Filaments, five on each side; segments of the three posterior cirri with triangular brushes of spines.

Var. (Donovani, of Leach.) Carina with the upper part flat, spear-shaped, externally with a narrow central ridge.

Var. (Villosa. [Pl. I], [figs. 6 b, c.]) Valves placed rather distant from each other; carina extremely narrow, with the upper part of nearly the same width throughout; terga with the lower part much acuminated; body of animal finely villose.

Coasts of Great Britain and France; Baltic Sea, according to Montagu Southern United States (from Agassiz); tropical Atlantic Ocean; East-Indian Archipelago, off Borneo and Celebes; Pacific Ocean, between the Sandwich and Mariana Archipelagos; New Zealand: attached to fuci, Spirulæ Janthinæ, Velellas, often to feathers and cork; often associated with the young of L. anserifera, (var. dilatata,) and L. pectinata.

General Appearance.—Capitulum highly variable in all its characters; thick and broad in proportion to its length, but the breadth is variable,—in some specimens, the capitulum being longer by one-fifth of its total length than broad; in others, one-fifth broader than long. Valves generally approximate; in some varieties, however, from the narrowness of the carina and terga, the valves stand far apart, there being an interval between the carina and scuta of nearly half the breadth of the latter. Valves excessively thin, brittle, transparent, colourless, smooth, but generally sinuous along the zones of growth, which are conspicuous: valves generally covered throughout by thin chitine membrane, which is thickly clothed, especially in the interspaces between the valves, with minute spines, barely visible to the naked eye. Scuta with the lower part of the tergo-carinal margin extremely protuberant; occludent margin, more or less, but slightly reflexed, with a depressed line running from the umbo to the apex; basal margin much reflexed, but to a variable extent and at a varying angle, even up to a right angle,—an external rim or collar being thus formed. There are no distinct internal teeth, but the basal margin under the umbones, is more or less distinctly produced into a rounded disc or projection, which is generally not so much outwardly reflexed as the rest of the basal margin: there is no distinct internal basal rim. The primordial valves are generally visible, but they do not lie, as in all other species, close to the basal margin, but a little above it,—the lower reflexed portion having been subsequently developed. Terga flat, with the occludent margin slightly arched, and not, as in the foregoing species, formed of two sides; apex bent towards the carina; width of the lower half highly variable, owing to the varying extent to which the scutal margin is hollowed out; in some specimens, the whole lower half beneath the apex of the scuta is of nearly the same width throughout; in other specimens this lower part is spear-shaped. The widest part of the tergum either equals in width, or is only two-thirds of the width of the widest part of the carina beneath its umbo. Carina ([Pl. I], [fig. 6 a]) highly variable in shape, with the part above the umbo either spear-shaped and slightly concave within, or nearly flat and furnished with a central external ridge; or the upper part (fig. 6 c) is of equal and extreme narrowness throughout, and deeply concave within, appearing as if only the central ridge had been developed. The part below the umbo, (answering to the fork in the foregoing species,) is about one-third of the length of the whole valve, and generally twice as wide as the upper part, but in the variety with the upper part of the carina equally narrow throughout, the lower part is thrice as wide as the upper; the disc, or lower part, is generally slightly concave within, exteriorly either with or without a central ridge; basal margin rounded; lateral margin more or less curved, according to the form of the upper part. The disc is not more deeply imbedded in membrane than is the upper part of the valve. The heel or umbo is either angular and prominent, or rounded. In very young specimens the carina is simply bowed, instead of being rectangularly bent.

Peduncle,—short, narrow, being abruptly inflected all round under the basal edges of the capitulum; lower part of very variable shape, being often suddenly contracted into a mere thread ([fig. 6 b]), which sometimes widens again at the extreme end. The external membrane is very thin, and is penetrated by the usual fine tubuli leading to the corium; its surface is wrinkled and destitute of spines, or with extremely few. The peduncle is often completely surrounded by a yellowish ball, (of which I have seen specimens from the coast of England, and from off Borneo,) sometimes half as wide as the capitulum, composed of very tender, vesicular, structureless membrane, and of a pulpy substance: perhaps the yellow colour may be owing to long immersion in spirits. Some authors have supposed that the ball was the ovisac of the animal; and for the first few minutes, deceived by the numerous included spores of, as I believe, Bacillariæ, I thought that this was the case; others have supposed that it consisted of some encrusting algæ or other foreign organism; but it is, in reality, a most singular development of the cement-tissue, which ordinarily serves to attach Cirripedes by their bases to some extraneous object, but here surrounding that object and the peduncle, gives buoyancy, by its vesicular structure, to the whole. The membrane of the ball falls to pieces in caustic potash, differently from the chitine membrane of the enclosed peduncle, and this shows that there is some difference in composition from ordinary cement. The ball, when cut in two, exhibits an obscure concentric structure. The whole is excreted by the two cement-ducts, through two rows of orifices, one on each side of the surrounded portion of the peduncle; and I actually traced, in one case, the yellow pulpy substance coming out of the cement-ducts. The upper apertures are in gradation larger than those below them, and they stand a little further apart from each other; these are figured as seen from the outside, much magnified, at [Pl. I], [fig. 6 d]. I did not succeed in finding the cement-glands, but I followed the ducts, of rather large size, running for a considerable distance as usual along and within the longitudinal muscles of the peduncle. Nearly opposite the uppermost aperture, on each side, the duct passes out through the corium, and becomes laterally attached to the outer membrane of the peduncle, at which point an aperture is formed (as in other cases, by some unknown process), thus giving exit to the contents of the duct. Beneath this upper aperture the duct runs down the peduncle, between the corium and the outer membrane, till it comes to the next aperture, to which it is also attached, and so on to all the lower ones; but I believe no cement tissue continues to pass out through these lower apertures. Beneath the lowest aperture the two ducts run into the two prehensile antennæ of the larva, which, as usual, terminate the peduncle. The antennæ are attached to some small foreign body in the centre of the vesicular ball, by the usual tough, light brown, transparent cement. The two upper apertures are nearly on a level with the outside surface of the ball; and it was evident that as the animal grows, new apertures are formed higher and higher up on the sides of the peduncle, and that out of these, fresh vesicular membrane proceeds, and grows over the old ball in a continuous layer. It appears that the growth of the vesicular ball is not regular,—that it is not always formed,—and that when formed the whole, or the lower part, sometimes disintegrates and is washed away. As that portion of the peduncle which is enclosed ceases to grow, and has its muscles absorbed, retaining only the underlying corium, whereas the upper unenclosed portion, and likewise, (as it appears) lower portions once enclosed but since denuded, continue to increase in diameter, the peduncle, when the vesicular ball is removed, often has the most irregular outline, contracting suddenly into a mere thread, and then occasionally expanding again at the basal point.

Frequently two or three specimens have their peduncles imbedded in one common ball, of which there is a fine specimen in the College of Surgeons ([Pl. I], [fig. 6]), the ball being about one inch and a quarter in diameter, with a slice cut off. In this specimen, it is seen that the vesicular membrane proceeding from several individuals, unites to form one more or less symmetrical whole, and that the original common object of attachment is entirely hidden. Dr. Coates[29] gives a curious account of the infinite number of specimens, through which he sailed during several days, in the Southern Atlantic Ocean: the balls appeared like bird’s eggs, and were mistaken for some fucus, which was supposed to have encrusted the scales of the Velellæ, to which the Cirripede had originally become attached. Several individuals had their peduncles imbedded in the same ball, “which floated like a cork on the water.” As this species grows into an unusually bulky animal, we here see a beautiful and unique contrivance, in the cement forming a vesicular membranous mass, serving as a buoy to float the individuals, which, when young and light, were supported on the small objects to which they originally had been cemented in the usual manner.

[29] Journal of the Acad. Nat. Sc., Philadelphia, vol. vi, p. 138, 1829.

Filamentary Appendages.—Five on each side, of which four lie in pairs at the base of the first cirrus (of these, only three are sometimes developed), and one on the flank of the prosoma.

Mouth.—Palpi much acuminated. Mandibles with five teeth; the first not far remote from the second; inferior point rather broad and finely pectinated. Maxillæ with two large, unequal, upper spines, and four regular steps.