Opercular Valves.—The scuta and terga are nearly of the same size and shape: they are mitre-formed, and higher than broad. They do not fill up the orifice of the shell. The scutum is a little larger than the tergum, and rather less symmetrical, the rostral corner of the valve being a little produced. There is no hollow or crest for the adductor muscle, which is small. In the tergum there is no trace of a spur. The two valves are not articulated together, but standing close to each other are united, as well as the scutum to the scutum, and tergum to the tergum, by thick, brown, tough, yet soft membrane, in layers continuous with, but differing in appearance from, the surrounding opercular membrane. The layers of shell, forming the valves, are thick, and only the three or four lower layers are usually preserved, the upper ones having symmetrically scaled off, leaving snow-white surfaces. Owing to the thickness of the successive shelly layers, and to the circumstance of each new layer being but very little larger than the last, the scaling off of the old upper layers is a quite necessary process; for otherwise the orifice into the sack would have been encumbered and almost closed by four long, slightly tapering points, prolonged upwards from the basal layers that form the four existing valves. The same scaling off process takes place in [Platylepas], and amongst pedunculated cirripedes in Lithotrya. Microscopical examination does not exhibit any fine spines on the membrane investing the valves, or any tubuli in the shelly layers after their dissolution in acid: in this respect the valves resemble those of [Coronula]. The summits of the valves project freely for about a third of their own height, above the level of the membrane by which they are surrounded. The orifice leading into the sack is bordered by very protuberant lips, standing up even considerably above the upper freely projecting portions of the valves.
The Opercular Membrane, connecting the valves and the top of the shell, is thick and tough, and deeply folded in concentric wrinkles. As in [Coronula], it consists of two or three separate membranes (each composed of many laminæ) one over the other, united to successive shelly layers of the opercular valves. As the upper shelly layer scales off, the membrane attached to it is likewise thrown off. The innermost laminæ of the last-formed opercular membrane extend down, closely attached to the sheath, to its basal edge, and therefore nearly to the basis of the shell; the outer and older laminæ, all closely attached one within the other and to the sheath, extend to a less and less distance downwards; consequently, the animal’s body is enclosed in a tube, thinning out downwards, formed by the laminæ of the successive opercular membranes, surrounded outside by the shell: only in the following genus, [Xenobalanus], shall we meet with a nearly analogous structure. As the shell of [Tubicinella] increases in diameter, from the growth of the radii, the opercular membrane lining the sheath is necessarily split along the six lines of suture, in the same manner as is the membrane externally investing the shell; in a like manner, also, it is repaired and added to by new longitudinal slips of membrane. Of this structure, in the opercular membrane, I have seen no other instance; for in most genera the old opercular membrane is moulted, and a new and larger one formed at each period of growth; in [Coronula], in which the opercular membrane is likewise for a time persistent, it does not run far down the inside of the shell, and each new membrane is formed large and extensible, so as to allow, without splitting, of some increase in the diameter of the shell. The opercular membrane at the summit of the shell, in [Tubicinella], is folded in concentric lines, and so deeply, that the basal edges of the opercular valves are generally hidden: this folding arises partly from each last deposited and innermost membrane being originally formed slightly folded, but chiefly from the rapid downward growth of the shell, and the consequent downward movement of the whole animal’s body, together with the opercular valves to which the body is attached, and this necessarily tends to wrinkle and fold the opercular membrane. Owing to the opercular membrane extending far down inside the shell, and being firmly attached to the sheath, as the upper part of the shell breaks away and disintegrates (which we shall presently see is constantly taking place), small particles of shell are left adherent to the circumferential and folded parts of the opercular membrane; and this at first much perplexed me.
Muscles of the Sack: these extend down almost to the base of the shell, but in the lower part they spread out and become thin and very irregular, not even corresponding on the opposite sides of the body. The fasciæ in the upper part show very distinct transverse striæ, but lower down these become either obscure or entirely deficient. In all these characters the muscles of [Tubicinella] and [Coronula] resemble each other. The rostral depressor muscles of the scuta consist each of four small bundles of fasciæ; the lateral depressores run not quite straight down, but in a curved course towards the carinal end of the sack: the tergal depressores are proportionally smaller than in ordinary sessile cirripedes, but they project and form two crests (with some fasciæ between them), which support the Branchiæ. The membrane lining the sack, I may here mention, is unusually strong.
Branchiæ.[128]—These are enormously developed; the two together covering two thirds of the area of the sack. Each consists of two folds, both deeply plicated. They are attached longitudinally to the two crests, including and formed by the muscles running from the terga to near the basis of the shell. The branchiæ are likewise attached transversely to the sack, under the basal margins of the terga.
[128] These have been described by Professor Owen in the second volume of the ‘Descriptive Catalogue of the Museum of the Royal College of Surgeons.’
Mouth.—The labrum is very finely hirsute, without teeth; the palpi have a short row of moderately long spines along their exterior basal margins. The mandibles have four rather narrow, sharp teeth, which (excepting the first) have double points: between the second and third, and again between the third and fourth teeth, there is a single small intermediate tooth: the inferior angle is irregularly pectinated. The maxillæ are small; there is a small notch beneath the two upper great spines, and a second notch near the inferior angle. Between the outer maxillæ, there is a square-topped mentum. Hence we see that the mouth in all its few peculiarities, resembles that of [Coronula].
Cirri.—These are short, with short and broad segments protuberant in front. The pedicel of the first cirrus is very broad, and exteriorly clothed with fine hairs: its rami are slightly unequal in length. The second and third cirri are very short. The three posterior pairs are remarkable from the pairs of main spines being placed so close one under the other, and in an oblique direction, that at first they appear to form a single crowded transverse row: the dorsal tufts are rather large.
Body.—The body is remarkable from its nearly vertical position, and from the much elongated pyramidal form of the prosoma, extending down nearly to the bottom of the sack. The membrane investing the prosoma, presents a few circular folds, falsely appearing like articulations. The œsophagus enters the stomach rather obliquely. With respect to the generative system, I have only to remark, that the vesiculæ seminales are of great length, and convoluted to a remarkable degree. The ovarian cæca, form a thick layer at the bottom of the sack; they do not appear to extend up the shell round the sack. The only other point, which I shall here mention, is that beneath the basal articulation of the first cirrus, there is a longitudinal swelling, ending in a freely projecting point, .06 of an inch in length; at first, I thought, that we here had a rudiment of a filamentary appendage like those found in several Lepadidæ; but closer examination showed an orifice at the apex, leading into the acoustic meatus, in which the singular, wrinkled, heart-shaped acoustic vesicle, mentioned in the [Introduction], hangs suspended. Alongside the freely depending point, with an orifice at its end, there is a smaller upward projecting point, without any orifice, but hollow within and lined by corium; I believe it opens internally into the acoustic meatus.
Attachment and general Growth of Shell.—All the specimens which I have seen have been attached in groups. They are buried up to the level of the operculum in the whale’s skin; and their summits, I suspect, lie even beneath the general surface of the body of the whale. It is certain that the shells grow much at their basal ends. As in the case of [Coronula], the flat membranous basis does not actually penetrate the skin; but the general pressure of the whole group of shells seems to push inwards the skin of the whale, and directly beneath each shell the formation of new epidermis is apparently checked. Between the shells, however, though close together, the epidermis continues to be formed, and is pushed upwards between them, in the same manner as it is forced into the flattened cavities on the under side of the shell of [Coronula].
The manner in which a full-grown shell assumes its ordinary shape, at first appears very perplexing:[129] it has to change from a cylinder, at first probably not much above the 1/50th of an inch in diameter, to a cylinder nearly one inch in diameter: and this is not effected by the growth of the radii, for the radii never reach the basis, and the basis of course has to increase in diameter like the rest of the cylindrical shell. The radii serve only to keep the summit of the shell wider than the basis, which is the natural shape of this species; and in large-sized specimens, this purpose is sometimes aided by the parietes during their downward growth decreasing slightly in width. In ordinary nearly full-sized specimens, the parietes are of the same width at the top and bottom, but in some large-sized specimens, as just stated, they even become narrower towards the bottom; as they grow only at the bottom, one does not at first see how they can ever increase in width, or how the older shells can have acquired their present diameter. But an examination of young specimens, from .1 to .3 of an inch in diameter, at once serves to show how the shell attains its full size and shape: for here the parietes are all found to increase downwards sensibly in width, though at a much slower ratio than in other sessile cirripedes; in larger, but not full-grown specimens, a similar increase can by care be detected: hence by long-continued growth at the base of the shell, with the removal of the upper part, a young [Tubicinella] of small diameter will be converted into an old one of large diameter, retaining during all the time its sub-cylindrical form, with its summit rather broader than its base. With respect to the removal of the upper part of the shell, this seems almost constantly going on, for the summit of every specimen invariably had a freshly broken aspect. The peculiar structure of the sheath, which is the strongest part of the shell, namely, its division into oblique layers, separable by a slight force, doubtless is subservient to the repeated breakage of the summit. In some species of [Tetraclita] and of [Balanus], gradual disintegration of the upper part of the shell is a necessary element in the growth of the animal, in order that the orifice may increase in size, and here we have mechanical breakage equally necessary.