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.