The shelly matter of which the valves are composed is translucent: it is remarkably destitute of any investing membrane. The under surface is marked with rows of minute approximate pores, parallel to the lines of growth, into which the corium enters: after a portion of shell has been dissolved in acid, these threads of corium are seen to change, a short distance within, into cylinders of yellow chitine, running obliquely through the substance of the valve. These cylinders are about 1/2000th of an inch in diameter, but in parts they are spindle-shaped and twice as thick: they vary in length, about 1/100th of an inch being the average length: these cylinders at their upper ends suddenly contract into a point, more or less long, or are produced into a very fine tortuous tubulus of chitine, imbedded in the shell: I have seen in no other Cirripedes tubuli of this structure. There are other ordinary tubuli, such as occur in the valves of most Cirripedes, about 1/6000th of an inch in diameter, and which sometimes alternate with the above-described thicker cylinders. There are no external spines. From the number and length of the tubuli of both kinds, the tissue left after the action of acid is singularly complicated.
Basis.—The basal membrane is thin, and is divided, but not very plainly, into concentric slips, marking the successive increments of growth. In the middle of it, in two young specimens, I found with great difficulty the pupal prehensile antennæ: they were of small size, measuring from the extreme edge of the main or second segment to the end of the disc, only 27/6000ths of an inch: the disc appeared narrow (as in Pollicipes and Scalpellum), with a single spine at the proximate end: the ultimate segment, placed as usual at about right angles to the disc, bore two groups of shorter and longer spines, but I could not count how many. The antennæ were enveloped in a mass of cement of a yellow colour, resembling in all its characters the cement of other Cirripedes. In only one case, I believe I saw bifurcating cement-ducts, of extreme tenuity, viz. 1/15,000th of an inch in diameter. The sheet of cement on the whole under side of the basal membrane, not rarely shows a very irregular reticulated structure. For convenience sake, it will be best to defer the discussion on the very anomalous, though slight, powers of excavation which this genus possesses, and which I must attribute to the effects of some substance secreted probably by the cement-organs. I will here only mention, that the specimens which have excavated a depression, are less firmly attached than those, which have not acted on their support; and that, in the former case, the basal membrane, for a considerable space in the middle, becomes quite detached.
Animal’s Body.—The body is much flattened and, owing to the little development of one side of the shell, lies parallel to the surface of attachment. The prosoma is but little protuberant. The articulations of the thorax are unusually straight and transverse. The Mouth is also much flattened: it is placed rather distantly from the adductor scutorum muscle, owing to the production of the lower margin of the labrum. The Labrum is not notched, or even hollowed out in the middle, or (excepting in [V. nexa]) bullate; its crest is surmounted by about eight (more numerous in [V. nexa]) little teeth, or by some fine bristles. The Palpi are of moderate size, with their tips nearly meeting; they are slightly curved, and have bristles only on their outer sides and extremities: they are apparently capable of being lifted up and down by a muscle attached to them, just outside the rounded swelling on each side of the labrum to which they are articulated: in [V. nexa], however, the palpi are very small and narrow, and their tips do not nearly meet. In this genus, therefore, we find the swollen state of the labrum and the size of the palpi—characters generally invariable and of high classificatory importance—variable. The mandibles have three upper main teeth, with two or three minute lower teeth, or, in [V. nexa], with the lower part pectinated with small spines: in [V. Strömia], I have seen traces of the second tooth being laterally double—a character of some importance. The Maxillæ have a notch under the upper pair of large spines, with the lower part bearing, as usual, a double row of bristles, and forming a large step-formed projection: these organs are furnished with the usual apodeme and muscles. The Outer Maxillæ are prominent, and deeply lobed on their inner surfaces, the two lobes being clothed with bristles.
Cirri.—The first pair are attached, as usual, on each side of the mouth, and stand some way apart from the five posterior pairs. The second and third pairs differ considerably in structure from the three posterior pairs, which are much elongated. The first pair (excepting in [V. nexa]) is short, with the two rami slightly unequal in length, and with the segments thickly clothed, as usual, with spines. The second pair is remarkable from the posterior ramus being more than twice as long, and containing thrice as many segments, as the anterior ramus, which is barely as long as the shorter ramus of the first pair: the segments in the anterior ramus of the second pair (only five in number in a full-sized specimen) are broader and more protuberant in front, and more thickly clothed with spines (the terminal spines being doubly pectinated), than are the segments on the posterior ramus; on the latter, the uppermost segments have their bristles arranged in front in simple pairs, with the dorsal spines long, the lower segments being more thickly clothed with bristles, owing to the development of lateral rows. The third pair resembles in every respect the second pair, except in being a little longer, and in the bristles on the posterior ramus being less crowded, more resembling the arrangement of those on the posterior cirri. In [V. nexa], however, there is not so great an inequality in length or dissimilarity in structure in the two rami of the second cirrus, and only a very slight difference of any kind in the two rami of the third pair. Fourth, fifth, and sixth pairs have numerous elongated segments, bearing four or three pairs of long slender spines in front, with a single minute bristle between each pair, and with two or three slender spines in the dorsal tuft.
There is a considerable amount of variation in the proportional length, and in the number of the segments, of the several cirri in [V. Strömia]; in some specimens the two rami of the fourth pair were unequal in length; in some, nearly all the cirri on the lower or attached side were shorter than those on the upper side.
Caudal Appendages.—These are of most unusual length, sometimes even exceeding those of Ibla quadrivalvis, which surpasses, in this respect, all other cirripedes. They arise on each side and over the anus. They consist of numerous (sometimes as many as twenty-four), unequal, cylindrical, thin segments, bearing, at their upper ends, a circle of long and very slender spines. They sometimes equal two thirds or even four fifths of the length of the sixth cirrus; but their length, and the number of their segments, (sometimes imperfectly divided), varies much in different specimens of the same species, and sometimes even on opposite sides of the same individual. In some very young shells, as big as a pin’s head, the caudal appendages were proportionally extremely short, and consisted of only two or three segments. No muscles enter these organs; and when the animal is taken out of its sack, they project straight out behind, instead of being curled in, like the cirri.
Anatomical Structure.—The animal’s body is attached to the two scuta by the adductor scutorum, and by the other usual muscles running towards the mouth, and surrounding the prosoma. The whole external covering or shell has no other muscles; [Verruca] thus differing from the [Balanidæ] and Lepadidæ; but the shell is attached all round, near its circumference, to the basal membrane, by a band of very short fibres, appearing like muscles, but really ligamentous, as determined for me by Professor Quekett. Branchiæ are entirely absent. The alimentary canal presents all the usual characters, but in the prosoma is rather abruptly bent back on itself. The orifices of the two olfactory pouches are not at all prominent; they are placed directly under the outer maxillæ, (homologically in their middle segment), just above a small, medial, tongue-like apodeme. The orifices of the acoustic sacks appeared to be in their usual position beneath the basal articulations of the first pair of cirri. The vesiculæ seminales occupy their usual position in the prosoma; they are not much convoluted; they unite before entering the penis. The probosciformed penis is imperfectly ringed; it is thick and short, and tapers much more abruptly than is usual; it supports a few very thin hairs. The ovarian cæca are spread over the basal membrane, at the bottom of the sack; hence they in fact lie almost on one side of the animal: they consist of two main trunks, proceeding out of the animal’s body at the rostral end of the sack, which then branch and inosculate. In specimens of [V. Strömia] collected by Mr. Peach for me, in Cornwall, during the first week of April, there were included two ovigerous lamellæ, placed transversely across the rostral and the carinal end of the sack: the lamellæ were .11 of an inch in length; they appeared loose and not attached, as in the Lepadidæ, to any ovigerous fræna. The ova, in their earliest age, have one end much pointed, and are 8/1000ths of an inch in length; they become blunter and increase a little in size before the larvæ burst forth. The larvæ, both during their earliest stage and after the first moult, have been excellently figured and described[137] by Mr. C. Spence Bate: they present no particular characters distinct from the larvæ of other Cirripedes. I will only further add, that the structure of the prehensile antennæ still adherent to the basal membrane, indicates that the larva in its last stage,—that is the locomotive pupa,—has a normal character.
[137] ‘Annals and Mag. of Nat. Hist.,’ 1851, Pl. 7, fig. 8-10.
Affinities.—These have been sufficiently discussed under the family; I need here only remark that all the species, with the exception of [V. nexa], are intimately allied together.