The little bristles above alluded to, which arise from the slips of membrane left adherent on the opercular valves, sheath, and walls, stand in rows; a row corresponding to each period of exuviation of the opercular membrane. The bristles are generally largest on the opercular valves and sheath; in [Balanus tintinnabulum], they are from 1 to 2/1000ths of an inch in length, but they are longer in some other species. I may here mention, as showing the connexion of these bristles with the opercular membrane, that similar bristles occur in [B. perforatus], scattered over the surface of that membrane, and are necessarily moulted with it. In the imbedded genera [Coronula] and [Tubicinella], none of these bristles exist. When a portion of valve or shell, furnished with bristles, is dissolved in acid, tough, sinuous, and apparently hollow, threads are seen to run from their bulb-like bases, into and up the corresponding layer, which, before dissolution, existed as shell; and they terminate internally in very fine points, which I believe are united to the underlying corium. These threads, or tubuli,[26] as I have called them in my volume on the Lepadidæ, are, in [Tetraclita porosa], about 1/5000ths of an inch in diameter, but only half that size in [B. tintinnabulum]. On parts of the shell where there are no bristles, similar tubuli penetrate the shelly layers, and come to the surface. The tubuli running to the lowest and last-formed row of bristles, just after a period of exuviation, are so delicate as hardly, or not at all, to be distinguished; in the row above, they are plain and longer, and for the next two or three upper rows they are, in some cases, as in [Tetraclita porosa], longer and longer, having been added to during each successive thickening of the valve. These tubuli consist of chitine, and no doubt first existed as threads of corium; they are so tough that they must serve to strengthen the successive layers of shell, but I imagine their chief function is to keep up the vitality of the newly-formed layers of shell. May we not, also, venture to suppose that by their means, some degree of sensibility is given to the bristles? I need only further remark, that in some species of [Balanus] and of [Chthamalus], the under side of the shell is penetrated by irregular pores, large enough to be visible to the naked eye, into which threads of corium penetrate; but these can hardly be said to appertain to the microscopical structure; and are more nearly related to those pores and furrows, formed by the greater or less development of the longitudinal septa, and in which the threads of corium deposit, or rather become changed into, transverse septa, or solid shelly matter, as previously described.
[26] I regret that I have used this term “tubuli”; for the threads thus designated, I believe, are not the same with the tubuli of Dr. Carpenter, which are not left after dissolution in acid. I have seen tubuli, as called by me, in the shell from the leg of a crab, after having been placed in acid.
Muscles of Sack.
In the pupa, the thorax, as we shall hereafter more fully see, is continuous with, and opens into the large anterior end or front part of the head; but during the metamorphosis (Pl. [30], fig. [2]), the thorax of the Cirripede becomes, owing to the almost transverse position occupied by the young animal within the pupa, to a great extent internally separated from the anterior end,—which anterior end forms, as we know, either the peduncle or the basis. Hence it comes to pass that the body or Thorax (Pl. [25], fig. [1]) is lodged within a sack (f) within the shell. The chitine membrane lining this sack is excessively thin and transparent, but less so in [Xenobalanus] and [Tubicinella]; it is obviously continuous with that investing the body of the animal; it is also essentially continuous with the opercular valves and membrane, and consequently with the whole shell. It is periodically moulted. It is lined by corium, as is likewise the surrounding shell; hence the corium is double round the sack, as indeed might have been expected from the shell and opercular valves (at least their upper parts) being formed by the prolongation, as is obvious in the pupa, of the posterior edges of the carapace. Between the two folds of corium, which are united together by transverse ligamentous fibres, branching out at both extremities, like the roots and branches of a tree, we have the longitudinal muscles, which go to the opercular valves; and likewise a layer-like mass of branching ovarian tubes (Pl. [25], fig. [1, g]): the ovarian tubes, however, are often confined to the base of the sack. In [Xenobalanus], the two folds of corium are united by longitudinal membranous septa, making a series of quite peculiar, square tubes.
The above-mentioned muscles are attached at their upper ends to the opercular valves, and at their lower ends to the basis. There are, in fact, three pairs, but the pair attached to the basi-carinal angles of the two terga (Pl. [25], fig. [1, i]), are almost invariably confluent, forming one great bundle; the second pair is attached to the lateral or basi-tergal corners of the two scuta, and are hidden in the figure; the third pair (h) is attached also to the scuta, to their rostral angles. These muscles can only act as depressores; they are often extremely powerful; they belong to the voluntary class, for they are transversely striped. By their action, the opercular valves are capable of varied slight movements, within the limit allowed by the width of the flexible opercular membrane. By the action of the lateral scutal depressores, the orifice leading into the sack is opened, the movement being generally aided by the protrusion of the cirri. By the sudden contraction of the rostral scutal depressores, the blows which are sometimes given by the beaked terga at the opposite end of the operculum, are probably effected. By the contraction of all three pairs of muscles, the opercular valves are held down with quite surprising force. The valves can be raised only by the action of the animal’s body against the basis.
In [Coronula] these muscles are more spread out, and do not extend down to the basis; their lower portions, as is likewise the case in [Tubicinella], do not exhibit transverse striæ, and hence tend to pass either into the involuntary class, or into ligament. This condition of the muscles, in the above two genera, accords with the little-developed state of their opercular valves. In [Xenobalanus], there is no longer any evidence of the muscles being collected into five or six bundles, for they are thinly and almost uniformly spread out, and show in no part transverse striæ. I may add that in much elongated specimens of [Balanus balanoides], these muscles become in their lower part ligamentous, and destitute of striæ.
Branchiæ.
In the [Balaninæ], a pair of Branchiæ is always present: they lie on each side, in a somewhat curved position, in the angle between the sides of the shell and the basis. In Pl. [25], fig. [1], they are exactly covered, on the further side, by the body of the animal. They are attached near each other at the carinal end of the sack in a vertical line, and likewise on each side in a transverse line, extending from close beneath the spur of the tergum towards the point of attachment of the body to the scutum. In [Balanus], as in the figure (Pl. [25], fig. [3]) of [B. tintinnabulum], each branchia consists of a medial fold of skin, a little curved conformably with the sack, and slightly tapering towards its rostral and free extremity; but this fold is almost hidden by the vertical sub-folds or membranous ridges, themselves plicated and sub-plicated, which project on both sides: these vertical folds are free at their tips: at their lower attached ends, they are thickest. On the side nearest the wall of the shell, the whole branchia has a bilobed appearance, owing to a very deep indentation caused by the projection of the scutal lateral depressor muscle; the sub-folds on this side are also more plicated. The branchia essentially is an inward plicated fold of the membranes of the sack; for its outer, very thin tunic is continuous with and moulted with that lining the sack; and within it we have two layers of delicate, pulpy, transparent corium, united together (as is best seen in [Coronula]) by ligamentous fibres, branched at their two ends, all exactly as in the corium surrounding the sack. There are here no distinct vessels, any more than in other parts of the body, but a fluid could easily circulate in the interspaces of the corium. From the large size of this organ, and its simplicity of internal structure, being adapted exclusively to expose a great surface of skin to the water, I do not doubt that it has been correctly considered as a respiratory organ. By the voluntary movements of the opercular valves (i. e. part of the carapace) the water is constantly being pumped in and out of the sack; the movement, indeed, may be almost compared to the heaving of a man’s chest. Moreover, the branchiæ on each side are attached so closely to the spur of the tergum, that each time the latter is moved, the whole branchia must, I think, be agitated, and the folds opened, as by the action of a lever.
In our two commonest, tidal, sessile Cirripedes, viz. [Balanus balanoides] and [Chthamalus stellatus], I have observed that, when left uncovered by water, they kept the orifice of their operculums a little open, with a bubble of air within their sacks, so that the orifice was in fact closed by a thin septum of water, with air beneath; when disturbed, they closed their operculums with force, and expelled the bubble of air with a clicking noise, which has been noticed by Dr. Coldstream,[27] and has been thought to be made by the movement of the operculum itself. [Bal. crenatus], a deep-water species, when out of water, keeps its operculum closed.