Chapter 26

Had the question been, whether the parasites which I have now described, were simply the males of the Cirripedes to which they are attached, the present summary and discussion would perhaps have been superfluous; but it is so novel a fact, that there should exist in the animal kingdom hermaphrodites, aided in their sexual functions by independent and, as I have called them, Complemental males, that a brief consideration of the evidence already advanced, and of some fresh points, will not be useless. These parasites are confined to the allied genera Ibla and Scalpellum; but they do not occur in Pollicipes,--a genus still more closely allied to Scalpellum; and it deserves notice, that their presence is only occasional in those species of Scalpellum which come nearest to Pollicipes. In the genera Ibla and Scalpellum, the facts present a singular parallelism; in both we have the simpler case of a female, with one or more males of an abnormal structure attached to her; and in both the far more extraordinary case of an hermaphrodite, with similarly attached Complemental males. In the two species of Ibla, the complemental and ordinary males resemble each other, as closely as do the corresponding hermaphrodite and female forms; so it is with two sets of the species of Scalpellum. But the males of Ibla and the males of Scalpellum certainly present no special relations to each other, as might have been expected, had they been distinct parasites independent of the animals to which they are attached, and considering that they are all Cirripedes having the same most unusual habits. On the contrary, it is certain that the animals which I consider to be the males and complemental males of the two species of Ibla, if classed by their own characters, would, from the reasons formerly assigned, form a new genus, nearer to Ibla than to the parasites of Scalpellum: so, again, the assumed males of the three latter species of Scalpellum would form two new genera, both of which would be more closely allied to Scalpellum, than to the parasites of Ibla. With respect to the parasites of the first three species of Scalpellum, they are in such an extraordinarily modified and embryonic condition, that they can hardly be compared with other Cirripedes; but certainly they do not approach the parasites of Ibla, more closely than the parasites of Scalpellum; and in the one important character of the antennæ, they are identical both with the parasitic and ordinary forms of Scalpellum. That two sets of parasites having closely similar habits, and belonging to the same sub-class, should be more closely related in their whole organisation to the animals to which they are respectively attached, than to each other, would, if the parasites were really distinct and independent creatures, be a most singular phenomenon; but on the view that they differ only sexually from the Cirripedes on which they are parasitic, this relationship is obviously what might have been expected.

The two species of Ibla differ extremely little from each other, and so, as above remarked, do the two males. In Scalpellum the species differ more from each other, and so do the males. In this latter genus the species may be divided into two groups, the first containing _S. vulgare_, _S. ornatum_ and _S. rutilum_, characterised by not having a sub-carina, by the rostrum being small, by the constant presence of four pair of latera, and by the peculiar shape of the carinal latera; the second group is characterised by having a sub-carina and a large rostrum, and may be subdivided into two little groups; viz., _S. rostratum_ having four pairs of latera, and _S. Peronii_ and _villosum_ having only three pairs of latera: now the males, if classed by themselves, would inevitably be divided in exactly the same manner, namely, into two main groups,--the one including the closely similar, sack-formed males of _S. vulgare_, _ornatum_, and _rutilum_, the other the pedunculated males of _S. rostratum_, _Peronii_, and _villosum_; but this latter group would have to be subdivided into two little sub-groups, the one containing the three-valved male of _S. rostratum_, and the other the six-valved males of _S. Peronii_ and _S. villosum_. It should not, however, be overlooked, that the two main groups of parasites differ from each other, far more than do the two corresponding groups of species to which they are attached; and, on the other hand, that the parasitic males of _S. Peronii_ and _S. villosum_ resemble each other more closely, than do the two hermaphrodite forms;--but it is very difficult to weigh the value of the differences in the different parts of species.

Besides these general, there are some closer relations between the parasites and the animals to which they are attached; thus the most conspicuous internal character by which _Ibla quadrivalvis_ is distinguished from _I. Cumingii_, is the length of the caudal appendages and the greater size of the parts of the mouth; in the parasites, we have exactly corresponding differences. Out of the six species of Scalpellum in their ordinary state, _S. ornatum_ is alone quite destitute of spines on the membrane connecting the valves; and had it not been for this circumstance, I should even have used the presence of spines as a generic character; on the other hand, _S. villosum_, in accordance with its specific name, has larger and more conspicuous spines than any other species. In the parasites we have an exactly parallel case; the parasite of _S. ornatum_ being the only one without spines, and the spines on the parasite of _S. villosum_ being much the largest! This latter species is highly singular in having no caudal appendages, and the parasite is destitute of these same organs, though present inn the parasites of _S. rostratum_ and _S. Peronii_. Again, _S. villosum_ approaches, in all its characters, very closely to the genus Pollicipes, and the parasite in having prehensile antennæ, with the disc but little pointed, and with spines at the further end, departs from Scalpellum and approaches Pollicipes! Will any one believe that these several parallel differences, between the Cirripedial parasites and the Cirripedes to which they are attached, are accidental, and without signification? yet, this must be admitted, if my view of their male sex and mature be rejected.

One more, and the most important special relation between the parasites and the cirripedes to which they are attached, remains to be noticed, namely that of their prehensile larval antennæ. I observed the antennæ more or less perfectly in the males of all, and except in _S. villosum_, in all the species, though so utterly different in general appearance and structure, I found the peculiar, pointed, hoof-like discs, which are confined, I believe, to the genera Ibla and Scalpellum. In the hermaphrodite forms of Scalpellum, I was enabled to examine the antennæ only in two species, _S. vulgare_ and _S. Peronii_, (belonging, fortunately, to the two most distinct sections of the genus,) and after the most careful measurements of every part, I can affirm that, in _S. vulgare_, the antennæ of the male and of the hermaphrodite are identical; but that they differ slightly in the proportional lengths of their segments, and in no other respect, from these same organs in _S. Peronii_,--in which again the antennæ of the male and of the hermaphrodite are identical. The importance of this agreement will be more fully appreciated, if the reader will consider the following table, in which the generic and specific differences of the antennæ in the Lepadidæ, as far as known to me, are given. These organs are of high functional importance; they serve the larva for crawling, and being furnished with long, sometimes plumose spines, they serve apparently as organs of touch; and lastly, they are indispensable as a means of permanent attachment, being adapted to the different objects, to which the larva adheres. Hence the antennæ might, _à priori_, have been deemed of high importance for classification. They are, moreover, embryonic in their nature; and embryonic parts, as is well known, possess the highest classificatory value. From these considerations, and looking to the actual facts as exhibited in the following table, the improbability that the parasites of _S. vulgare_ and _S. Peronii_, so utterly different in external structure and habits one from the other, and from the Cirripedes to which they are attached, should yet have absolutely similar prehensile antennæ with these Cirripedes, appears to me, on the supposition of the parasites being really independent creatures, and not, as I fully believe, merely in a different state of sexual development, insurmountably great.

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 | | |-- | | | |20 | | | | |23 | | | | | |22 | | | | | | |-- | | | | | | | |--

|_L. australis_, | |111 | | |-- | | | |40 | | | | |42 | | | | | |39 | | | | | | |18 | | | | | | | |30

|_L. pectinata_, | |51 | | |-- | | | |23 | | | | |16 | | | | | |14 | | | | | | |9 | | | | | | | |16

|_L. fascicularis_, | |60 | | |40 | | | |22 | | | | |16 | | | | | |15 | | | | | | |-- | | | | | | | |--

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 | | |-- | | | |11 | | | | |7-8 | | | | | |7-8 | | | | | | |6 | | | | | | | |13-14

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_, | |82 | | |40 | | | |28 | | | | |25 | | | | | |35 | | | | | | |12 | | | | | | | |26

|_C. aurita._ | |-- | | |-- | | | |-- | | | | |28 | | | | | |40 | | | | | | |11 | | | | | | | |26

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 | | |-- | | | |24 | | | | |14 | | | | | |12 | | | | | | |8 | | | | | | | |20

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 | | |-- | | | |7-8 | | | | |7 | | | | | |-- | | | | | | |3-4 | | | | | | | |7-8

|_I. quadrivalvis_, | |32-33 | | |-- | | | |10 | | | | |8 | | | | | |5 | | | | | | |4 | | | | | | | |8

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_, | |39 | | |19 | | | |10 | | | | |10-11 | | | | | |5-6 | | | | | | |6 | | | | | | | |7

|_S. ornatum_, | |36 | | |21 | | | |10 | | | | |12 | | | | | |-- | | | | | | |-- | | | | | | | |--

|_S. Peronii_, | |30 | | |19 | | | |-- | | | | |9 | | | | | |6 | | | | | | |5 | | | | | | | |10

POLLICIPES: disc small, _hoof-like_, not pointed, with a single spine on the hinder margin; end segment, as in _Scalpellum_.

|_P. cornucopia_, | |20 | | |-- | | | |6 | | | | |6 | | | | | |6 | | | | | | |6 | | | | | | | |8

[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 dioecious 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.