could learn nothing about
their ways;--if indeed they have any.