From the kindness of Professor Owen, Mr. Gray, and Mr. Cuming, I have been enabled to examine six specimens of this species; and on two of them I found Complemental males. They were attached in the same position as in S. Peronii; namely, beneath the adductor muscle, in the fold between the scuta, so as to be protected by the latter when closed. This parasite is six-valved, and has a close general resemblance with that of S. Peronii, but differs in very many points of detail. It is represented of the natural size at á fig. 4. The capitulum is 43/1000ths of an inch, measured across the scuta and terga; and the same measured from the base of the carina to the top of the capitulum; hence it is broader, by a quarter of the above measurement, and considerably higher than the male of S. Peronii. From the capitulum being higher, that is, not so much truncated, the orifice is placed more obliquely. The membrane connecting the valves is finely villose, and is besides furnished with spines, conspicuously thicker and longer than those on the male S. Peronii. The scuta and terga are much more elongated, a scutum being here 35/1000ths of an inch in length. The carina descends only just below the basal points of the terga, instead of far below them. The rostrum is a little broader and more arched than the carina; it is 2/1000ths in length, and therefore more than two thirds of the length of the carina, the latter being 28/1000ths of an inch from the apex to the basal margin. The primordial valves, with the usual hexagonal tissue, are seated on the tips of the scuta, terga, and carina, but not on the rostrum; so that these valves follow the same law of development, as in the ordinary and hermaphrodite form of Scalpellum. The scuta (a, fig. 4, greatly enlarged), the terga (b), and carina (c) of the male, resemble the same valves in the hermaphrodite, much more closely than do these valves in the male and hermaphrodite S. Peronii. The rostrum has not its basal margin hollowed out, and is very much larger relatively to the carina, than in the hermaphrodite. The large relative size of the rostrum in the complemental male both of this species and of S. Peronii, is a remarkable character, which I can in no way account for.
The peduncle is narrow and short, but in a different degree in the two specimens examined. It is naked. The prehensile antennæ were not in a good state of preservation: the disc is narrower than the basal segment, and only slightly pointed, in which important respect it differs from the same part in the foregoing species; at its distal end, rather on the inner side, there are two or three spines, apparently in place of the excessively minute hairs, which are found at the same spot in some or in all the other species of Scalpellum, and in Ibla: similar strong spines occur in Pollicipes. Unfortunately, for the sake of comparison, I was not able to find the prehensile antennæ in the hermaphrodite S. villosum.
Mouth. — Labrum bullate, with teeth on the crest. Palpi blunt, spinose.
Mandibles, with three teeth; inferior point rather strongly pectinated.
Maxillæ, with a considerable notch under the upper pair of large spines; inferior part of the edge not prominent.
Outer Maxillæ, with the spines on the inner edge arranged into two groups. Olfactory orifices tubular and prominent, with some long bristles near their bases. In the mandibles having only three teeth, in the maxillæ being notched and in the lower part not being prominent, and, lastly, in the bristles on the inner face of the outer maxillæ being arranged in two groups, these several organs differ from those in the hermaphrodite.
Cirri. — First pair short, with only three or four segments in each ramus: second cirrus, with the basal segments not very thickly clothed with spines: sixth cirrus with seven segments, not protuberant in front, each bearing four pairs of spines, without intermediate tufts.
Caudal appendages, none. This is an interesting fact, considering that these organs are likewise absent in the hermaphrodite S. villosum, — an absence highly remarkable, and confined to the genus Conchoderma and the one species of Anelasma.
Penis thick, not tapering, rather exceeding in length the pedicel of the sixth cirrus, square at the end, and furnished with some spines. In one specimen, I believe I distinguished the vesiculæ seminales: if so, they contained only pulpy matter, and not spermatozoa. There were no ovarian tubes within the peduncle, which was lined by the usual muscles; I traced the two delicate cement-ducts, running from within the antennæ close up to the animal’s body. Hence in this case, as in that of S. Peronii, I dare positively affirm that ovarian tubes do not occur; for it is out of the question that I could have traced the cement-ducts, and, at the same time, overlooked the far larger and more conspicuous ovarian tubes, into which, moreover, the ducts, had they existed, would have run. Consequently, these parasites are not females; but judging from the probosciformed penis, and from the presence, as I believe, of vesiculæ seminales, they are males.
The complemental males of the present species, and of S. Peronii, so closely resemble each other, that what I have stated regarding the affinities of the latter, are here quite applicable. It is singular how much more alike the parts of the mouth and the cirri of these two complemental males are, than the corresponding parts in the two hermaphrodites: this no doubt is due to the two males having been arrested in their development, at a corresponding early period of growth. Several of the characters, by which the hermaphrodite S. villosum so closely approaches, and almost blends into the genus Pollicipes, — such as the thicker cirri, with the intermediate tufts of bristles, the small second tooth of the mandibles, and the little brush-like prominence on the maxillæ, — are not in the least apparent in the complemental male.
SUMMARY ON THE NATURE AND RELATIONS OF THE MALES AND COMPLEMENTAL MALES, IN IBLA AND SCALPELLUM.
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.
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 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!
‘Annals of Natural History,’ vol. ii, (2d series, 1848,) , Pl. vi. Mr. Dalrymple has published a very interesting paper on the same subject in the ‘Philosophical Transactions,’ (,) 1849; and there is another Memoir by Mr. Gosse in the ‘Annals of Natural History,’ vol. vi, (1850,) .
Genus — Pollicipes. Pl. VII.
Pollicies. Leach. Journal de Physique, tom. lxxxv, Julius, 1817.
Lepas. Linn. Systema Naturæ, 1767.
Anatifa. Brugière. Encyclop. Méthod. (des Vers), 1789.
Mitella. Oken. Lehrbuch der Naturgeschichte, 1815.
Ramphidiona. Schumacher. Essai d’un Nouveau Syst. &c., 1817 (ante Julium).
Polylepas. De Blainville. Dict. des Sc. Nat., 1824.
Capitulum (secundum Klein). J. E. Gray. Annals of Philos., tom. x, new series, Aug. 1825.
This is one of the rare cases in which, after much deliberation, and with the advice of several distinguished naturalists, I have departed from the Rules of the British Association; for it will be seen that Mitella of Oken, and Ramphidiona of Schumacher, are both prior to Pollicipes of Leach; yet, as the latter name has been universally adopted throughout Europe and North America, and has been extensively used in geological works, it appears to me to be as useless as hopeless to attempt any change. It may be observed that the genus Pollicipes was originally proposed by Sir John Hill (‘History of Animals,’ vol. iii, ), in 1752, but as this was before the discovery of the binomial system, by the Rules it is absolutely excluded as of any authority. In my opinion, under all these circumstances, it would be mere pedantry to go back to Oken’s ‘Lehrbuch der Naturgeschichte’ for the name Mitella, — a work little known, and displaying entire ignorance regarding the Cirripedia.
Valvæ ab 18 usque ad 100 et amplius: lateribus verticilli inferioris multis; lineis incrementi deorsùm ordinatis: sub-rostrum semper adest: pedunculus squamiferus.
Valves from 18 to above 100 in number: latera of the lower whorl numerous, with their lines of growth directed downwards: sub-rostrum always present: peduncle squamiferous.
Hermaphrodite; filamentary appendages either none, or numerous and seated on the prosoma and at the bases of the first pair of cirri; labrum bullate; trophi various; olfactory orifices generally highly prominent; caudal appendages uni-articulate and spinose, or multi-articulate.
Attached to fixed, or less commonly to floating objects, in the warmer temperate, and tropical seas.
It has been remarked, under Scalpellum, how imperfectly that genus is separated from Pollicipes; and we have seen under Scalpellum villosum that the addition of a few small valves to the lower whorl, would convert it into a Pollicipes, most closely allied to P. sertus and spinosus. It has also been shown, that the six recent species of Pollicipes might be divided into three genera, of which P. cornucopia, P. elegans, and P. polymerus, would form one thoroughly natural genus, as natural as Lepas and the earlier genera; P. mitella would form a second; and P. sertus and P. spinosus a third; but I have acted to the best of my judgment in at present retaining the six species together. As far as the valves of the capitulum are concerned, it would be very difficult to separate P. mitella from P. sertus and spinosus.
Description. The number of valves in the capitulum has in this genus acquired its maximum. The number varies considerably in the same species, and even on opposite sides of the same individual, and generally increases with age. It is more important, that the number of the whorls in P. cornucopia, and in the two following closely-allied forms, also increases with age. In P. sertus and P. spinosus, even the number of the whorls varies in different individuals, independently of age. The valves are arranged alternately with those above and below; they are generally thick and strong, making the capitulum somewhat massive; in some species they are subject to much disintegration; but in others, the apices of the several valves, especially of the carina and rostrum, are well preserved, and project freely: they are covered with membrane, which, differently from in most species of Scalpellum, either does not bear any spines, or only exceedingly minute points. In all the species there is a sub-rostrum and sub-carina, and often beneath these a second sub-rostrum and sub-carina. In medium-sized specimens there are at least 20 valves in the lowermost whorl. The carina is either straight or curved, but never rectangularly bent, and is always of considerable breadth. None of the valves are added to at their upper ends. The scuta have a deep pit for the adductor muscle. The valves lie either some little way apart, or more commonly close together. In P. mitella the scuta and terga are locked together by a fold, and the valves of the lower whorl overlap each other in a peculiar manner, resembling that in which the compartments in the shells of Sessile Cirripedes fold over each other.
The Peduncle is of considerable length in some of the species, and rather short in others; it is, in every case, clothed with calcified scales. The scales in the first four species are placed alternately and symmetrically; they are formed and added to in the same manner as in Scalpellum; they differ in size according to the size of the individual, and consequently the lower scales on the peduncle, formed when the specimen was young, are smaller than the upper scales; the lower scales are separated from each other by wide interspaces of membrane, owing to the continued growth of the peduncle by the formation of new layers of membrane, and the disintegration of the old outer layers. Each scale is invested by tough membrane (or has been, for it is often abraded off), in the same manner as the valves; each is furnished with one or more tubuli, in connection with the underlying corium. In P. sertus and P. spinosus, the scales are small, spindle-shaped, and not of equal sizes, and the rows are distant from each other, so that their alternate arrangement is not distinguishable; in these two species, new scales are formed round the summit of the peduncle, and the growth of each is completed whilst remaining in the uppermost row; but, besides these normal scales, such as exist in the other species of Pollicipes and in Scalpellum, new scales are formed in the lower part of the peduncle, which are generally of very irregular shapes, are often larger than the upper ones, are crowded together, and sometimes do not reach the outer surface of the membrane. This formation of scales in the lower part of the peduncle, independently of the regular rows round the uppermost part, is perhaps a feeble representation of the calcareous cup at the bottom of the peduncle in the genus Lithotrya. The prehensile antennæ will be described under P. cornucopia.
Size. — Most of the species are large: and P. mitella is the most massive of the Pedunculated Cirripedes.
The Mouth is not placed far from the adductor muscle. The labrum is highly bullate. The mandibles have either three or four main teeth (Pl. X, fig. 1), with often either one or two smaller teeth inserted between the first and second. The maxillæ (Pl. X, figs. 13, 14), have their edges either straight and square, or notched, or more commonly with two or three prominences bearing tufts of finer spines. The outer maxillæ (fig. 17) generally have a deep notch on their inner edges, but this is not invariable. The olfactory orifices in most of the species are highly prominent.
Cirri. — The first pair is never placed far distant from the second. The posterior cirri have strong, somewhat protuberant segments; and between each of the four or five pair of main spines (Pl. X, fig. 27), there is a rather large tuft of straight, fine, short bristles. The second and third pair have the basal segments, either of the anterior rami, or of both rami, so thickly clothed with spines (fig. 25), as to be brush-like: in P. mitella, however, the third pair is like the three posterior pair in the arrangement of its spines, in this respect resembling the sessile Chthamalinæ. The caudal appendages are either uni-articulate and spinose, or multi-articulate: it is remarkable that there should be this difference in such closely allied species as P. cornucopia and P. polymerus: the short, obtuse, obscurely-articulated caudal appendage of the former species (fig. 22) makes an excellent passage from the uni-articulate (fig. 19) to the multi-articulate form, as in P. mitella.
The stomach, in those species which I opened, is destitute of cæca; the hepatic glands are arranged in straight lines; the rectum is unusually short. The prosoma is well developed.
In P. cornucopia, P. elegans, and P. polymerus, there are numerous filamentary appendages both on the prosoma, and at the bases of the first pair of cirri: these appendages are occupied by testes, and I suspect stand in relation to the length of the peduncle and consequent great development of the ovaria. In order to give space for the filamentary appendages, the sack (generally roughened by small inwardly-pointing papillæ) penetrates more deeply than usual into the upper part of the peduncle. There are small ovigerous fræna in P. sertus, P. spinosus, and P. mitella: in the three other species, the frænum or fold occupies the usual position on each side, and is large; but in one specimen carefully examined by me, I was unable to see any glands; and in another specimen, the ovigerous lamellæ were not attached to the fræna; hence I conclude that the fræna are functionless in these three species.
Affinities. — I have already remarked on the close relationship between this genus and Scalpellum; there is also some affinity with Lithotrya.
Distribution. — All over the world. The P. cornucopia ranges from Scotland to Teneriffe: the P. polymerus is found in opposite hemispheres in the Pacific Ocean, extending from California to at least as far as 32° south of the Equator.
Geological History. — Having so lately given, in the ‘Memoirs of the Palæontographical Society,’ a full account of all the fossil species known, I will not repeat here the conclusions there arrived at. I will only state, that species of Pollicipes are found in all the formations, extending from the Lower Oolite to the Upper Tertiary beds.