[68] This conclusion is supported by the structure of [Proteolepas]: in this Cirripede there is not a vestige of a carapace, and as the whole of the animal in front of the mouth is almost utterly aborted, being reduced to a mere covering to the two cement-ducts, and as, on the other hand, the mouth with the mandibles, though peculiarly modified, is not at all aborted, there is a strong probability, that the abortion of the carapace is connected with the aborted state of the three anterior cephalic segments; and that the carapace in its origin is not any way related to the fourth or mandibular segment.
The whole of the head in front of the mouth, together with the carapace, is, as we know, formed of three segments; and each of these segments, homologically, ought to consist of eight elements; I recall to mind these facts, inasmuch as the transverse separation between the peduncle and capitulum in the Lepadidæ, and between the basis, the shell, and the opercular valves in the [Balanidæ], might be thought to be connected with the separation of the three cephalic segments. So again, as in the [Balanidæ] the shell normally consists of eight compartments, these might be thought to be related to the eight elements of one or other of the three segments. But I see no reason for admitting this view; and in the case of the carina and rostrum, two of the most persistent and important of the compartments, they exactly cover the sutures which ought to separate the two tergal and two sternal elements of the segment. The valves, moreover, often form many more whorls than three, or the number of the true cephalic segments in front of the mouth; and in each whorl the valves tend to stand in tile-like or alternate order, with respect to those in the whorls both above and below, which would not be the case, if they were the eight elements of the segments.
For the true homologies of the sclerodermic plates, or Shelly valves, with which the external covering of Cirripedes is so generally strengthened, we must, I believe, look to the carapace of the Podophthalmia. In these latter, we find the carapace composed of sclerodermic plates, which, though closely joined and only occasionally separated by sutures, yet in their origin are distinct;[69] they tend, also, to be arranged in alternate or tile-like order. As the animal grows, the old sclerodermic plates, all joined together, are moulted, and new ones, also all joined together, of a larger size, are formed beneath. Now let us imagine the growth to be more gradual but yet periodical, and the new and larger sclerodermic plates, when formed under the old ones, to adhere firmly to them; the older plates would thus be prevented from becoming confluent, and instead of being all moulted together, as is now the case, they would be almost continually separated from each other, owing to the almost continuous increase in size of the new underlying plates. Consequently lines of splitting would run between the several plates, however numerous they might be, instead of there being, as now, a single line of splitting extending down the back. In fact, we should have the identical manner of growth of the shell or carapace, which occurs in Cirripedes. It is on this ground, and from the several points of homological resemblance incidentally mentioned in the last few paragraphs, that, in the early part of this [Introduction] (p. [13]), when discussing the whole class, I stated that I believed that the carapace of Cirripedes presented more real resemblance with the carapaces of the Podophthalmia, or higher Crustacea, than with those of the lower Crustacea, though in mere shape they more nearly resembled the latter.
[69] ‘Annales des Sciences Naturelles,’ 3d series, tom. xvi, pp. 233, 236, 237.
Cementing Apparatus. (Plate [28].)
I have already (p. [128]) given an account of the manner in which, in the pupa of Lepas, the cement-tissue escapes from the prehensile antennæ, and of the structure of the cement-ducts, and of the cement-glands or incipient ovaria; and likewise of the changes by which these organs assume their ultimate form in the mature Cirripede. In my former volume, on the Lepadidæ, I described the cement-glands and the cement-tissue in several genera, and I have there shown (singular as the fact is) that the two cement-glands, with their contents, actually consist of ovarian tubes with their contents (for there seemed to be a relation in the state of fulness in both) in a modified condition. In the [Balanidæ], I am not able, from the difficulty of the dissection, to confirm these conclusions, excepting in so far that the tubes on which the cement-glands are formed, run into the mass of ovarian cæca; but, I may add, that in the abnormal [Proteolepas], belonging to another Order (see the section, Pl. [24], fig. [1]), nothing could be plainer than that the membrane of the ovarian sack (b) formed the cement-ducts, and that their cellular contents, which within the sack (a) were in process of conversion into ova, within the ducts were converted into the cement-tissue. This cement, by some unknown power, travels down the ducts, and debouches at the antennæ.
In the Lepadidæ, there are only two cement-glands, which are situated high up in the midst of the ovarian cæca, with one cement-duct proceeding from each: both the glands and ducts increase in size with the age of the animal:[70] the cement issues either permanently from the prehensile antenna, or, after a short period, through apertures in the peduncle, arranged irregularly or in straight lines,—the last formed apertures being furthest from the central and basal point of the peduncle. In the [Balaninæ], on the other hand, at each period of growth, a pair of new cement-glands is developed, larger than those last formed, and making, with the older glands, a chain, connected together by what I have called the cement-trunk. The cement-trunk consists of a tube, which generally becomes enlarged just before entering each gland. The glands, the cement-trunk, and cement-ducts, all adhere to the basal membrane or basal shelly plate. Each gland gives rise to two cement-ducts, these often bifurcate, and sometimes repeatedly bifurcate and inosculate before pouring out their contents round the circumference of the basis; and sometimes they all first enter a circumferential cement-duct. The probable cause of the greater complexity of the cementing apparatus and of the greater number of the excretory orifices in the [Balanidæ], compared with the Lepadidæ, is that the entire surface of the broad basis, which answers to the whole peduncle in the Lepadidæ, is firmly cemented down to the supporting object, instead of merely the basal end of the peduncle. The cement issues either in a cellular condition, or more commonly as a fine network, which, at a short distance from the orifices (Pl. [28], fig. [4 a], z), becomes so fine as to form a sheet or layer: I may here recall the fact, that in the cement proceeding from the disc of the antennæ, in some species of Lepas, a similar structure was observed. The cement itself presents the same transparent, brown, laminated, structureless appearance, and the same chemical reaction, as described in my former volume. The cement has the capacity of occupying and filling up all inequalities in the supporting surface; I have seen it, when spread over an encrusting Flustra, present an exact model of every cell; in the case of [Coronula], it seems, as we shall immediately see, to have the power of penetrating into, and even almost blending with the epidermis of the supporting Cetacean. The last-formed cement-glands and cement-ducts present a delicate and transparent appearance, and contain cellular matter; whereas the old cement-glands, and sometimes the old cement-ducts, are filled with brownish cement, not acted on by boiling potash. The foregoing remarks are confined to the sub-family [Balaninæ], for I have not been able to examine thoroughly the [Chthamalinæ], and can only affirm, that in [Chthamalus] and [Pachylasma] the cement-ducts repeatedly bifurcate and inosculate, in the same manner as in the [Balaninæ]. I will now proceed to describe, in some detail, the cementing apparatus in the several following genera.
[70] I had some slight reason to suspect in Pollicipes that new cement-glands were successively formed: this is more probable in this genus than in the others, inasmuch as it is the most nearly related to the [Balanidæ].
[Coronula.]—The cementing apparatus is here more simple than in any other genus of the [Balaninæ], and I have studied it more carefully. The basal membrane of [Coronula balænaris] is figured in Pl. [28], fig. [1 a], and must first be described; its relation to the shell will hardly be understood without looking at the outline of the folded walls of this species, in Pl. [16], fig. [5]. The basal membrane closes the central circular hollow, and is continuous with rays (not represented in Pl. [28]) extending under the doubled walls and terminal transverse loops. It has eighteen concave sides, corresponding with the inner ends of the folded walls, for each of the six compartments is trebly folded. The membrane consists of successive, conformable slips (c′, c′), bordered exteriorly by thickened yellowish rims, and internally overlapping (when viewed from the inner side) the few last-formed slips, and then thinning out. The membrane forming each slip is itself laminated. The middle portion, about 1/50th of an inch in diameter, is rather opaque, owing to the slips being so close together. Beyond this central part, the slips suddenly increase in size, but yet have a different shape from the 18-sided outline, which they ultimately assume: this difference is owing to the great changes in shape, as explained under the genus [Coronula], which the shell undergoes, when the walls at first assume their folded structure. The walls are invested by longitudinally striated membrane (p, p, p, fig. [1 a]), which turns in under their basal edges; and this membrane is united with the basal membrane, by what I shall call the circumferential slip (b), and which is shaded in fig. [1 a], simply for the sake of catching the eye. It is the circumferential slip of membrane which sends rays under the spoke-like folded walls: thin as it is, this slip is yet laminated, but is not bordered by thickened edges. The membrane investing the walls is, like the basal membrane, formed of successive slips with thickened edges, which overlapping (viewed from the inside) the last-formed slips, project beyond them, and so face the edges of the slips in the basal membrane; they are only obscurely indicated in fig. [1 a]. The circumferential slip (b) lies over (as viewed from within) both the basal and wall membrane. This whole structure will, perhaps, be best understood by the sectional diagram (fig. [1 b]), in which the letters (c′, c′) show the slips of basal membrane; (p) the parietal membrane, coating the outside surface of the walls of the shell, not here represented; (b) the circumferential slip overlying both; and (z, z) the layers of cement, which may for the present be disregarded. In order to allow, of the growth of the shell, the circumferential slip (b) periodically splits in the middle, all round, in a line exactly conformable to the edge of the last-formed slip of basal membrane; and likewise in straight, medial lines under the spoke-like (cut off in fig. [1 a]) doubled walls. I have seen, under a high power, the line of splitting, very shortly after its formation, with the two edges ragged and near together, with an extremely narrow, new circumferential slip just formed, between and over the edges of the previously formed slip. What causes the circumferential slip to split so symmetrically, I cannot say: the opposed edges, after a time, become thickened, apparently from adhering to the underlying layer of cement, as will presently be described. The circumferential slip continues increasing in breadth till the period of its splitting arrives, by which time it has become much broader than the last-formed slip of basal membrane; and after the splitting takes place, the interior half towards the basal membrane, forms a new basal slip all round the basis, and the exterior half adds a new slip to the membrane investing the walls. This latter membrane being inflected under the basal edges of the walls, is, during the growth of their edges, drawn straight down, the newly-formed portion taking the inflected position.
In the sectional diagram, ([1 b]) the circumferential slip is not yet broad enough to split; when it has become so, it will split under the letter (b). The slips of basal membrane are, as may be seen in fig. [1 a], narrower towards the circumference; but the two or three last-formed slips, are out of proportion narrower than the others; and it is certain, from the comparison of the basal membranes of specimens of different ages, that these will afterwards increase in width.[71] I have seen no other instance, in Cirripedes, of growth in membranes, except at their extreme margins: I suspect that these last-formed slips are pulled, during the downward and outward growth of the shell, a little from over the last-formed slips, new and larger laminæ being all the time thrown down, so as to prevent any fissure being formed. I also suspect that the gradual increase in width of the circumferential slip itself, is due to the opposed edges of the underlying and last-formed circumferential slip being dragged further apart from each other, new and wider laminæ of membrane being continually thrown down; the new circumferential slip being thus, also, all the time thickened, as well as rendered broader.