This Cirripede, in appearance the most anomalous of its family, has affinities distinctly pronounced. Four years ago the Rev. R. T. Lowe sent me some specimens, which he had obtained from a porpoise between Madeira and England; and I named them in MS. Siphonicella, from their relationship to [Tubicinella],—a fact which I mention only because Sir C. Lyell has alluded to this genus under the above name (without any description), in his anniversary address to the Geological Society, as have I, in my volume (p. 156) on the Lepadidæ. Since that time Professor Steenstrup has described and named the genus, fully recognising its place and affinities, and has most kindly sent me a magnificent group of specimens.
This genus singularly resembles, in general appearance, some of the pedunculated Cirripedes, so much so that in the specimens sent me by Mr. Lowe, in which the almost rudimentary shell was, from disintegration and its deep imbedment, not plainly visible, I did not in the least doubt that I was examining a new genus of Lepadidæ. I may mention, as a proof how truly all the parts and organs are correlated in Cirripedes, that I was at first in despair when I found a species to all appearance pedunculated, with its labrum not bullate, its palpi of large size, its third pair of cirri totally unlike the fourth and succeeding pairs, and with only a single layer of muscles round the peduncle; but when, in addition, I found that there were branchiæ, and that these were double, I felt convinced that I was dissecting a disguised sessile cirripede, and that its true place was near [Coronula]: soon, I found the imbedded and almost rudimentary shell, of which a mere fragment would equally well have declared the true position and relationship of the whole animal. Though [Xenobalanus], in external aspect, is so completely masked, yet in its habits, namely, in living attached on Cetaceans, as in its essential structure, it displays its real affinities. In the course of the following description, it will be seen that in the shell, the affinity is almost equally close to [Coronula] and [Platylepas], but that, considering the whole animal, the affinity is somewhat closer to [Tubicinella]. [Xenobalanus] may indeed be described as a [Tubicinella] without opercular valves,—with the opercular membrane thickened down to the basis,—and with the shell, excepting the few last-formed basal zones of growth, almost wholly removed by the breakage of its upper end; this remnant of a shell, however, presenting some strong points of resemblance to [Coronula].
1. [XENOBALANUS] GLOBICIPITIS. Pl. [17], fig. [4 a]-[4 c].
XENOBALANUS GLOBICIPITIS. Steenstrup. Videnskabelige Meddelelser fra den Naturhist. Forening i Kjöbenhavn, for Aaret, 1851. Tab. 3, fig. 11-15.
General Appearance.—The shell is in an almost rudimentary condition, and appears like a small white irregular star, imbedded up to its top in the skin of the porpoise. Out of this thin, star-shaped shell, a cylindrical, flexible, peduncle-formed body springs, which forms the main part of the animal; it is narrow where coming out of the central cavity of the star, but soon acquires its full diameter; at the upper end it has a reflexed hood, and hence is broader, and this has the appearance of forming a capitulum, like that of a pedunculated Cirripede. This pseudo-capitulum is formed by a membranous reflexed collar or hood, which is very narrow at the lower end of the orifice, close under the mouth, and becomes wider and wider towards the upper and carinal or posterior end of the orifice; hence the lower reflexed edge of the hood is only slightly oblique or even nearly transverse. The orifice leading into the sack is large, and nearly in the same straight line with the peduncle; it is a little hollowed out in the middle at the upper end, and on each side of this medial hollow, there is a small rounded projection or horn, not perforated, but hollow, as may be seen by turning up the hood and looking at its under side. These two little horns curiously bring to mind the ear-like appendages in Conchoderma aurita (Otion), but these latter are perforated, open into the sack, and point outwards. The peduncle-formed body answers, as we shall presently see, to the main part of the shell in [Tubicinella], and the hood, as it would appear, to the lips of the sack-aperture, which project between its scuta and terga; of these valves there is not here a trace. The whole surface is smooth, and is formed by rather thin membrane, of an orange colour; but from the colour of the underlying corium, the whole appears of a dark chocolate red, the reflexed hood being rather lighter coloured. It is singular how closely the colour resembles that of some dark varieties of the above-mentioned Conchoderma aurita, and likewise of Anelasma squalicola, both pedunculated Cirripedes, having oceanic habits, and destitute, to a remarkable degree, like [Xenobalanus], of shelly valves.
The largest specimen which I have seen was very nearly two inches in length: in this specimen the star-shaped shell measured, from extreme point to point, nearly a quarter of an inch in diameter, but the internal cavity only about one eighth of an inch. This latter measure gives also the diameter of the peduncle, where coming out of the shell; the diameter just beneath the hood, was in this same specimen rather more than a quarter of an inch, and therefore greater than the diameter of the points of the shell. The depth of the shell from the upper rim to the basal membrane, in one specimen which I measured, was only one twentieth of an inch, and this specimen had its pseudo-peduncle one inch and three quarters in length, consequently thirty-five times as long as the shell was deep.
Structure of Shell.—The almost rudimentary shell (fig. [4 b]) consists of a small, thin, six-rayed disc, formed of six compartments, each of which, instead of being outwardly convex, as in ordinary Cirripedes, is deeply bowed inwards. The narrow sutures (s s) separating the six compartments, run along the middle of the six rays, each ray being composed of the bowed ends of the walls of the adjoining compartments. The rays are a little curved towards the carinal end of the shell. It is remarkable that the rostrum is smaller and less deeply folded inwards than the other compartments, and the lateral compartments are a little smaller than the carino-lateral compartments, which is exactly the reverse of what is usually the case. Only about four zones of growth have been preserved in any specimen, and consequently the shell is very nearly of the same diameter at the top and bottom; for the upper end of the shell is rapidly removed, as in [Tubicinella], by the scaling off of the upper rims of the sheath, and by the disintegration of the walls. The zones of growth are commonly not piled exactly over each other, but rather obliquely, as represented in fig. [4 b]. Each zone projects, forming a prominent, sharp, toothed ridge round the shell. In [Coronula] (Pl. [16], fig. [6]) and its allies, the outer lamina of the wall is formed by the union, a little above the basal margin, of ledges running along the sides of the longitudinal septa. In [Xenobalanus] (Pl. [17], fig. [4 c]) similar ledges are less perfectly joined, and apertures seem always to be left in transverse rows under the transverse toothed ridges, which latter are best seen in fig. [4 b]. The apertures, of course, are covered by membrane. The transverse ridges are surmounted by knobs arising from the longitudinal septa; and the knobs themselves are capped by other little heads, which are not represented in the drawing. Owing to these projections, and to the prominence of the transverse ridges and of the longitudinal septa, the external membrane is attached so firmly to the shell that even with the aid of caustic potash it can hardly be separated.
The internal cavity of the shell is small: it approaches a hexagon in shape, with the rostral side very short, and the lateral sides curved inwards. It is lined by a rather thick sheath, which descends very near to the basal membrane; the sheath is divided into very distinct, successive convex zones of growth. The external membrane of the pseudo-peduncle is attached with remarkable strength to these rib-like zones of the sheath. The alæ (a in fig. [4 b]) are represented by mere angular shoulders, received into very slight notches, and placed at the inner ends or entrances of the double walls, or rays as I have called them. With respect to the radii, they also are in an extremely rudimentary condition; but a thin layer of shell, apparently continuous and homologous with the sheath, extends from the sheath along both sides of each ray, and on the rostral side (whence the radius ought to arise), about half-way from the end of the ray, gives rise to a projection or ridge (d, in fig. [4 b]) which runs from the top to the base of the shallow shell. From this longitudinal ridge, septa, parallel to the basis, extend to nearly the extremity of the ray or double wall. These represent the radii; but they never grow, so as to increase the diameter of the shell. These radii evidently correspond to the additional or pseudo-radii in [Coronula], which in that genus lie between the parallel, as here, and folded parietes. Of the true radius, having the same thickness as the paries, I here saw only traces in an internal, very slight, longitudinal ridge running up the shell, close to the outer extremity of each ray or double wall.
The membranous Basis is united all round to a rather wide flap of membrane which is inflected from the outer surface of the shell. The cement-glands appear to be mere enlargements of the cement-trunks, which latter extend in two nearly straight and parallel lines. From each gland two cement-ducts proceed, one of which runs parallel to the cement-trunk.