The parasites of S. vulgare take advantage of a pre-existing fold on the edge of the scutum, where the chitine border is thicker; and in this respect there is nothing different from what would naturally happen with an independent parasite; but in S. ornatum the case is very different, for here the two scuta are specially modified, before the attachment of the parasites, in a manner which it is impossible to believe can be of any service to the species itself, irrespectively of the lodgment thus afforded for the males. So again in S. rutilum, the shape of the scutum seems adapted for the reception of the male, in a manner which must be attributed to its own growth, and not to the pressure or attachment of a foreign body. Now there is a strong and manifest improbability in an animal being specially modified to favour the parasitism of another, though there are innumerable instances in which parasites take advantage of pre-existing structures in the animals to which they are attached. On the other hand, there is no greater improbability in the female being modified for the attachment of the male, in a class in which all the individuals are attached to some object, than in the mutual organs of copulation being adapted to each other throughout the animal kingdom.

Generic Characters of the larval prehensile Antennæ, in the Lepadidæ, as far as known from their imperfect state of preservation, and the number of species examined.Name of Species.Length of, from end of disc to the further margin of the oblique basal articulation: Scale, fractions of the 1/6000ths of an inch.Length of, from end of disc to the inner margin of the basal articulation. Scale same.Width of basal segment, in widest part. Scale same.Disc, length of. Scale same.Disc, width of. Scale same.Ultimate segment, length of. Scale same.Ultimate segment, width of. Scale, fractions of the 1/20,000ths of an inch.
Lepas: disc large, thin, almost circular, slightly elongated, with several long spines on the hinder margin; end segment with three very long, plumose spines on the upper exterior angle.[61]L. anatifera (?)62—202322——
L. australis,111—4042391830
L. pectinata,51—231614916
L. fascicularis,6040221615——
Dichelaspis: disc small, thin, circular, with several spines on the hinder margin; end segment, with two long spines on the upper exterior angle.D. Warwickii,54—117-87-8613-14
Conchoderma: disc large, thin, transversely elongated, with several long spines on the hinder margin; end segment, with two excessively long, plumose spines on the upper exterior corner.C. virgata,82402825351226
C. aurita.———28401126
Alepas: disc small, slightly elongated, with two or more spines on the hinder margin; end segment, with two long spines on the upper inner corner, and four shorter ones on the exterior corner.A. cornuta,60—241412820
Ibla (parasitic males of): disc, hoof-like, pointed, elongated, with a single spine on the hinder margin; end segment, with four short spines on the upper exterior corner.I. Cumingii,22—7-87—3-47-8
I. quadrivalvis,32-33—108548
Scalpellum: disc hoof-like, generally pointed and elongated, with a single spine on the hinder margin; end segment, with a notch on the inner[61] side, bearing two spines, longer than on the exterior corner.S. vulgare,39191010-115-667
S. ornatum,36211012———
S. Peronii,3019—96510
Pollicipes: disc small, hoof-like, not pointed, with a single spine on the hinder margin; end segment, as in Scalpellum.P. cornucopia,20—66668

[61] In the diameter of the disc, the thin membranous border, which is present in the first three genera, is included; but I have some doubts, whether this border be not the first rim of cementing tissue, as all the specimens, of which measurements are here given, had been removed after attachment. In using the terms inner and outer sides of the end segment, it is supposed, that this segment is stretched straight forwards, instead of being bent rectangularly outwards, as in its natural position; and then there can be no doubt which is the inner and outer sides.

It should be observed that the evidence in this summary is of a cumulative nature. If we think it highly, or in some degree probable,—from the ordinary form of [Ibla Cumingii] having been shown on good evidence to be exclusively female,—from the absence of ova and ovaria in the assumed males of both species of Ibla, at the period when their vesiculæ seminales were gorged with spermatozoa,—from the close general resemblance between the parts of the mouth in the parasites and in the Iblas to which they are attached,—from the differences between the two parasites being strictly analogous to the differences between the two species of Ibla,—from the generic character of their prehensile antennæ,—and from other such points,—if from these several considerations, we admit that these parasites really are the males of the two species to which they adhere, then in some degree the occurrence of parasitic males in the allied genus Scalpellum is rendered more probable. So the absolute similarity in the antennæ of the males and hermaphrodites both in S. vulgare and S. Peronii; and such relations as that of the relative villosity of the several species in this same genus, all in return strengthen the case in Ibla. Again, the six-valved parasites of S. Peronii and S. villosum are so closely similar, that their nature, whatever it may be, must be the same; hence we may add up the evidence derived from the identity of the antennæ in the parasite and hermaphrodite S. Peronii, with that from the antennæ in the male S. villosum, approaching in character to Pollicipes, to which genus the hermaphrodite is so closely allied; and to this evidence, again, may be added the singular coincident absence of caudal appendages in the male and hermaphrodite S. villosum. If these two six-valved parasites be received as the complemental males of their respective species, no one, probably, will doubt regarding the nature of the parasite of S. rostratum, in which the direct evidence is the weakest; but even in this case, the particular point of attachment, and the state of development of the valves, form a link connecting in some degree, the parasites of the first three species with the last two species of Scalpellum, in accordance with the affinities of the hermaphrodites.

When first examining the parasites of S. rostratum, S. Peronii, and S. villosum, before the weight of the cumulative evidence had struck me, and noting their apparent state of immaturity, it occurred to me that possibly they were the young of their respective species, in their normal state of development, attached to old individuals, as may often be seen in Lepas; this, however, would be a surprising fact, considering that S. rostratum and S. Peronii are ordinarily attached, in a certain definite position, to horny corallines, and considering that the exact points of attachment in these three parasites, (of which I have seen no other instance amongst common Cirripedes,) namely, between the scuta, would inevitably cause their early destruction, either directly or indirectly, by their living supports being destroyed. Nevertheless, I carefully examined a young specimen of S. rostratum only thrice as large as the parasite; and not having very young specimens of S. Peronii and villosum, I procured the young of closely-allied forms, namely, of S. vulgare, (with a capitulum only 4/100th of an inch in length,) and of [Pollicipes polymerus], (with a capitulum of less size than that of one of the parasites,) and there was not the least sign of anything abnormal in the development of the valves. In S. vulgare, at a period when the calcified scuta could have been only 1/100th of an inch in length, (and therefore considerably less than the scuta in the parasites,) the upper latera must have been as much as 4/1000ths of an inch in length, and the valves of the lower whorl certainly distinguishable.

To sum up the evidence on the sex of the parasites, I was not able to discover a vestige of ova or ovaria in the two male Iblas; and I can venture to affirm positively, that the parasites of S. Peronii and S. villosum are not female. On the other hand, in the two male Iblas, I was enabled to demonstrate all the male organs, and I most distinctly saw spermatozoa. In the parasitic complemental male of S. vulgare, I also most plainly saw spermatozoa. In the parasites of S. rostratum, S. Peronii, and S. villosum, the external male organs were present. I may here just allude to the facts given in detail under Ibla, showing that it was hardly possible that I could be mistaken regarding the exclusively female sex of the ordinary form of I. Cumingii, seeing how immediately I perceived all the male organs in the hermaphrodite I. quadrivalvis; and as the parasite contained spermatozoa and no ova, the only possible way to escape from the conclusion that it was the male and I. Cumingii the female of the same species, was to invent two hypothetical creatures, of opposite sexes to the Ibla and its parasite, and which, though Cirripedes, would have to be locomotive! I insisted upon this alternative, because if the parasite of I. Cumingii be the male of that species, then unquestionably we have in I. quadrivalvis a male, complemental to an hermaphrodite,—a conclusion, as we have seen, hardly to be avoided in the genus Scalpellum, even if we trust exclusively to the facts therein exhibited.

With respect to the positions of the parasitic males, in relation to the impregnation of the ova in the females and hermaphrodites, it may be observed that in the two male Iblas, the elongated moveable body seems perfectly adapted for this end; in the males of the first three species of Scalpellum, the spermatozoa, owing to the manner in which the thorax is bent when protruded, would be easily discharged into the sack of the female or hermaphrodite; this would likewise probably happen with the complemental male of S. rostratum, considering its position within the orifice of the capitulum, between the mouth and the adductor scutorum muscle. The males of S. Peronii and villosum being fixed a little way beneath the orifice of the sack, below the adductor muscle, are less favorably situated, but the spermatozoa would probably be drawn into the sack by the ordinary action of the cirri of the hermaphrodite, and therefore would at least have as good a chance of fertilising some of the ova, as the pollen of many diœcious plants, trusted to the wind, has of reaching the stigmas of the female plants. Regarding the final cause, both of the simpler case of the separation of the sexes, notwithstanding that the two individuals, after the metamorphosis of the male, become indissolubly united together, and of the much more singular fact of the existence of Complemental males, I can throw no light; I will only repeat the observation made more than once, that in some of the hermaphrodites, the vesiculæ seminales were small, and that in others the probosciformed penis was unusually short and thin.

Viewing the parasitic males, in relation to the structure and appearance of the species to which they belong, they present a singular series. In S. Peronii and S. villosum, the internal organs have the appearance of immaturity; the shape of the capitulum is specially modified for its reception between the scuta of the hermaphrodite, and several of the valves have not been developed. This atrophy of the valves, is carried much further in S. rostratum. In Ibla, many of the parts are embryonic in character, but others mature and perfect; some parts, as the capitulum, thorax, and cirri, are in a quite extraordinary state of atrophy; in fact, the parasitic males of Ibla consist almost exclusively of a mouth, mounted on the summit of the three anterior segments of the 21 normal segments of the archetype crustacean. In the males of the first three species of Scalpellum, some of the characters are embryonic,—as the absence of a mouth, the presence of the abdominal lobe, and the position of the few existing internal organs; other characters, such as the general external form, the four bead-like valves, the narrow orifice, the peculiar thorax and limbs, are special developments. These three latter parasites, certainly, are wonderfully unlike the hermaphrodites or females to which they belong; if classed as independent animals, they would assuredly be placed not in another family, but in another Order. When mature they may be said essentially to be mere bags of spermatozoa.

In looking for analogies to the facts here described, I have already referred to the minute male Lerneidæ which cling to their females,—to the worm-like males of certain Cephalopoda, parasitic on the females,—and to certain Entozoons, in which the sexes cohere, or even are organically blended by one extremity of their bodies. The females in certain insects depart in structure, nearly or quite as widely from the Order to which they belong, as do these male parasitic Cirripedes; some of these females, like the males of the first three species of Scalpellum, do not feed, and some, I believe, have their mouths in a rudimentary condition; but in this latter respect, we have, amongst the Rotifera, a closely analogous case in the male of the Asplanchna of Gosse, which was discovered by Mr. Brightwell[62] to be entirely destitute of mouth and stomach, exactly as I find to be the case with the parasitic male of S. vulgare, and doubtless with its two close allies. For any analogy to the existence of males, complemental to hermaphrodites, we must look to the vegetable kingdom.

Finally, the simple fact of the diversity in the sexual relations, displayed within the limits of the general Ibla and Scalpellum, appears to me eminently curious; we have (1st) a female, with a male (or rarely two) permanently attached to her, protected by her, and nourished by any minute animals which may enter her sack; (2d) a female, with successive pairs of short-lived males, destitute of mouth and stomach, inhabiting two pouches formed on the under sides of her valves; (3d) an hermaphrodite, with from one or two, up to five or six similar short-lived males without mouth or stomach, attached to one particular spot on each side of the orifice of the capitulum; and (4th) hermaphrodites, with occasionally one, two, or three males, capable of seizing and devouring their prey in the ordinary Cirripedial method, attached to two different parts of the capitulum, in both cases being protected by the closing of the scuta. As I am summing up the singularity of the phenomena here presented, I will allude to the marvellous assemblage of beings seen by me within the sack of an [Ibla quadrivalvis],—namely, an old and young male, both minute, worm-like, destitute of a capitulum, with a great mouth, and rudimentary thorax and limbs, attached to each other and to the hermaphrodite, which latter is utterly different in appearance and structure; secondly, the four or five, free, boat-shaped larvæ, with their curious prehensile antennæ, two great compound eyes, no mouth, and six natatory legs; and lastly, several hundreds of the larvæ in their first stage of development, globular, with horn-shaped projections on their carapaces, minute single eyes, filiformed antennæ, probosciformed mouths, and only three pair of natatory legs; what diverse beings, with scarcely anything in common, and yet all belonging to the same species!