Affinities.—In the preliminary remarks, we have seen how this genus blends into Pollicipes; and under the head of Oxynaspis, I have shown its close affinity to that genus. If, indeed, we take Pollicipes spinosus, and destroy all but six of the already minute and almost rudimentary latera, we shall, as far as the capitulum is concerned, convert it into a Scalpellum, closely similar to S. villosum. If we take any species of Scalpellum, (excepting S. villosum and S. rutilum,) and destroy all the valves, but the scuta, terga and carina, we shall convert it into an Oxynaspis. Lastly, I have shown under Ibla, that in several most remarkable peculiarities of structure, there is a manifest affinity between Scalpellum and that genus.

Geological History.—Full details on this subject have been given in the Memoirs of the Palæontographical Society. I will here only state, that the oldest known form of Scalpellum occurs in the Lower Green Sand.

[† SUB-CARINÂ NULLÂ.]

1. Scalpellum Vulgare. Pl. V, fig. 15.

S. (Herm.) valvis 14, si rostrum pæne rudimentale includatur: lateribus superioribus inæqualiter ovatis.

(Herm.) Capitulum with 14 valves, including the rudimentary rostrum: upper latera irregularly oval.

Mandibles, with four or five teeth: maxillæ, with the edge straight, bearing numerous spines.

Complemental Male flask-formed, with four rudimentary valves; no mouth; cirri not prehensile; attached to the occludent margin of the scutum, near the umbo.

HERMAPHRODITE.

Description.—Capitulum much flattened with the apex produced, of a pale brown colour, sometimes faintly tinted purple, composed of 14 valves, of which the rostrum is rudimentary and barely visible externally; valves thin, white, translucent, smooth, slightly marked by the lines of growth, separated from each other by rather wide interspaces of colourless membrane, which is thickly clothed by small, articulated spines of unequal length. The valves, excepting sometimes their umbones, are also covered with membrane, bearing spines, placed in rows parallel to the lines of growth; the spines are particularly numerous round the orifice of the sack.

Scuta slightly convex, thrice as long as broad; upper part much acuminated; occludent margin almost straight; basal margin nearly at right angles to the occludent margin; the tergal margin is separated from the lateral margin by an angle more or less prominent; a slight curved ridge runs from the umbo to this angle, and this deserves especial notice, inasmuch as it indicates the outline which the valve assumed in its earliest growth, and which is permanently retained in most of the older fossil species. Along the occludent margin, there is a trace of a ledge, developed in a variable degree, and which is noticed only on account of the plainly visible ledge along this same margin, in the allied genus Oxynaspis. The umbo, or centre of calcification, is seated close to the occludent margin, and at about one fourth of the length of the valve from the apex. Internally, (fig. 15, a´, Pl. V,) the part above the umbo is flat; and beneath this upper part, there is a large rounded hollow (d) for the adductor muscle: a fold or indentation (a) running downwards from the umbo, extends in a very oblique line across the occludent margin. This fold is of high interest as giving lodgment to the Complemental Males, and will hereafter often be referred to.

Terga, triangular, flat; occludent margin, very slightly arched.

Carina much bent, with the umbo placed at barely one third of the entire length of the valve from the apex. Two very slight ridges can be perceived, one on each side, running from the umbo to the basal margin, and separating the roof from the parietes of the valve; these ridges are of great use in distinguishing the fossil carinæ of Scalpellum, from the carinæ of Pollicipes. The part above the umbo is formed by the upward production of a marginal slip along each side of the valve, which slips in the fossil species (C in the woodcut, fig. 1, given in the Introduction,) I have designated as the intra-parietes. The lower part of the valve gradually widens from the umbo downwards; internally, the whole is deeply concave, and continuously curved. The angle varies at which the upper and lower portions externally meet each other; but is never less than 135°. The upper part of the carina runs up between the terga for three-quarters of their length; the basal margin does not extend down low enough to pass between the carinal latera.

Rostrum, (fig. 15 b´, seen externally, and highly magnified,) minute, almost hidden by the enveloping membrane and by the small prominent umbones of the rostral latera; in area equalling about one fourth of the rostral latera; externally pyramidal, with the upper side rather longer than the lower; internally slightly concave, square, with the upper margin and sometimes with the lower margin, slightly hollowed out. Umbo of growth nearly central.

Upper Latera, flat, irregularly oval, with an almost rectangular shoulder under the basal angle of the terga; in area, about one third larger than the largest valve of the lower whorl; the exact degree of elongation of the oval figure varies a little. Umbo seated a little above the central point.

Lower Whorl,—Rostral Latera, nearly twice as long as broad, lying under the basal margins of the scuta: umbo seated over the rostrum; opposite end, towards which the valve widens either sensibly or but little, is either square or rounded; in area, less than any of the other valves, excepting the rostrum; in breadth, equalling either half or one third of the height of the infra-median latera; growth, directed chiefly towards the infra-median latera. The freely-projecting umbo is about one sixth part of the entire length of the valve.

Infra-median Latera, rather larger than the carinal latera; their shape varies from elongated pentagonal with the angles rounded, to oval, with the longer axis directed upwards. The umbo is seated a little above the middle of the basal margin, so that there is some little growth downwards, but the main growth is upwards. The upper point generally stands a little above that of the carinal latera.

Carinal Latera, flat, less in area than the infra-median latera; basal margin nearly straight; carinal margin slightly hollowed out, terminal margin arched and protuberant. The umbones of the two valves almost touch each other under the middle of the carina; main growth towards the infra-median latera and upwards; umbones projecting not above one fifth of the entire length of the valve.

Peduncle, much flattened, rarely as long as the capitulum, with the upper end nearly as wide as it; the lower end is either blunt, or tapers to a very fine point. The calcareous scales are transversely elongated, and are about four times as wide as high; their internal surfaces are slightly concave, and their external, convex; the two ends are pointed. Viewed internally, the scales approach in shape to rhomboids. There are, in a medium-sized specimen, about twenty scales in each whorl, their tips overlapping each other: the whorls are placed not very near each other and at rather unequal distances, except round the uppermost part, where, being in process of formation, they are packed closely together. The membrane uniting the scales, supports numerous transverse rows of articulated spines, varying from 1/100th to 1/500th of an inch in length, and each furnished with a long sinuous tubulus, 1/10,000th of an inch in diameter, running through the membrane to the underlying corium.

Attachment.—Specimens are attached to various horny corallines, and occasionally to the peduncles of each other.[51] In both cases, supposing the coralline to be erect, the capitulum is placed upwards, with its orifice towards the branch to which it is attached, and consequently with its carina outwards. Where several are crowded in a group, their peduncles often become twisted and their positions irregular, with their orifices facing in any direction. This uniform position is simply the consequence of the larva attaching itself head-downwards, and from the position of the prehensile antennæ, necessarily with its sternal surface parallel and close to the branch of the coralline; hence the dorsal surface, which afterwards is converted into the carina, faces outwards. The peduncle, as already stated, often tapers, at its basal extremity, to a sharp point. In very young specimens, for instance in one with a capitulum only 1/20th of an inch in length, the method of attachment is the same as in Lepas and many other genera, namely, by cement proceeding exclusively from the antennæ of the larva; but in older and full-grown specimens, instead of the whole bottom of the peduncle becoming flattened and broadly attached, which would be here impossible, the cement is poured out through a straight row of orifices along the rostral edge, thus causing, by an excellent adaptation, a narrow margin to adhere firmly to the thin and cylindrical branches of the coralline. These orifices are represented, magnified seven times, in Pl. IX, fig. 7, in which the lower attached portion of the peduncle is split open and exhibited; they are circular, and stand at regular intervals, in a straight line; the higher orifices are larger, but further apart from each other than the lower ones; in one full-grown specimen, I counted ten of these orifices in a length of exactly a quarter of an inch. At each period of growth, the corium recedes a little from the attached portion of the peduncle; of which portion, the greater part is thus left empty and as incapable of further growth, as are the larval antennæ at the extreme point: in the specimen figured, the corium extended a little below the upper orifice. The prehensile antennæ, however, I must remark, do not strictly rise from the extreme point of the peduncle, but at a little distance from it, on the rostral surface; this simply ensues from the antennæ in the larva, being situated on the sternal surface, close to, but not actually on the front of the head. The two cement glands are seated high up on the sides of the peduncle, and remote from each other; they are small, unusually globular and transparent. The two cement-ducts (fig. 7 a a) proceeding from them, are 3/2000ths of an inch in diameter, and run in a zig-zag line; at the point where they pass through the corium to enter the lower attached portion of the peduncle, they become closely approximated, and partially imbedded in the membrane of the peduncle. Together they run along the rostral edge, giving out through each orifice a little disc of brownish cement, and finally they enter the larval antennæ. The peduncle, just above the attached portion, where still lined by corium, no doubt increases in diameter at each period of growth, and must, I presume, become pressed against the almost parallel branch of the coralline. The corium, at this same period, shrinks, or is absorbed, and the two cement-ducts come in contact with, and adhere to, the inner surface of the outer membrane of the peduncle; and then, by a process which I do not understand in this or any other Cirripede, apertures are formed both in the ducts and through the membrane, so that the cement passes through, firmly fastening the outer surface of the peduncle with its calcareous scales and spines, to the coralline.

The structure of the larval prehensile antennæ will be most conveniently described when we come to the Complemental male; and figures (10-12, Pl. V) will be given.

Size and Colours.—Montagu states (‘Test. Brit.,’ p. 18) that British specimens rarely have a capitulum .62 of an inch in length; I have, however, seen an Irish specimen, .7 long; and several specimens, from the Bay of Naples, .8 long, and including the peduncle, 1.3 in length. The valves in all the specimens are white, and the membrane connecting them either nearly white, or dirty pale yellowish, or purplish-brown. Within the sack the corium under the valves is tinted pale purple, and two very faint bands of the same colour can generally be distinguished running down the two sides of the peduncle. Body, coloured yellowish-white, with the upper segments of the pedicels of the cirri, tinted in front with purple.

Body, much flattened, the prosoma is very little developed; the mouth placed far from the adductor muscle, and is directed in a remarkable manner towards the ventral surface of the thorax: the first pair of cirri stands far separated from the second pair.

Mouth.—Labrum with the upper part highly bullate, forming an overhanging projection equalling the longitudinal axis of the mouth; basal margin much produced; crest with a row of bead-like teeth.

Palpi rather small, with their external margin straight, and internal margin oblique: the bristles on the two palpi just meet each other.

Mandibles, with five or six teeth, with the second, (or second and third, when there are six teeth,) smaller than the others; in two specimens, there were five teeth on one side and six on the other; inferior angle rather broad and strongly pectinated.

Maxillæ with the edge nearly straight, without any notch, but with the inferior portion very slightly projecting; there are twelve or thirteen pairs of unequal spines, of which some of the middle ones are rather longer than the others, and almost as long as the two upper great spines.

Outer Maxillæ.—On the inner margin the bristles are divided into two separate tufts; exteriorly, near the base, there is a distinct rounded swelling with bristles. The olfactory orifices are highly protuberant, approximate, flattened, scarcely tapering towards their upper ends.

Cirri.—The five posterior pair are elongated, very little curled, with short pedicels; their segments are long, not at all protuberant in front, bearing five or six pairs of long, slightly serrated spines, with a very minute tuft of bristles between each pair, and with some short lateral spines on the inner side of each segment; on the fourth pair of cirri, these lateral spines are considerably developed; dorsal tufts consist of fine spines, with one much longer than the others. First pair short, separated by a wide interval from the second; rami unequal in length, by between two and four segments; longer ramus having nine segments, scarcely half as long as the rami of the second cirrus; shorter ramus with seven segments; in the same individual there were twenty segments in the sixth cirrus. The segments in the shorter ramus of the first cirrus are oblong in a transverse direction, and may be compared to a set of shields placed transversely and strung together; in the longer ramus the segments are longitudinally oblong; in both they are thickly covered with spines. Second cirrus; the anterior ramus is a little broader than the posterior ramus, with the segments bearing about five rows of bristles; fifteen segments in the shorter ramus. Third pair, with the two rami equal in thickness, and with the segments differing very little from those of the posterior cirri, excepting that the serrated spines in the external lateral rows are rather larger. The fourth pair is remarkable by having, on the inner side of the upper edge of each segment, a little tuft of minute smooth spines, flattened, and a little enlarged near their ends, so as to be spear-shaped; I could not see these singular spines on the other cirri. The lower segments of the pedicels of all the cirri, excepting the sixth pair, are remarkable from having their inner edges, in the middle, produced into a considerable, abrupt, rounded projection, irregularly covered with spines.

Caudal Appendages, (Pl. X, fig. 21,) very small, flattened, of nearly the same width throughout; in a medium-sized specimen, only 1/100th of an inch in length; each bears from ten to twenty small bristles placed distantly from each other, of which those on the rounded apex are the longest.

Generative System.—The penis is remarkably acuminated; the vesiculæ seminales are unusually small, and enter only for a short distance into the prosoma; the testes are large. The ovarian tubes are of large diameter; the ova are nearly spherical and large, namely, 9/400ths of an inch in diameter; they are not numerous, and lie in single layers in the two lamellæ. The ovigerous fræna are well developed, and lie under the scuta; one I measured was 5/100ths of an inch in length and 2/100ths in width; the margin is obliquely truncated and slightly sinuous. This species breeds late in the autumn, and even in mid-winter; I have examined a specimen from Cornwall with ova containing larvæ, taken on the 26th of October; again, in another specimen from Belfast, sent to me by Mr. Thompson, taken in January, there were ova in the lamellæ, and therefore no doubt impregnated; and on February the 12th I received from Mr. Peach, from Cornwall, specimens so very young that they must have become attached during the first days of the month.

Varieties.—The specimens from near Naples, (which I owe to the kindness of the Rev. F. W. Hope,) are somewhat larger, and differ slightly from those of Britain: they form, I imagine, the S. Siciliæ of Chenu. After carefully examining them internally and externally, I think it is quite impossible to consider them specifically distinct, for although in several specimens, the valves were placed a little further apart from each other,—the upper latera a little more elongated,—the carinal latera rather narrower in their upper half,—the infra-median latera rather more rounded,—and, lastly, in the scuta, the tergal margin extended almost in the same line with the lateral margin; nevertheless in other specimens, I could perceive no difference whatever. It is, however, remarkable that in several full-grown Neapolitan specimens there were no Complemental males, whereas I have never seen a single full-grown British specimen without such being present. In some specimens in the British Museum, without any given locality, I have observed considerable variation in the breadth of the carinal and rostral latera.

COMPLEMENTAL MALE. Pl. V, figs. 9-14.

When first dissecting Scalpellum vulgare, I was surprised at the almost constant presence of one or more very minute parasites, on the margins of both scuta, close to the umbones: these are represented, but rendered darker and therefore more conspicuous than in nature, in the drawing, Pl. V, fig. 15, which is three times the natural size. I carelessly dissected one or two specimens, and concluded that they belonged to some new class or order amongst the Articulata; but did not at that time even conjecture, that they were Cirripedes. Many months afterwards, when I had seen in Ibla, that an hermaphrodite could have a complemental male, I remembered that I had been surprised at the small size of the vesiculæ seminales in the hermaphrodite S. vulgare, so that I resolved to look with care at these parasites; on doing so, I soon discovered that they were Cirripedes, for I found that they adhered by cement, and were furnished with prehensile antennæ, which latter, I observed with astonishment, agreed in every minute character, and in size, with those of S. vulgare: the importance of this agreement will not at present be fully appreciated. I also found, that these parasites were destitute of a mouth and stomach; that consequently they were short-lived, but that they reached maturity; and that all were males. Subsequently the five other species of the genus Scalpellum were found to present more or less closely analogous phenomena. These facts, together with those given under Ibla (and had it not been for this latter genus, I never probably should have even struck on the right track in my investigation,) appear sufficient to justify me, in provisionally considering the truly wonderful parasites of the several species of Scalpellum, as Males and Complemental Males. When these parasites are fully described, will be the proper time to discuss and weigh the evidence on their sexual relations and nature. I will now describe the parasite of S. vulgare.

General Appearance.—Shape, flask-like, compressed (Pl. V, fig. 9, magnified 36 times), with a short neck: the outline is usually symmetrical, but sometimes is a little distorted on the under side. The creature is imbedded more than half its length or depth in the transparent, spine-bearing chitine border of the scutum of the hermaphrodite. Its length, or longer axis, varies from 10 to 11/400ths; its breadth, or transverse axis, is 6 to 7/400ths; and its thickness, for it is much flattened, is only 4/400ths of an inch. On the summit, there is a fimbriated orifice (a), the size of which can rarely be made out quite distinctly, owing to the extreme thinness of the membranous edges. A little way beneath the orifice, there are four little blunt, bristly points (b), generally rather more than the 1/1000th of an inch in length; they are rather variable in size, and seem to be of no functional importance; directly beneath them, there are four little calcareous beads (as may be known by their dissolving with effervescence in any acid, and breaking easily under the needle); these are the 3/2000ths of an inch in their larger external diameter; they are rather deeply imbedded in the outer integument, and taper a little downwards ending in a concave terminal point, into which a minute tubulus enters, like those passing into and through the valves of ordinary Cirripedia: along the axis of imbedment, they are often 4/2000ths of an inch in length. These calcareous beads or rudimental valves are seated in pairs, at the two ends of the flattened animal, so that when the animal is laid on one side, the upper bead in each pair exactly covers and hides the lower one. The outer integument is composed of chitine, as may be inferred from boiling caustic potash having no effect on it; the upper part is thicker than the imbedded portion and is wrinkled transversely; it is covered with minute spines 4/10,000ths of an inch in length, either single or in groups of two and three, (Pl. V, fig. 14.) This outer tunic is lined by corium, sometimes slightly mottled with dull purple; and this by delicate, longitudinal, striæ-less muscles, running from the base up to the under edge of the orifice; these longitudinal muscles are crossed, at least, in the upper part, by still finer transverse muscles.

Thorax and Abdomen.—When the external integument is cut open, the thorax (Pl. V, fig. 13) is found lodged within an inner sack or rather tube, extending from near the bottom of the animal, up to the external orifice. The whole thorax is sometimes forced through the orifice, owing perhaps to the action of the spirits of wine and consequent endosmose, and is thus well displayed without dissection. The thorax tapers a little, is much flattened and straight; its length, together with the terminal abdominal lobe, is about 6/400ths of an inch; it is formed of very thin, most finely hirsute membrane, transversely wrinkled and so extensible, that when everted by the internal muscles being seized, it stretches to twice its former length; in this condition, five transverse articulations are displayed. The abdominal lobe is smooth, and cannot be stretched, or turned inside out by pulling the above muscles. On the thorax, corresponding with the interspaces between the five transverse articulations, there are four pair of short limbs, but their bases, I believe, are prolonged across the inner or ventral surface of the thorax, so as almost to touch each other. These limbs, I believe, have no articulations, except, perhaps, where united to the thorax. The anterior or lowest limb, on each side, supports two or sometimes only a single spine; this pair is rather smaller than the second, and is placed a little more distant from it, than are the upper pairs from each other. The second pair differs from the upper two, only in having its three spines a very little shorter. The two upper or posterior pair exactly resemble each other; each has two spines on the summit, and a third seated lower down, on a little notch on the outer side, but with its point on a level with the others. The points of the spines of the two upper limbs, stand on a level with the external spines at the end of the abdomen. All the spines are of excessive tenuity and sharpness; they are straight, long, and not plumose.

The abdominal lobe is square, and from not being wrinkled, has a different appearance from the thorax: on each of the posterior angles, there are three moderately long, very sharp spines, with the tips of the outer pair bent a little inwards; in the middle between them, there are two little spines, and a little below and outside these latter, on the ventral surface, there are two other longer spines with their tips bent inwards; and again, lower down, two other pair, one beneath the other, of short spines. Perhaps, the three pair of spines on the ventral surface, mark the three segments, which are distinct on the abdomen of the larva in the last stage of its development, in Lepas and other genera. In the same way, it is probable that the lateral spine on the notch in each limb, marks the point where, in the larva, there is an articulation. Altogether, there are seven pairs of spines on the abdomen, and eleven pairs on the thoracic limbs.

A little way beneath the lower or anterior pair of limbs, the thorax is abruptly bent, and becomes confluent with the lower internal parts of the whole animal. Here, the very delicate membrane of chitine which lines the sack or tube, extending from the external orifice, can be seen to be continuous, as in all Cirripedes, with the outer tunic of the thorax. Within the thorax, there are some longitudinal muscles, without transverse striæ, which, I believe, enter the short limbs, but not the abdomen, as I infer from the latter not being everted when they are pulled. At their lower ends these muscles terminate abruptly, and from being contracted are often a little enlarged. They extend a short way beneath the lower pair of limbs, and are, I suspect, attached to the outer integument of the animal, near the base.

After the most careful dissection of very many specimens, and their examination in many different methods (as by caustic potash, &c.), I can venture positively to assert that there is no vestige of a mouth, or masticatory organs, or stomach: I did not see any anus, but I will not affirm that such does not exist.

In the upper part of the animal, lying under the superficial muscles, and close beneath the upper line of their attachment, I found in all the specimens, an eye, of a pointed oval form, rather less than 11/12,000ths of an inch in diameter, formed of an outer capsule, lined with purple pigment-cells, and surrounding, as it appeared, a lens. The eye is not introduced in fig. 9, for I could not see it, except by dissection, and therefore do not know its exact relative position.

Generative System.—The contents of the animal, between the sack containing the thorax and the outer integuments, and directly under the thorax, varied much in condition: in young and lately attached specimens the whole consisted of a pulpy mass with numerous oil-globules; in other specimens, apparently more mature, there were vast numbers of cells, sometimes cohering in sheets, about 3/10,000ths of an inch in diameter, and having darkish granular centres; these I believe to be the testes, for in a specimen presently to be mentioned, in which the vesicula seminalis was gorged with spermatozoa, I found adhering to its outside, a mass of cells of exactly the same diameter, but now empty and transparent instead of having brownish centres. Lastly, in several other specimens, at the very bottom of the sack-formed animal, there was a brownish, pear-shaped bag, of different sizes in different individuals, and occasionally broader even than the thorax. This bag contained either pulpy matter, or a great mass of spermatozoa. Before being disturbed, these spermatozoa lay parallel to each other in flocks, and they yielded to the needle in a peculiar manner, so that I found (having had experience with these bodies in living Cirripedia) I could almost tell before examination under the compound microscope, whether or not I should see spermatozoa. Many had distinct heads,[52] which were two or three times as broad as the filamentary bodies; the latter when placed between glass were the 1/20,000th of an inch in diameter. I compared these spermatozoa with others taken out of the vesiculæ seminales of the individual hermaphrodite S. vulgare, to which the parasite was attached, and could not perceive the slightest difference in them. The brownish pear-shaped bag, or vesicula seminalis, the coat of which seems fibrous, could sometimes be distinctly traced, sending a chord or prolongation far up the thorax: at the end of the abdominal lobe, no doubt there is an orifice; and this, I believe, I once distinguished. Owing to this chord, the bag often adheres to the thorax, when the latter is dissected out of the general integuments; in this condition, I twice clearly made out that it was single: in one other specimen, however, there appeared to be two small vesiculæ seminales. By using a condenser and very brilliant light, the outline of the vesicula seminalis could sometimes be distinguished before dissection, at the bottom of the sack-formed animal; and such was the case in the specimen drawn in fig. 9.

Although I have dissected, at least, thirty specimens, taken at different times of the year, and from different localities, and when many of the specimens were mature and ready for the impregnation of ova, as clearly shown by the presence of innumerable spermatozoa, I have never seen even a trace of an ovum or ovaria.

Antennæ and Attachment.—The prehensile antennæ (Pl. V, fig. 10), are seated a little above the very base of the sack-like animal; and this might have been expected from the antennæ in the larva, being seated on the ventral surface, not at the very extremity of the head. By a very strong light, they can sometimes just be seen whilst the parasite is attached to the hermaphrodite (the scutum of the latter having been cleaned on the under side), and are thus represented in fig. 9. They are formed of thicker membrane than the general integument of the body: the second segment, or disc, is pointed and hoof-like; when seen in profile (fig. 11), the upper convex surface has a uniform slope with the upper surface of the basal segment; it is furnished with a single backward pointing spine, attached, I believe, on the under side, nearly opposite the articulation of the ultimate segment: at the apex, there are some excessively minute hairs or down. The ultimate segment projects rectangularly outwards as usual, and has on its inner side, rather beneath the middle, a conspicuous notch (fig. 12), which bears two or three long, non-plumose spines; on the summit there are three or four rather shorter spines. On the outside of the great basal segment there is a single spine curving backwards. The importance of the following measurements (in fractions of an inch) will hereafter be seen.

I did not see the cement-ducts, which, perhaps, was owing to the corium extending from the inside of the whole animal some way into the antennæ, thus rendering them rather less transparent than in common Cirripedes. That the ducts and cement-glands exist, is certain, for the antennæ in every case were enveloped in a little irregular mass or capsule of the usual, brown, transparent, laminated cement. When several of these parasites were attached close together, the cement ran up between them.

I may here state, that I found on one Scalpellum, three males very lately attached, and not as yet imbedded in the chitine border; they were white, opaque, pulpy, and full of oily globules; the lower part was considerably more pointed, and extended further beyond the prehensile antennæ, than in the older and imbedded specimens. There were distinct remnants of two great reddish-brown eyes, showing that in this respect the larvæ of the male in their last stage of development, are characterised like the larvæ of other Lepadidæ. The male larva would, probably, be a little larger than the male itself; but yet compared with the larva in the earliest stage, there can have been unusually little increase of size during the several intermediate metamorphoses; I judge of this from the dimensions of the larva of the hermaphrodite in the first stage, namely, 9/400ths of an inch, exactly the size of some of the smaller males. In the allied genus Ibla, the increase is also less than is usual, namely, from 15/1000ths of an inch, the diameter of the ovum, to only 25/1000ths of an inch, the length of the boat-shaped larva, just before its final metamorphosis.

Habits and Concluding Remarks.—The males are imbedded in the spinose chitine border of the occludent margin of the scuta, exactly over an oblique fold or notch (fig. 15 á a), close by the umbo. This fold has no direct relation to the males, but being present is taken advantage of by them; for it occurs in the young hermaphrodite, before the attachment of the males, and in species of the genus in which the males are attached to other parts. It occurs, also, in fossil species of Pollicipes, and in these it seems caused by the upper inner part of the valve being rendered more and more prominent during growth: in the present species, I suspect, its origin is connected with the formation of a ridge bounding the outer side of the pit for the adductor scutorum muscle: we shall see in the next species, that this fold is of the highest importance in relation to the position of the Males. The transparent chitine border of the scuta is broad, and fills up the fold in the shell, so that the outline of the occludent margin is not affected by it: in the drawing (fig. 9) some of the inner layers of chitine (e e), which dipped into and filled up the fold, have been removed, that the lower part of the animal might be more plainly exhibited. The chitine bears numerous spines of various lengths, which must afford some protection to the males, rudely arranged in lines, parallel to the edge of the valve, indicating the successively-formed layers of chitine; each spine has a fine, tortuous tubulus connecting its base with the underlying corium. The extreme outer edge of the border is thin, forming a kind of lip, close beneath which the delicate tunic lining the sack is attached. During continued growth, the valve is added to in thickness, and so is the chitine border, and likewise in breadth. It appears that the larva of the male must attach itself on the under side of this border, on the edge of the tunic of the sack, and that by the action of the cement, the corium beneath is killed (as I believe always is the case with other parasitic Cirripedia), whereas on both sides, the chitine continues to be added to, so that the male, excepting the upper and always projecting portion, becomes imbedded at first laterally, and ultimately all round: I have seen specimens in several different stages of imbedment. Hence, in old specimens, with a thick and broad chitine border, it might and does come to pass that one male is imbedded (the valve being laid flat) directly beneath another.

I have examined a great number of specimens from various localities, taken at different times of the year,—some dozen specimens from Cornwall,[53] and several from unknown localities in various collections; some from Ireland, from the Shetland Islands, from Norway, and from near Naples. Every one of these specimens, with the exception of some of the Neapolitan ones, had parasitic males attached to them: I must also except very young specimens, on which they never occur. On a Cornish specimen, with a capitulum a little more than one fifth of an inch in length, it may be mentioned as unusual that there were three males. In young specimens there is generally one male on each scutum, but sometimes there are two, and sometimes none on one side. In large old Cornish specimens I have counted on the two sides together, six, seven, and eight males, and in one Irish specimen no less than ten, seven all close together on one valve and three on the other, but I do not suppose that all these were alive at the same time. In the Neapolitan specimens, however, which are the largest that I have seen, there was in no case more than two; and out of seven or eight specimens, four had not any male; so that it would appear there is something in this locality hostile to the development of the parasitic males. I have noticed only one instance (that given in fig. 9) in which the males were imbedded a little way apart; generally they touch each other, and are cemented together: where there are several males, they occur at different levels, as measured from the under or upper surface of the chitine border: in one instance of four males adhering to one valve, I distinctly perceived that the lowest one was white, pulpy, and recently attached; the two above, which were placed close together and between the same laminæ of chitine, were mature; and the third still higher up, was dead, empty, transparent, and half decayed: in some other instances, I have found the uppermost parasites dead, and, together with the surrounding chitine, partially worn away.

The larva of the male must have a different instinct from the larva of the hermaphrodite; for the latter attaches itself head downwards to a coralline, whilst the male larva crawling on the scuta of the hermaphrodite, discovers, I presume by eye-sight, the fold in the shell beneath the translucent border of chitine, and there invariably attaches itself. Its object in choosing this particular spot, I believe, simply is that the depth or thickness of the chitine is there greater, and sufficient for its imbedment, which would hardly be the case elsewhere. This parasite has, as we have seen, no mouth or stomach, and indeed, considering its fixed position and the non-prehensile condition of its limbs or cirri, a mouth would have been of no service to it, without it had been extraordinarily elongated. The male must live on the nourishment acquired during its locomotive larval condition; and its life no doubt is short, but yet not very short, as I infer from the depth to which mature specimens are buried in the chitine border. The full development of the spermatozoa consumes, I suppose, some considerable lapse of time. The thorax and limbs, though furnished with muscles, are obviously, as already remarked, of no use for prehension; these parts serve, probably, to defend the little creature, when its eye announces the passing shadow of some enemy, and for this purpose they are well adapted from the extreme sharpness of the spines. The thorax, into which I traced the vesicula seminalis, no doubt also serves for the emission and first direction of the spermatozoa; and hence, perhaps, its singularly extensible structure. I have already remarked, that in specimens preserved in spirits, the thorax is often largely protruded, and bent down at right angles to the orifice. I presume this is caused by endosmose; nevertheless it deserves notice, that it was in these protruded specimens that the vesicula seminalis was most conspicuously gorged with spermatozoa. I suspect the longitudinal and transverse muscles lining the upper part of the outer integuments of the whole animal, can be of little use to the creature, without it be to aid in the protrusion of the thorax, and perhaps in the violent expulsion of the spermatozoa, thus causing them to reach the ovigerous lamellæ within the sack of the hermaphrodite. It is also probable, that the action of the cirri of the hermaphrodite, would tend to draw inwards the spermatozoa in the right direction. In one specimen, the spermatozoa in the hermaphrodite and in the male were mature at the same time; in another this was not the case; and as the males, apparently, become attached at all periods of the year, this want of coincidence in maturity must often occur. Can the males retain their spermatozoa, till told by some instinct, that the ova in the sack of the often fecundated hermaphrodite are ready for impregnation; or are the spermatozoa sometimes wasted, as must annually happen with such incalculable quantities of the pollen of many diœcious plants?

This little Cirripede is, in many respects, in a partially embryonic condition. There is no separation between the capitulum and peduncle; there is no mouth; and the thorax, throughout its whole width, opens into the anterior part of the animal: the limbs differ greatly from those both of the mature Cirripede and of the larva, but come closest to the latter: the preservation of the abdomen is a well-marked embryonic character. On the other hand, the four rudimentary calcareous valves, the narrow orifice, the hirsute outer integument, the two muscular layers, the single eye, and male internal organs, are all characteristic of the fully-developed condition. The four little valves, as I believe, represent the scuta and terga, though they are placed considerably below the orifice: the little bristly points have no homological signification, and are absent in the male of the following closely allied species. The four pairs of limbs answer to the four posterior cirri, as may be inferred from their proximity to the abdominal lobe, and from the three posterior pairs closely resembling each other, and differing a little from the first pair; this latter pair corresponds with the third pair in the hermaphrodite form of Scalpellum. If I am right in believing that only a single vesicula seminalis is ordinarily developed in the male, this is a special and singular character.

As stated in the beginning of this description, from the one great fact of the absolute correspondence of the prehensile antennæ of the parasite, with those of the hermaphrodite Scalpellum vulgare, together with its fixed condition, its short existence, and exclusively male sex, I have thought myself justified in provisionally considering it as the Complemental Male of the Cirripede to which it is attached; but I hope final judgment will not be passed on this view, until the whole case is summed up at the end of the genus.[54]