[ALCIPPE] LAMPAS, Hancock ut suprà.
Hab.—North-eastern shores of England, fifteen to twenty fathoms, imbedded in dead shells of Fusus antiquus and Buccinum undatum (A. Hancock); south-eastern shores, off the Eddystone, Lighthouse (C. S. Bate).
FEMALE. Pl. [22].
I may premise that after the sketch of the leading peculiarities of [Alcippe], and after the discussion on its affinities, just given under the Family, I think it would be superfluous to institute a full generic description, separately from the following detailed account of this most anomalous cirripede.
General Appearance.—The whole animal is from .2 to .3 of an inch in length, of a soft texture, colourless or yellowish, and lives concealed in a cavity of its own formation in the shells of certain Gasteropods. This cavity communicates with the water by a narrow fissure-like orifice (Pl. [22], fig. [4]), broadest at the posterior end, where the cirri are exserted; narrow, closed, and generally curved at the other (a) end: the two sides of the fissure (b) are commonly bordered by a calcareous inorganic deposit: the walls of the cavity are worn so thin over the peduncle, at the narrow end of the fissure, that the orange-coloured ovaria can generally be seen through the shell of the mollusc, and hence there is here a distinct fan-shaped stain (fig. [3]) on the surface. The animal consists of a compressed capitulum, without valves, and of a sort of peduncle depressed on its rostral face, and covered with a broad, oval, thin, horny disc. We must remember that in the Lepadidæ the peduncle does not essentially differ from the capitulum, being only the flexible lower or anterior end of the animal, and is separated from the capitulum only by shape, and generally by the direction of the lines of growth. The disc, when most regular (fig. [1], H), lies in a plane at right angles to the sides of the capitulum, and almost in a line with the orifice leading into the sack; but the peduncle is often very irregular (fig. [2]), and the disc comes even to occupy a position nearly parallel to one or the other side of the capitulum. On the carinal side, the capitulum is generally separated from the peduncle by a rather deep fold (f, in the section fig. [5]), but this depends in some degree upon the state of distension of the mass of ovarian cæca. I have given a drawing, fig. [1] (partly taken from Mr. Hancock), of a very regular individual, and of an extremely distorted specimen (fig. [2]). The distortion, I believe, is generally caused by the animal, during its excavation, breaking into some old cavity.
External Structure.—The orifice leading into the sack is about one third of the total length of the animal: its edges or lips are thickened, horny, and brownish: at the lower end, exactly where the orifice ends, the lips are formed, from being deeply notched, into two sharp projections (a, figs. [1], [5], [6]), unlike anything occurring in any other Cirripede. The external membrane (c, fig. [6]) of the lip supports an irregular but nearly straight band of sharp, thick spines of chitine, about 1/1000th of an inch in length, together with a few hairs: at the carinal or upper end of the orifice the spines are largest and most numerous; at the other and lower end, they decrease in size; and on the two projections (a, fig. [6]), and on the adjoining parts of the external membrane, they graduate into the small dentated points which cover the whole surface of the animal. The inner tunic of the sack (b), on each side along the upper half of the orifice, is remarkable from having a moderately broad, curved band of short, sharp spines, not quite so thick as those on the external surface, closely adpressed together and pointing upwards, like the javelins of an ancient phalanx, thus probably preventing the ingress of any intruding animal. This band of spines curves at the upper end, conformably with the shape of the orifice. The inner tunic of the sack in this upper part is yellowish, and, what is very unusual, is thicker than the external membrane. A little way down, within the orifice, and more especially in front of an elegant row of hairs on the two sides of the great labrum, there is a band of very fine but stiff hairs (5/1000ths of an inch in length), pointing upwards, and making together with those on the labrum a hedge, barring ingress into the sack.
The external membrane over the whole animal, excepting the horny disc which covers the rostral face of the peduncle, is very thin and transparent; it is periodically and often moulted, as may be inferred from the many old lines of junction round the edges of the horny disc: it is irregularly and pretty thickly (but not so thickly as in fig. [7]) studded with star-headed, minute points, from 2 to 5/10,000ths of an inch in diameter, composed of hard chitine, seated on a short footstall, and this on a circular, yellowish, slightly thickened disc of the general investing membrane, appearing like a halo surrounding each little point. These points are directed obliquely upwards. There are none on the horny disc, though particularly numerous close to its margin. Their state varies much: just after a moult, when newly formed, the spines are regularly star-headed, with quite sharp rays, from two to six in number, with some of them occasionally bifid; but these points or rays soon become blunted, and ultimately half the star is worn away, so that the appearance then presented is that of a crescent with a few blunt points on its convex side. At each exuviation, the thickened membrane of the orifice with its strong external spines (the condition of which also varies according to the period elapsed since the last moult), and of course the whole internal tunic of the sack, with its spines and hairs, are all moulted, together with the external membrane and the little star-shaped points. In most specimens a barely distinguishable band or bar of yellowish, slightly thickened membrane, runs from the points (a), at the lower end of the orifice, for some way obliquely downwards; and at the lower end of this bar the weak adductor scutorum muscle (having transverse striæ) is attached. This bar is often strengthened by a prominent external fold of membrane, but yet it is so flexible, and as it is united only to the lower end of the orifice, I can hardly believe that it can, by means of the adductor muscle attached to its opposite extremity, have much power in closing the orifice. I believe that this muscle acts simply in narrowing the whole animal, so as to favour its movement within the cavity in which it is imbedded. Owing to this position of the adductor muscle, and its consequent little power in closing the orifice, we can understand the necessity for the defence afforded by the bands of spines and hairs on the inner tunic of the sack and on the labrum, which do not occur in other Cirripedes.
Horny disc.—The general shape of the disc, its irregularity and position, have been already described. It never extends, as remarked by Mr. Hancock, to the extreme lower point of the peduncle; upwards it reaches to a little below the lower end of the orifice. It consists of successive layers of membrane, either moderately thick and opaque, or only a little thicker than the general membrane of the body, but never furnished with the little sharp points; it increases in size, in like manner as the calcareous valves of other Cirripedes, the undermost and last formed layer extending beyond the others, with its edge united, till the next exuviation, to the general membrane of the body. The disc is attached, at its upper end, apparently in the usual way, by cement, to the roof of the cavity of the shell in which it is imbedded; but the lower parts of the disc are also slightly and partially attached, chiefly along the lines of growth or exuviation; and this, I suspect, is effected by an inorganic calcareous deposit; anyhow I could not perceive here any cement or cement-ducts. Beyond the circumference of the disc the whole animal lies free in its cavity. The lines of growth in the middle part of the disc are generally obliterated by the decay of the older and outer layers. These lines, though of course ordinarily conformable with the general outline of the disc, are not always so, for the disc sometimes becomes during growth slightly changed in form, and the animal, consequently, slightly changed in position; sometimes either one or the other side or the upper end of the disc is left deserted by the new layers of the growing disc; these being formed on the deserted side of less size or extension, instead of larger size, as they normally should be all round the disc.
The upper end of the disc is always produced into a projection of not regular shape, but generally hollowed out or embayed in front (fig. [1]), and almost always hollowed out on the two sides. This projection stands directly over the adductor muscle (b in fig. [5]), and on the exterior surface is generally convex, being concave on the under side for the attachment of several muscles presently to be described. The horny layers are in this part usually thicker than elsewhere. The disc is thus upwardly produced, owing apparently to the fissure which leads into the cavity of the shell of the mollusc becoming, during the process of excavation, considerably longer than is necessary,—that is longer than the orifice leading into the sack; and consequently, for the protection of the imbedded animal, the lower and narrow end of the fissure is closed on its under side by this upward production of the horny disc, formed of layers of membrane of unusual thickness. A deposition, also, of lime, hereafter to be mentioned, gives further protection.