As after the most careful and repeated examinations of various Lepadidæ, I was convinced that there were no oviducts, so I have come to a similar conclusion in regard to the [Balanidæ]; the ova being brought to the surface, by the formation of a new membrane round the sack underneath them, and by the subsequent exuviation of the old membrane. The ova are united together by a most delicate tunic investing each egg; the ovigerous lamellæ being thus formed, as in the Lepadidæ. In the cases of [Chthamalus stellatus], [Balanus balanoides], and [Platylepas decorata], I saw a pair of very distinct but fragile lamellæ. In [Xenobalanus], the two ovigerous lamellæ form two sub-cylindrical packets, pointed at their lower ends and often cohering. There are no ovigerous fræna, for the attachment of the lamellæ; the ova being sufficiently well retained, as it would appear, by the well-closed shell. I have elsewhere stated my full belief that it is the ovigerous fræna which have been metamorphosed into the branchiæ of the [Balanidæ]. Most sessile cirripedes breed when very young; and I have every reason to believe that they breed several times in the year. The ova are ovate, and vary in length from 14/2000th of an inch in [Chthamalus], to 19/2000th in some species of [Balanus], in which this greater length was owing to a more elongated shape,—up to 25/2000th in some other species of [Balanus]. The ova are wonderfully numerous, especially in the genus [Coronula].
I may here mention the singular case of some elongated specimens of [Balanus balanoides], from Tenby, in South Wales: some of these presented nothing abnormal; but in no less than seven specimens, the two, three, or four posterior pairs of cirri, either on one or both sides, were in an almost rudimentary condition, being of small size and having a shrunk and wasted appearance. In six out of these seven specimens, the probosciformed penis was quite short and abruptly truncated, as if from abortion. By cutting off the truncated apex, and cleaning the external tissue, I ascertained that it was imperforate, apparently in all the cases, and I am certain of this fact in several of the cases. In three of the specimens, I examined the vesiculæ seminales; in one, I found some spermatozoa, but cohering together in a peculiar manner; in the second, there were no spermatozoa; and in the third, the vesiculæ were shrunk, empty, and quite rudimentary in size. So that these three individuals certainly could not have impregnated their own eggs; nevertheless, within the shell of these very three, there were perfectly developed larvæ: I am led to conclude from this fact, that adjoining specimens in a perfect condition had, by means of their long probosciformed penis, effected the fecundation of their imperfect neighbours. I need only further add, that some out of the above six specimens, with more or less aborted cirri and imperforate male organs, were infested by a peculiar parasite, allied to Bopyrus,[50] and that these specimens did not contain ova.
[50] I have given a short notice on this parasite, in my former volume on the Lepadidæ, in a foot-note to p. 55.
Metamorphoses and Homologies, throughout the Order of [Thoracica].
In my former volume, the metamorphoses were described under three principal stages or heads; but whether these three included all the main changes, I was then hardly able to conjecture. But now I have reason to believe that such is the case, for in the genus [Cryptophialus], belonging to the [Abdominalia], the whole course of the metamorphosis, from the egg to the pupa, takes place within the sack of the parent; and I found, when having, on the coast of South America, numerous specimens to examine, that the egg-like larvæ (Pl. [24], fig. [15]-[18]) could be naturally grouped into two main stages, but with many transitional intermediate grades (answering to the successive moults in the first stage of ordinary larvæ), before they passed into the third or pupal stage. And the first two stages in these egg-like larvæ of [Cryptophialus], clearly seem to correspond with the first two stages in ordinary larvæ; for in both the chief changes are, the shortening of the terminal projection—the increase in size and approximation on the ventral surface of the anterior horns or cases for the antennæ—and the compression of the whole body. In all members of the [Thoracica], the metamorphosis seems to run a remarkably uniform course. The larva in the first stage undergoes several moults and slighter changes—how many is not known—before arriving at its second main stage, which has been observed only in one single instance; and judging from [Cryptophialus], this second stage passes abruptly by one moult into the pupal stage; and this, certainly, passes abruptly into the Cirripedial or mature stage.
Larva, First Stage.
The larvæ in this stage are known, amongst the [Balanidæ], in [Balanus], [Pyrgoma], [Coronula], [Platylepas], and [Chthamalus]; and these genera include all the principal forms. Amongst the [Verrucidæ] they are known in its one genus, [Verruca]. Amongst the Lepadidæ, in Scalpellum, Ibla, [Alcippe], Lepas, Conchoderma, &c.; and in all these genera the larvæ present no important difference—hardly any difference which could be viewed as generic, were these larvæ independent animals,—as may be inferred, chiefly, from Mr. C. S. Bate’s descriptions.[51] The abstract given in my former volume was not accompanied by any illustrations, and I have consequently here given (Pl. [29], fig. [8]), a view of the larva, in the first stage before moulting, of Scalpellum vulgare: the natatory legs are not drawn with accuracy, only the relative position of the several organs having been carefully attended to. I have also had copied from Mr. Bate’s memoir, a figure of the larva (Pl. [29], fig. [9]) of [Balanus balanoides], in its first stage, before moulting, with its ventral surface exhibited; and another figure (with a few trifling alterations made after examining specimens most kindly sent me by Mr. Bate) of the larva of [Chthamalus stellatus] (fig. [10]), in its first stage, but after moulting once. It should be observed that Mr. Bate has given a drawing of the larva of this latter cirripede, in the first stage, before moulting; and it does not differ essentially from that just referred to (fig. [9]), of [B. balanoides], but is rather more fully developed. These drawings suffice to show the character of the larvæ in the first stage, both before and after the first moult, and even after the second moult, throughout the Order of [Thoracica]. The larvæ sometimes undergo their first moult within the sack of their parent, as I have been informed by Mr. Bate, and as I have observed in [Coronula].
[51] ‘Annals and Magazine of Natural History,’ vol. viii (2d series), 1851, Plates 6, 7, and 8.
I will now make a few remarks on these larvæ in the first stage, before and after the first moult, supplemental to those in my former volume. Their shape is oval, and the whole dorsal surface is evidently covered by a carapace. It is remarkable that the body exhibits no distinct articulations; those given by Goodsir[52] being certainly erroneous. Commencing at the anterior extremity, the eye varies considerably in the state of its development; in [Platylepas decorata] it is nearly circular, and in most of the specimens very distinct; whereas in the allied [Coronula balænaris], before the first moult, it is very imperfect, but afterwards square and of considerable size. In [Balanus galeatus], in the immature larvæ dissected out of the egg, the cellular matter which was in process of conversion into the eye, formed a transverse band, obscurely divided into two portions, and this seems to indicate that the single eye is in fact formed by the confluence of two eyes. In Scalpellum vulgare, this heart-shaped eye lies between a V-shaped muscle, the nature of which I cannot understand, and which has not been represented in (Pl. [29], fig. [8], a). I need only further add, that in [Chthamalus stellatus], after the first moult, the eye exhibits, in specimens sent me by Mr. Bate, some appearance of tending to become double.
[52] ‘Edinburgh New Philosophical Journal,’ July, 1843, Pl. 3, 4.