Cirripedes are commonly bisexual or hermaphrodite, but in Ibla, Scalpellum, and Alcippe, members of the Lepadidæ in the order Thoracica, and in Cryptophialus in the order Abdominalia, the sexes are separate. As two of these genera were described in my former volume, and two others (Alcippe and Cryptophialus) are described in this volume, I may as well here give a brief summary of the facts as yet known on this very curious subject. The Males, in the above four genera, present a wonderful range of structure; they are attached in the usual way by cement proceeding from the not-moulted antennæ of the pupa, to different parts, in the different species, of the female. These males are minute, often exceedingly minute, and consequently generally more than one is attached to a single female; and I have seen as many as fourteen adhering on one female! In several species the males are short-lived, for they cannot feed, being destitute of a mouth and stomach. As the females are longer lived, successive crops of males, at each period of propagation, become attached to her. It is the females in the above genera which retain the characters of the genus, family, and order to which they belong; the males often departing widely from the normal type. Some of the males are rudimentary to a degree, which I believe can hardly be equalled in the whole animal kingdom; they may, in fact, be said to exist as mere bags of spermatozoa. So widely do some of them depart in every character from their class, that twice it has happened to me to examine specimens with a little care, and not even to suspect, until a long period afterwards, that these males were Cirripedes.
In my volume on the Lepadidæ () in searching for analogies for the permanently epizoic and rudimentary condition of the male Cirripedes, I quoted two cases, which I believe are now known not to be analogous; namely, the Syngamus trachealis of Von Siebold, and the worm-like Hectocotyle, which latter was quite lately supposed to be a male Cephalopod, but has now been ascertained to be only one of the arms of the male wonderfully adapted and organised as a sperm-receptacle. The Asplanchna, the mouthless male of a Rotifer, () alone remains for me.
In Scalpellum Peronii, and villosum, the males are but little abnormal, for if classified independently of their sexual relations, they would be considered as immature specimens of a new genus, standing next to Scalpellum; in Scalpellum rostratum, the male would form another and rather more distinct genus. The males, in the latter, are attached to the other sex, between the basal edge of the labrum and the adductor scutorum muscle; but in S. Peronii and villosum they are attached lower down, in the furrow between the two scuta, and are thus protected: in these three species, the internal parts of the male present nothing particular. In Ibla, the males are attached low down within the sack of the female; they may be said to consist of a mouth surmounted on a long peduncle, for there is no capitulum or general covering, and the whole thorax is in a rudimentary condition, the cirri being reduced to two distorted pairs. As these males certainly moult several times and grow a little, they must feed; and as they have no cirri fit for action, they must seize their food by the contortions of their peduncle, which we know homologically consists of the three first segments of the head. The movements of the peduncle must, also, supply those of the probosciformed penis, almost invariably present with other Cirripedes, but here absent. If compelled to class these males without regard to the female, great difficulty would be experienced; we could hardly place in the family of the Lepadidæ, a Cirripede without a capitulum, and without cirri, those very organs which give their name to the class, and with a thorax reduced to the dimensions of a lower lip; yet, if the presence of a peduncle did determine the classifier to place these males amongst the Lepadidæ, then undoubtedly the character of the mouth, &c. would fix their position next to Ibla.
The males of Scalpellum vulgare, ornatum, and rutilum, resemble each other in all essential points, and differ wonderfully in appearance and structure from all ordinary Cirripedes. They consist of a minute flattened bag with a small orifice at the summit, and at the lower end attached by the cemented pupal antennæ. On each side of the orifice, there is a pair of calcareous beads, representing the two scuta and two terga of ordinary Cirripedes; and between the scuta a minute black eye is generally conspicuous. In S. ornatum the beads, I may remark, on the two sides are not equal; those either on the right or on the left side, being larger than those on the opposite side, so that the animal externally is asymmetrical. Inside, within a tubular sack, the thorax is lodged, supporting four (instead of six) pairs of limbs; and these, instead of forming biramous, multiarticulated, captorial cirri, are reduced to almost a rudiment, supporting a few long sharp spines, which apparently act only as defensive organs. At the end of the thorax there is seated a large abdominal lobe, which does not occur in the other sex. Hence the thorax, though rudimental, has been specially modified. Of the mouth and stomach there is not a vestige. Constructed as these males are, assuredly they have no claim to be ranked amongst the Lepadidæ or pedunculated Cirripedes; nor is it possible to class them in any group whatever of ordinary Cirripedes. In S. vulgare the males are attached, often several together, to the extreme edges of the two scuta, and therefore immediately over the orifice leading into the sack; in S. rutilum and ornatum, they are attached in concavities on the under side of both scuta, just above the depression for the adductor scutorum muscle. In the former of these species, the pit for the reception of the male is formed by shelly matter not having been deposited over a certain space on the under side of the valve; and the pit is converted by a covering of membrane into a pouch. As there are two scuta so there are two pouches, in each of which a male is lodged; hence, according to the Linnean nomenclature, Scalpellum ornatum may be said to belong to Diandria monogynia. As these males, from being mouthless, soon die, they are succeeded by successive pairs; the pupa being led by a wonderful instinct to crawl into the pouch, and there undergo its metamorphosis.
Lastly, the males of Alcippe and Cryptophialus (Pl. 23, fig. 19, and Pl. 24, fig. 19) are remarkable for their similarity to each other, considering the essential dissimilarity of the two females. The females live in cavities which they excavate in the shells of Molluscs, and within which they are attached by a horny disc; this disc is the only part of the outer integument which is not frequently moulted, and, apparently in consequence, the males are attached to its edges. It results from this position, that the males are protected by being enclosed within the cavity excavated by the female; and it further results, that the males are attached at a considerable distance from the orifice of the sack of the female, into which the spermatozoa have to be conveyed; and to effect this, the probosciformed penis is wonderfully developed, so that in Cryptophialus, when fully extended, it must equal between eight and nine times the entire length of the animal! These males, like those last mentioned of Scalpellum, consist of a mere bag, lined by a few muscles, enclosing an eye, and attached at the lower end by the pupal antennæ; it has an orifice at its upper end, and within it there lies coiled up, like a great worm, the probosciformed penis, and beneath it a single testis, with a single vesicula seminalis. These organs complete the whole organisation of the male; for there is no mouth, no stomach, no thorax, no abdomen, and no appendages or limbs of any kind. Yet all these parts are present in the female. I know of no other instance in the animal kingdom of such an amount of abortion. The whole exterior of these males evidently is composed, as in all ordinary Cirripedes, of the three first cephalic segments; of the fourteen succeeding segments of the archetype Cirripede we have not a vestige, excepting the probosciformed penis, which, from analogy, should arise from the ventral apex of the seventeenth segment, the first three segments of the head being counted in the seventeen. Here, then, fourteen out of seventeen segments have aborted, the tip of the seventeenth having coalesced with the third cephalic segment! I am tempted just to notice the case of Proteolepas, in the order Apoda, as showing, within the limits of the same sub-class, a wonderful amount and diversity in abortion; for in Proteolepas, the three anterior cephalic segments are reduced to the merest rudiment, encasing the cement-ducts, the fourteen succeeding segments being unusually well developed; whereas in the above described males, we have just seen the three anterior segments fully developed, whilst the fourteen succeeding segments are lost or have coalesced with the others; so that within the same sub-class all seventeen segments of the archetype have almost disappeared.
It may be asked how I know that the several above described rudimentary epizoons are really the males of the Cirripedes to which they are attached. Even if the whole course of the metamorphoses had not been known in three of the cases, the mere fact of these epizoons being cemented by the three terminal segments of their peculiar, pupal antennæ, would have been sufficient to have shown that they belonged to the class of Cirripedes. In nearly every case, I was able to demonstrate, and not in one or two but in many specimens, that these epizoons were males; and as in several instances the spermatozoa were developed, and as, notwithstanding, in no instance was there a vestige of ova or ovaria, it may safely be concluded that they were not hermaphrodites, and therefore required females of some kind. If these epizoic Cirripedes had been independent animals, as they all belong to the same sub-class, and all have such peculiar habits, it might have been expected that they would have shown some special affinity towards each other; but this is not the case; the epizoon of Ibla is more nearly related to Ibla, and the epizoon of Scalpellum more nearly related to Scalpellum, than are these epizoons to each other. If the several epizoons were classed by themselves, they would be grouped in divisions, corresponding with those of the Cirripedes on which they are attached, which is just what might have been expected if these latter were their females. There are, also, many special relations between the male epizoons and the Cirripedes to which they are attached; thus, the mouth of the epizoon of Ibla, is so like the mouth of Ibla, which is peculiar in several respects, that I should easily have recognised it as belonging to a member of that genus. Scalpellum villosum is remarkable as one out of only two or three members of the whole Family, which is destitute of caudal appendages, so is its male epizoon; again, S. villosum is unusually spinose, so is its male epizoon; on the other hand, Scalpellum ornatum is remarkably smooth, so is its male epizoon; and I could give other similar instances. Will it be believed that these coincidences are accidental, and that the epizoons have no special or sexual relation to the Cirripedes to which they are attached?
One other instance of coincident structure is so important, that it must, even in this sketch, be noticed; the prehensile antennæ of the pupa are most important and complicated organs, and differ in the different genera of the same family; they are preserved in a functionless condition throughout life, and in two instances I was able accurately to compare these organs in the epizoon and in the Cirripede to which it was attached, and they were identical in every particular. The full force of the excessive improbability of this resemblance, and of the above coincidences in structure, on the supposition of the epizoon and its support not being sexually related, will hardly be perceived without referring to the facts given in detail in my former volume.
Lastly, in the case of Cryptophialus (and indirectly in that of Alcippe) the nature of the male epizoon is, I think, actually demonstrated; for I traced both it and the female or ordinary form of Cryptophialus, through the same several larval stages, from the egg, enclosed within the sack of the female, to the pupa and mature animal. Moreover, if the male nature and sexual relation to the supporting Cirripede, be admitted in any one of these epizoons, then so close is the agreement in habits, and to a certain extent in structure, in all the foregoing epizoons, that probably no one admitting one instance would dispute the others, and further evidence would even be superfluous. Indeed, had it not been for the following facts, I should not have brought forward, either here in abstract, or in other places in detail, so many arguments and so much evidence.
I have as yet not entered in detail on the sex of the supporting Cirripede: in Cryptophialus, Alcippe, and in one species of Ibla, I was able to demonstrate in many specimens, that all the male organs, internal and external, were entirely absent; and consequently that these Cirripedes were exclusively female. In Scalpellum ornatum, also, there is no trace of external male organs (the state of the four dried specimens not allowing the internal organs to be examined), and there cannot be any reasonable doubt that this species likewise is exclusively female. It should be borne in mind that the male organs, external and internal, are most easily discovered, and that in the above cases I had an abundant supply of excellent specimens. On the other hand, in Ibla Cumingii, and in four species of Scalpellum, I was able to demonstrate in the supporting Cirripede the presence of all the male organs, as well as of the female; and in the vesiculæ seminales of several specimens, both in the Ibla and in Scalpellum vulgare, spermatozoa were contained; the male organs, however, not being very amply developed. These species, consequently, are not exclusively female, but are hermaphrodite, though having male epizoons attached to them. This statement, I am well aware, is enough, at first, to cast a doubt on all that I have said; but let any one reflect on the evidence, of which I have here given a summary, and which has been elsewhere given in full, and I think he must admit that at least those epizoons which are exclusively male, and which are attached to Cirripedes exclusively female, are sexually related and form one species; but if he admit this, he cannot possibly escape from the conclusion that some of the other epizoons, for instance that of Ibla quadrivalvis, are the males of the hermaphrodites to which they are attached, — these epizoons not exclusively impregnating the ova of a female, but aiding the self-impregnation of an hermaphrodite. Hence I have called these males Complemental Males, to show that they do not pair with a female, but with a bisexual individual. Nothing strictly analogous is known in the animal kingdom, but amongst plants, in the Linnean class, Polygamia, closely similar instances abound.
In the series of facts now given, we have one curious illustration more to the many already known, how gradually nature changes from one condition to the other, — in this case from bisexuality to unisexuality. Finally, in the four genera so often named, we meet the following several cases, some of them even the most diverse, occurring in closely allied species. (1st), a female, with a single male (rarely with two) permanently attached to her, protected by her, and capable of seizing, by the movements of its pedunculated body, any minute animals or substances found within her sack; (2d), a female with successive pairs of short-lived, mouthless males, inhabiting pouches on each side under her scutal valves; (3d), a female with many, in one instance fourteen, short-lived males, destitute of mouth, thorax, and appendages, but furnished with a stupendously long male organ, attached to a thickened portion of her outer integuments, but lying within the cavity which she has excavated; (4th), an hermaphrodite with a male attached within the sack, capable of feeding itself, as in the first case; (5th), an hermaphrodite with from one to three males, organised like ordinary Cirripedes, and apparently capable of seizing prey in the common way; and attached between the scuta, and thus protected; (6th and lastly), an hermaphrodite with from one or two up to five or six, short-lived, mouthless males, like those in the second case, attached in one particular spot, on each side of the orifice leading into the sack.