Olfactory Sacks.

I can add nothing to the account given of these organs under the Lepadidæ: I saw them in all the genera which I examined for this object. In Coronula diadema the orifices are large; they are seated in the usual position (Pl. 26, fig. 4, n), in the confluent segments, beneath the free part of the outer maxillæ, and somewhat exteriorly, or as near as possible to the inner maxillæ. In no sessile cirripede are the orifices produced or tubular, as is the case with several genera amongst the Lepadidæ. I failed, as heretofore, in tracing with certainty the nerve, which appears to enter the base of the sack, to its ganglion.

Male Organs of Generation.

All the Cirripedes of the family we are now describing, are bisexual or hermaphrodite; and no instance has been observed of the presence of males or complemental males. I have very little to add to the observations made by M. Martin St. Ange and R. Wagner,[44] and to those given in my former volume. The testes seem always to be confined to within the thorax, including the prosoma. With their ducts, they resemble club-moss or stag’s horns, with the extremities a little enlarged: a figure[45] of a small portion from Balanus perforatus is given in Pl. 25, fig. 2. It is quite surprising how like in structure and appearance the branching ovarian tubes often are to the testes with their ducts; but the latter are of smaller diameter. Two main ducts generally unite just before entering the broad, often reflexed, end of the vesicula seminalis: in Coronula balænaris, however, I observed four ducts entering this receptacle. The two vesiculæ seminales, lying within the thorax and prosoma, are usually very long and tortuous: they are formed of a thin inner tunic, which is strengthened by thicker reticulated lines, and of an outer layer of transverse fibres, which are either elastic, or probably muscular, as they serve to expel the contents with force when the end is cut off. The inner tunic is prolonged up the probosciformed penis, at the base of which the two vesiculæ unite.[46] The contents of the vesiculæ are commonly pulpy and cellular; and from the cells the spermatozoa are developed; soon after their development, they are, as it appears, expelled.

I have seen the spermatozoa in Balanus crenatus, perforatus, and balanoides, and in Chthamalus stellatus. The cells, from which the spermatozoa are developed, and which are often found in vast numbers within the vesiculæ, are on an average about 1/5000th of an inch in diameter. The spermatozoa differ remarkably within the vesicula of the same individual, according to their state of development. I have observed in B. perforatus and in the Chthamalus, that the shortest, and therefore, I presume, the youngest (Pl. 29, fig. 7, a), had a globular head with no projection in front: as they increased in size, this head became less in diameter, and a short tapering filament, (a, b,) like the tail, projected out of it. This anterior filament does not lie in exactly the same line with the posterior filament, which is straight as an arrow. In Bal. crenatus, the anterior filament was 1/2000th of an inch in length, and the posterior filament 4/2000th, giving a total length of 5/2000th: in the longest and best developed specimens of Chthamalus stellatus, the nodular enlargement was much elongated and spindle-shaped, and not above half the diameter it had in the earliest stage; the posterior filament (measured from the front of the enlargement, this consequently being included) was 5/2000th in length, and the front part only 1/4000th, giving a total length of 11/4000ths of an inch. These observations agree pretty well with Kölliker’s;[47] but this author states, that perfectly developed spermatozoa are absolutely without any nodular enlargement: if this be the case, I have never chanced to see the spermatozoa in their perfect condition. Mr. Bate, also, figures some (Pl. 29, fig. 7, c) in this state, without any enlargement.

The probosciformed penis lies adpressed on the under side of the thorax, with its apex generally projecting between the first and second pairs of cirri. It presents the same ringed or articulated structure as in the Lepadidæ: it arises from an unarticulated projection or support, which also forms the posterior border to the anus. This support often terminates, as first observed by Poli, in a very sharp point; but this point cannot be of much functional importance, for though present in Balanus balanoides, it is absent in the closely allied B. crenatus; in Tubicinella there is only a rudiment of this point; I have not observed it in any member of the Chthamalinæ. The strong, transverse and longitudinal muscles with which the penis is furnished, are attached to this support. The apex or orifice of the penis is, I believe, invariably surrounded by some bristles. Its length varies much, according to its state of contraction or relaxation; and this again, I believe, is dependent on the condition of the male secreting organs. In a small specimen of Elminius modestus, the penis was actually thrice as long as the whole thorax, including the prosoma: in Pachylasma and in Octomeris angulosa, the penis is very short, being equal only to once and a half the length of the pedicel of the sixth cirrus: in Octomeris brunnea, the unarticulated support is much elongated, being as long as the pedicel of the sixth cirrus, in which respect this organ resembles that of Ibla quadrivalvis, and of no other Cirripede. From the attachment of the penis at the posterior end and on the under side of the anus—from the position of the caudal appendages (where such occur) over the anus—from the position of these same appendages in the pupa—and lastly, from the position of the papilla-like penis in the abnormal Proteolepas, I infer that, homologically, the penis is situated at the apex of the abdomen, on its ventral surface; and that, consequently, this organ cannot be considered as the abdomen itself in a modified condition.

Female Organs of Generation.

I have scarcely anything to add to the statements in my former volume. These organs consist of the true ovaria, or glandular bodies seated on each side, not far from the basal edge of the labrum; of the main or unbranched ovarian ducts; and of the (Pl. 25, fig. 1, g) ovarian branching tubes and cæca. I traced distinctly in Balanus, Tetraclita, and Coronula, the two main ovarian ducts, running from within the prosoma to the layer of inosculating, branching, ovarian cæca[48] which overlie the basis. In Coronula diadema one of these main ducts was 1/100th of an inch in diameter. Though I traced these ducts near to the grape-like, glandular masses,[49] which I cannot doubt are the true ovaria, I did not succeed in tracing them into actual connection. As in the Lepadidæ, these ovarian glands lie on the sides, near the basal margin of the labrum, and almost under, but rather to the outside of the antennular nerves. The branching and inosculating ovarian cæca form a layer, which corresponds with the mass filling up the peduncle in the Lepadidæ. In Tetraclita they do not cover the whole basis, but are confined to the circumference; they, however, likewise extend up between the two layers of corium round the walls of the shell, and chiefly in the interspaces between the depressor muscles of the opercular valves. In Chelonobia, they enter between the radiating septa in the thickness of the walls: in Coronula diadema, they extend from over the basal membrane into the six large square chambers (Pl. 16, fig. 7, v) separating the radii and alæ: in Tubicinella they are confined to the basis: in Xenobalanus, they form a layer over the basis and likewise round the upper part of the peduncle-like body, which answers to the shell of other sessile cirripedes.

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.

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.

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.

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]

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.

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.