Each individual consists of a body enclosed in a shell, not unlike that of a mussel in figure, and of a fleshy worm-like stem or peduncle, the extremity of which is fixed to the object upon which the animal is stationed. This stem is tubular, tolerably firm, and has a fleshy feel; it is composed exteriorly of a fine coriaceous outer membrane, bedewed with a watery fluid, and beneath this, of an inner membrane of considerable density, apparently consisting of muscular fibres, running for the most part in
parallel longitudinal lines. That these fibres are muscular we may conclude from the animal having the power of contracting the stem, or of twisting it in various directions. Within the tube there is a fluid secretion.
The shell is composed of five pieces, four of which are lateral, two on each side; while between the valves down the back is interposed a single narrow slip uniting them together. Their colour is white, more or less tinged with purplish blue. Along the anterior margin the valves are but partially connected by a membrane, leaving a large fissure, through which emerge the plumose and jointed arms or cirrhi.
The food of the Barnacles consists of small crustacea and mollusca. These are entangled by the many-jointed plumose cirrhi, which are perpetually thrown out and folded again, so as to serve the purpose of a casting-net, and drag the prey to the mouth, where it is seized and crushed.
With regard to the reproduction of these creatures, the eggs are seen enclosed at certain times within the hollow of the peduncle, where they appear of a blue colour, and render the pedicle opaque; from this they pass through a minute conduit into the cavity of the mantle, where they are arranged like two leaflets, attached to the septum between the body and the peduncle. They are enclosed in a film, out of which they fall when ready to hatch.
It is a remarkable fact, as we learn from Mr. Thompson[147], that the young barnacles and other cirrhipeda
on emerging from the egg are quite free, and very different from their parents. “They possess locomotive organs, consisting of a large anterior pair of limbs, provided with a sucker, and hooks for the purpose of mooring themselves at pleasure to various objects—and also of six pairs of swimming-limbs, acting in concert like oars. Besides these, they have a tail bent under the body, consisting of two joints and terminating in four bristles: this is an additional locomotive organ. Thus endowed, they swim along in a series of bounds, the oars and tail giving in measured time successive impulses. They have, moreover, large lateral eyes set on peduncles, and the body is covered with a sort of shell, as in certain crustacea (e.g. Cyclops), which they closely resemble,” and for which Mr. Thompson at first mistook them.
In due time a metamorphosis takes place; the shell is thrown off, the eyes disappear, the limbs become transformed to cirrhi, the regular valves develop themselves, the peduncle shoots forth, and the animal becomes permanently fixed.
Believing these little creatures to be the larvæ of some crustaceous animal, some of them, says Mr. Thompson, were collected in the spring, and in order to see what changes they might undergo, were kept in a glass vessel, covered by such a depth of sea-water, that they could be examined at any time by means of a common magnifying-glass. They were taken May 1st, and on the night of the 8th the author had the satisfaction to find
that two of them had thrown off their exuviæ, and, wonderful to say, were firmly adhering to the bottom of the vessel, and changed to young barnacles. In this stage the sutures between the valves of the shell and of the operculum were visible, and the movements of the arms of the animal within, although these last were not completely developed: the eyes also were still perceptible, although the principal part of the colouring-matter appeared to have been thrown off with the exuviæ. On the 10th another individual was seen in the act of throwing off its shell, and attaching itself as the others to the bottom of the glass. It only remains to add, that as the secretion of the calcareous matter goes on in the compartments destined for the valves of the shelly covering, the eyes gradually disappear, from the increasing opacity thence produced, and the visual ray is extinguished for the remainder of the animal’s life; the arms at the same time acquire their usual ciliated appearance. Thus, then, an animal originally natatory and locomotive, and provided with a distinct organ of sight, becomes permanently and immovably fixed, and its optic apparatus obliterated; and furnishes not only a new and important physiological fact, but is the only instance in nature of so extraordinary a metamorphosis.