The mouth in its earliest condition alone remains to be described; in S. vulgare, it is seated on a very slight prominence, in a most remarkable situation, namely, in a central point between the bases of the three pairs of legs. I traced by dissection the œsophagus for some little way, until lost in the cellular and oily matter filling the whole animal, and it was directed anteriorly, which is the direction that might have been expected, from the course followed by the œsophagus in the larva in the last stage, and in mature Cirripedes. Mr. A. Hancock has called my attention to a probosciformed projection on the under side of the larva of [Lepas fascicularis], when just escaped from the egg. Mr. Bate has described this same proboscis in Balanus and Chthamalus, and states the important fact, that it is capable of being moved by the animal; and, lastly, I have seen it in an Australian Chthamalus, and in Ibla, of remarkable size. This proboscis, which is always directed posteriorly, (like the mouth in the mature animal,) certainly answers to the mouth as made out by dissection in Scalpellum; and I believe I saw, as has Mr. Bate, a terminal orifice: it certainly does not possess any trophi. In Ibla (in which the larva is large enough for dissection), the base of the proboscis arises posteriorly to the first pair of legs, and the orifice at the other end reaches beyond or posteriorly to the point, where the mouth in Scalpellum opens, namely between the middle pair of legs. The mouth being either so largely probosciformed or seated only on a slight eminence, in two genera so closely allied as Ibla and Scalpellum, and (judging from Mr. Thompson’s figures, and from what I have seen myself,) in the species of the same genus Lepas, is a singular difference: in the cases in which, at first, the proboscis is absent, it would probably soon be developed. I cannot but suppose that the inwardly directed spines on the bases of the two posterior legs, which are so rapidly developed, serve some important end, namely, as organs of prehension for the larvæ, like the mandibles and maxillæ of mature Cirripedes, for seizing their prey, and conveying it to their moveable mouths, conveniently seated for this purpose.

The first pair of legs answers, as I believe from reasons hereafter to be assigned, to the outer pair of maxillipods in the higher crustacea; and the other four legs to the first two pair of thoracic limbs in these same crustacea; this being the case, the highly remarkable position of the mouth in the larva, either between the bases of the two posterior pair of legs, or at least posteriorly to the first pair, together with the probable functions of the spiny points springing from the basal segments of the two hinder pair of true thoracic limbs, forcibly bring to mind the anomalous structure of the mouth being situated in the middle of the under side of the thorax, in Limulus,—that most ancient of crustaceans, and therefore one likely to exhibit a structure now embryonic in other orders. I will only further remark, that I suspect that the truncation of the anterior end of the carapace, has been effected by the segments having been driven inwards, and consequently, that the larger antennæ within the lateral horns, though standing more in front than the little approximate pair, are normally the posterior of the two pair. According to Milne Edwards, the posterior pair are normally seated outside the anterior pair, and this is the case with those within the lateral horns.

Larva in the Second Stage.—Notwithstanding the considerable changes, already briefly given, which the larva undergoes during the first two or three exuviations after leaving the egg, all these forms may be conveniently classed under the first stage. The larva in the Second stage is known only from a single specimen described, figured, and found by Burmeister,[8] adhering to sea-weed in the midst of other larvæ of Lepas in the last stage. In its general shape and compressed form, it seems to come nearer to the last than to the first stage. It has only three pair of legs, situated much more posteriorly on the body than in the first stage, and all directed posteriorly; they are much shorter than heretofore, and resemble rather closely those of the last stage, with the important exception that the first pair has only one ramus. It is this circumstance which leaves no doubt on my mind, that we here have the three pair of limbs, of the first stage, metamorphosed. The body is prolonged some way behind these limbs, and ends in a blunt, rounded point, in which, probably, are developed the three posterior pair of legs and the abdomen of the larva in the last stage. The mouth is now seated some way anteriorly to the limbs, is large and probosciformed, and is, I presume, still destitute of trophi. There are now two closely approximate eyes, but as yet both are simple. The smaller pair of antennæ has disappeared. The whole animal was attached to the sea-weed by a (I presume, pair of,) “fleischigen Fortsatz,” which Burmeister considers as the prehensile antennæ, to be presently described, in an early state of development. I have little doubt that this is correct, for in an abnormal Cirripede of another order, in which the larva appears in the first stage with prehensile antennæ, the eggs have two great projecting horns including these organs, and attached by their tips, through some unknown means, to the sack of the parent, apparently in the same manner as Burmeister’s larva was attached to the sea-weed. I will only further remark on the larva of this Second stage, that its chief development since the first stage, has been towards its anterior end. The next great development, to be immediately described, is towards the posterior end of the animal.

[8] Beiträge zur Naturgeschichte der Rankenfüsser, s. 16, Tab. i, figs. 3, 4.

Larva, Last Stage.—My chief examination has been directed, at this stage of development, to the larvæ of [Lepas australis], which are of unusual size, namely, from .065 to even almost .1 of an inch in length; I examined, however, the larvæ of several other species of Lepas, of Ibla and of Balanus, with less care, but sufficiently to show that in all essential points of organisation they were identical; this, indeed, might have been inferred from the similarity of the larval prehensile antennæ, preserved in the bases of all mature Cirripedes, and which I have carefully inspected in almost every genus. The larvæ in this final stage, in most of the genera, have increased many times in size since their exclusion from the egg; for instance, in [Lepas australis], from .007 to .065, or even to .1 of an inch. They are now much compressed, nearly of the shape of a cypris or mussel-shell, with the anterior end the thickest, the sternal surface nearly or quite straight, and the dorsal arched. Almost the whole of what is externally visible consists of the carapace; for the thorax and limbs are hidden and enclosed by its backward prolongation; and even at the anterior end of the animal, the narrow sternal surface can be drawn up, so as to be likewise enclosed. As in several Stomapod crustaceans, the part of the head bearing the antennæ and organs of sense, in front of the mouth, equals, or even exceeds in length, and more than exceeds in bulk, the posterior part of the body, consisting of the enclosed thorax and abdomen. I will now briefly describe, in the following order, the carapace, the organs of sense, mouth, thorax and limbs, abdomen, and internal viscera.

The form of the Carapace has been sufficiently described; it consists of thick chitine membrane, marked with lines, and sometimes with stars and other patterns; it is obscurely divided into two halves by a line or suture along part of the dorsal margin; these halves or two valves are drawn together by an adductor muscle, in the same relative position as in the mature Cirripede. The part overhanging and enclosing the thorax is lined by an excessively delicate membrane, obviously homologous with the lining of the sack in the mature animal, and is nothing but a duplicature of the carapace, rendered very thin from being on the under or protected side: a layer of true skin or corium, probably double, separates these two folds.

Acoustic Organs.—On the borders of the carapace, at the anterior end, on the sternal surface, there are two minute orifices, in L. australis .002 in diameter, sometimes having a distinct border round them; the membrane of the carapace on the inside is prolonged upwards and inwards in two short funnel-shaped tubes, lodged in closed sacks of the corium: within these sacks on each side a delicate bag is suspended, and hangs in the mouth of the above funnel; at the upper end a large nerve could be distinctly seen to enter the bag: I cannot doubt that this is a sense-organ; from its position and from the animal not feeding (as we shall presently see), I conclude that it is an acoustic organ.

Antennæ.—These are large and conspicuous; they are attached very obliquely on the sternal surface, a little way from the anterior end of the carapace, beyond which, when exserted, they extend;[9] they can (at least in Ibla) be retracted within the carapace. They consist of three segments: the first or basal one is much larger than the others, and apparently always has a single spine on the outer distal margin. The second segment consists either of a large, thin, circular, sucking disc, or is hoof-like ([Tab. V], [figs. 5, 10, 11, 12]); in all cases it is furnished with one or more spines, (seven very long ones in Lepas,) on the exterior-hinder margin. The third and ultimate segment is small; it is articulated on the upper surface of the disc, and is directed rectangularly outwards; it is sometimes notched, and even shows traces of being bifid; it bears about seven spines at the end; some of these spines are hooked, others simple, and in Lepas and Conchoderma, two or three are very long, highly flexible, and plumose, a double row of excessively fine hairs being articulated on them. I can hardly doubt that these latter spines, (within which the purple corium could be seen to enter a little way,) floating laterally outwards, serve as feelers. The antennæ, at first, are well furnished with muscles. They serve, in Lepas, according to Mr. King, and in Balanus, according to Mr. Bate, and as I saw myself in another unnamed order, for the purpose of walking, one limb being stretched out before the other; but their main function is to attach the larva for its final metamorphosis into a Cirripede. The disc can adhere even to so smooth a surface as a glass tumbler.[10] The attachment is at first manifestly voluntary, but soon becomes involuntary and permanent, being effected by special and most remarkable means, which will be most conveniently described in a later part of this Introduction. I will here only state that I traced with ease the two cement-ducts running from two large glandular bodies, to within the antennæ up to the discs.

[9] Mr. J. D. Dana, who has examined these organs in the larvæ of Lepas, informs me in a letter, that in his opinion they “correspond with the inferior antennæ, the superior being wanting, as in most Daphnidæ.” He continues—“I know of no case in which the inferior are obsolete when the superior are developed; but the reverse is often true.” In position these antennæ certainly correspond to the inferior and central pair of the larva in the first stage, which belong, as it would appear, to the first segment of the body; but judging from the drawing by Burmeister of the larva in the second stage, I am, in some respects, more inclined to consider that they correspond to the larger pair seen within the lateral horns of the carapace in the first stage.

[10] Rev. B. L. King. Annual Report of B. Institution of Cornwall, 1848, p. 55.