Arising posteriorly to the eye, we see, in Scalpellum vulgare, a pair of minute curved horns (b′), directed backwards; and within these horns I distinctly saw an articulated organ. These horns are difficult to be distinguished, and probably could not be made out previously to the first moult, in any larva of less size than that of Scalpellum vulgare. But after the first moult, Mr. Bate has seen, in two species of [Balanus], in [Verruca] and in [Chthamalus] (fig. [10], b), a pair of articulated organs, in this same position, evidently now forming antennæ, and directed anteriorly, and free from any envelope. It is somewhat important, as we shall presently see, to bear in mind that these antennæ first appear within an envelope or horn; and that I detected that they included an articulated organ, before I had heard of Mr. Bate’s observations. These antennæ, from their small size, from being seated internally with respect to the horns containing the other pair of antennæ, and from the position which the latter assume in the later stages of the larva, I believe to be the first or anterior pair. Their position in appearance posteriorly to the large lateral horns, containing the second pair of antennæ, is probably due to the anterior cephalic segments having been driven inwards, the truncated outline of the front of the head, and likewise, probably, the position of the mouth between the bases of the natatory legs being thus caused.
In this same larva of Scalpellum vulgare, within the great lateral horns just alluded to (fig. [8], c), filiform organs, supporting rows of spines, could be distinguished; and these appeared to me to be antennæ. These horns or cases resemble in structure the smaller pair just described; they arise from the ventral surface, and can hardly, therefore, be considered as prolongations of the carapace. After the first moult (fig. [10], c) they are seen to have increased much in length: in some cases they are of considerable length before the first moult, as in Lepas: in the [Balanidæ] they seem to be generally shorter than in the Lepadidæ; but in [Balanus galeatus] I found them one third of the entire length of the animal. Whilst within the egg, these horns are adpressed laterally to the body, and so point posteriorly; afterwards they project rectangularly from the sides, or, as in Scalpellum vulgare, are directed somewhat anteriorly. As in the larvæ of all ordinary Crustaceans, as yet known, the antennæ are amongst the earliest developed organs; and as the first pair of natatory legs (Pl. [29], figs. [8]-[10], e) in these Cirripedial larvæ, might so very naturally be thought to be antennæ (as has been remarked to me by Mr. Dana), both from their structure and from their position a little anteriorly to the mouth, I am well aware that to prove my view correct, namely, that these horns are the second pair of antennæ in process of formation, it is not sufficient merely to have seen organs resembling antennæ within them; nor is it sufficient to advance the strictly analogical fact of the first-mentioned pair of antennæ, which in Scalpellum indisputably appear in their earliest condition within an envelope or horn. Further evidence is required, and this is presented in [Cryptophialus], in which the lateral horns of the egg-like larva, in its first stage (Pl. [24], fig. [16]), can be actually followed step by step until, in the second stage (fig. [17]), just before passing into the pupa, the horns are seen to have become larger and more nearly approximated to each other on the ventral surface; and whilst in this condition, I several times dissected out the prehensile antennæ of the future pupa with every character perfectly recognisable. Hence I cannot doubt that in the larvæ of Cirripedes the law of development is, that in their very earliest condition, the small first pair of antennæ are enclosed in cases; and that the large second pair remains thus enveloped until the pupal stage. This conclusion, we shall immediately see, is in harmony with the late development of the succeeding appendages or organs of the mouth, which certainly do not appear in the first larval stage, and are not known to appear even till after the final metamorphosis.[53]
[53] According to M. Joly, (‘Annales des Sciences Naturelles,’ 2d series, tom. xix, p. 59) in the larva of the macrourous Caridina, the natatory legs appear before the gnathites or parts of the mouth; so that in ordinary Crustaceans there is no invariable order of development from the anterior towards the posterior end of the body, as has sometimes been supposed.
The mouth is more or less probosciformed (Pl. [29], figs. [8]-[10], d), differing considerably in this respect in different species of the Lepadidæ; and this, probably, is due to the larva being born in a more or less mature condition. Its exact position likewise varies, for it arises either between the first or second pairs of natatory legs. It is known, from Mr. Bate’s observations, to have the power of movement. It is directed posteriorly, the œsophagus extending anteriorly; both these directions being the same as in the mature cirripede. Certainly during these early stages there are no jaws or gnathites; but the margin, answering to the labrum, is furnished with some short, thick, sharp spines, and with hairs. In Scalpellum vulgare the orifice of the œsophagus seems to lie rather beneath the upper prominent spinose edge, which, as just remarked, probably answers to the labrum; but this is one of the species in which the probosciformed mouth, at least before the first moult, is not much developed.
We come, now, to the three pairs of natatory legs: the first (Pl. [29], figs. [8]-[10], e) has throughout the order only one ramus, whereas the two succeeding pairs (f, g) are biramous. I must here remark that the straight and strong, and the curved plumose spines, with which these limbs, after the first moult, become furnished, now appear to me as more probably prehensile, rather than masticatory as I imagined in my former volume. That these spines are important organs to the larvæ I do not doubt. With regard to the homologies of these three pairs of limbs, my first impression was that they were the mandibles and the two pairs of maxillæ in their earliest condition; but I consider this view as quite untenable, for several reasons; viz., the wide interval between their bases and the mouth itself,—the somewhat variable position of the mouth with respect to the legs,—and the position which the latter occupy in the second larval stage.[54] A far more tenable view is that these three pairs of legs are the three pairs of maxillipeds, in their earliest condition, in accordance with the view of M. Joly[55] on the nature of the three very similar pairs of natatory legs in the larva of Caridina, a macrourous Crustacean. But, in Cirripedes, the three pairs of natatory legs, in the larva in the first stage, are apparently the very same as the first three pairs, in the larva in the second stage, and in the pupa. And in the pupa the first three pairs, which certainly correspond with the first three pairs of cirri in the mature animal, seem to me, for reasons presently to be assigned, to be the second, third, and fourth thoracic limbs. Hence I am led to the conclusion that the first pair of legs in the larva in the first stage, are homologically the second thoracic (answering to the third pair of maxillipeds in the higher Crustaceans), and that the two succeeding pairs are the third and fourth thoracic limbs; to be succeeded, in the pupal stage, by the fifth, sixth, and seventh thoracic appendages.
[54] Mr. Dana, moreover, has remarked, (‘Crustacea: United States Exploring Expedition,’ p. 1386), “that he knows of no instance of a mandible becoming so completely a leg, as to lose wholly the mandibular function even of its basal portion.”
[55] ‘Annales des Sciences Naturelles,’ 2d series, tom. xix, 1843, p. 34. M. Joly’s observations were made on the Caridina. I owe to the great kindness of Mr. C. Spence Bate, an examination of some larvæ of the allied genus Hippolyte varians, and I found, on dissection, the view of M. Joly, that the three pairs of natatory legs are the maxillipeds, so far strongly confirmed, that they followed closely, with equal intervals, the mandibles and two pairs of maxillæ. The first pair of natatory legs in Caradina, in its earliest condition within the egg, is uniramous, like the first pair in the larvæ of Cirripedes. There is one fact which seems rather strongly opposed to the view of these three pairs of legs in the larvæ of the macrourous Crustaceans being the maxillipeds, which is that Capt. Du Cane (‘Annals of Nat. Hist.,’ 1838, vol. ii, pl. 6, and 7) observed only three pairs of limbs in process of formation posteriorly to the first three pairs, whereas there should be found, in accordance with M. Joly’s view, five pairs, i. e. all five pairs of ambulatory legs. This one fact countenances the view, which I hold on the nature of the legs in the larvæ of Cirripedes during their early stages, namely, that they are the second, third, and fourth thoracic limbs, to be succeeded by only three additional pairs.
Lastly, behind the natatory legs, on the ventral surface, (Pl. [29], figs. [8], [9], i), the body is much produced, and terminates in a horny fork, which, after the first moult (fig. [10], i), becomes much elongated. Anteriorly to this fork, on the ventral surface, there is another fork (l), and again above this I could distinguish, in [Chthamalus stellatus], after the first moult, another fork (m), or at least a pair of short thick spines. From the structure of the forked abdomen in the known larvæ of the Podophthalmia, I presume that this portion of the body is the abdomen of the young Cirripede, but it is not at all plainly articulated. After the first moult, the posterior end of the carapace (h), which is always pointed, becomes much elongated and serrated on both sides;[56] reminding one of the structure of the carapace of the so-called Zoea, or larva of certain Podophthalmia. Situated under this posterior prolongation of the carapace, there is a swelling (n, with long hairs on both sides), which apparently lies on the dorsal surface of the spinose and forked abdomen; here, when the larva is compressed, the cellular and oily contents of the body burst forth; and I suspect that this swelling is the anus, for it is known from the researches of Rathke,[57] that the anus in the higher Crustaceans opens during the earliest periods dorsally.
[56] I suspect that the account given by Goodsir (‘Edinburgh New Phil. Journal,’ 1848) of the posterior points of the carapace and abdomen in the larva of a [Balanus], is not quite accurate.
[57] ‘Annales des Scienc. Nat.,’ tom. xx, p. 451.