In [Balanus tintinnabulum], the basis is calcareous: when its upper surface is cleaned, dried, and examined under a good light, the numerous larger cement-ducts can be seen, even by the naked eye, or under a weak lens, and present an elegant appearance. These larger ducts run in parallel lines from the two chains of glands towards the circumference. They are all encrusted with calcareous matter, and in the more central parts are hidden under it; at each period of growth, when the basis is added to round the circumference, it would appear that a layer of excessive tenuity of shell is thrown down over the whole surface, just in the same way as in [Tubicinella], at each period, a new and larger disc of membrane was thrown down over the pre-existing membranes with their cementing apparatus. The cement-glands, in the middle of the basal plate, seem often to give rise to small abnormal depositions of calcareous matter. When the basis (it is best to take a young specimen) is slowly dissolved in acid, all the cementing apparatus is left uninjured, adhering to the delicate tissue which before existed in a calcified condition. Near the middle I saw the two antennæ of the pupa; and from them the two cement-trunks extended about half-way towards the circumference. These two chains of glands are often placed very irregularly, but they tend to form, as in [Coronula], a large angle, open towards the rostral end of the shell. The glands, close to the old antennæ, commence abruptly, of rather large size: the later-formed glands, with their ducts, are in regular order larger than the younger ones, and stand much closer together. After immersion in acid all the glands and ducts appeared empty, instead of the older ones being, as in [Coronula], filled with cement. In one case I counted on each trunk twenty-five glands, besides some smaller obscure ones close to the centre.
In Pl. [28], fig. [4 b], I have given a drawing of two of the cement glands: the cement-trunk (f f) is smooth and apparently cylindrical: it becomes enlarged (at g) before entering the gland: it seems even to be prolonged across the gland under the form of a narrow bar (not represented), which apparently serves to keep the two ends of the trunk, on the two sides of the gland, in their proper relative places and distances. The gland itself is an elongated bag (h), which properly lies exactly over the enlarged portion (g) of the trunk, but in the drawing has been purposely displaced: it gives rise, in the later-formed glands, to a sort of neck (see the upper gland), which is either so long as to deserve rather to be called a duct and which soon bifurcates, or is quite short (see the lower gland) and gives rise to two separate ducts. On the opposite side of these glands, there is a spur (m), of greater or shorter length, which is evidently a rudimentary duct, for in the younger glands it existed as a perfect duct. Moreover, the first-mentioned duct often gives off branches (t′), having an exactly similar appearance with the spur (m). The membrane of which the cement-trunk (f), with the enlargements (g), is composed, is smooth, but that of the glands and of all the ducts, presents a very peculiar appearance, which at first would be called scaled, but more properly perhaps notched,—each notch being apparently formed by a line of thickened membrane, extending obliquely round only a short portion of the tube, and indenting it. The ducts, which I measured, were between 1/3000th and 4/3000ths of an inch in diameter.
In fig. [4 a], I have given a drawing of the two chains of glands, but with only those ducts figured which proceeded from the last-formed pair of glands. The specimen here drawn was old; and it is rare to find the structure of the ducts so simple. From both glands[72] a neck or thick duct arises, which soon bifurcates; one branch runs direct into the circumferential duct, and the other (t) bifurcates again; of the latter, one branch unites with its fellow from the opposite gland, and then forming a single duct (t′) enters, as do the two other branches, the circumferential duct. Thus, into the latter, five main ducts enter: the position of their points of entrance, with respect to the shell, varies considerably; but I think the five points tend to face the middle of the rostrum, and middle of the two lateral compartments on each side. In some other specimens, in which the ducts were nearly as simple, I observed that the neck or main duct at once divided into three branches, instead of into two, with one soon bifurcating; and on one side a rudimentary branch or spur was given off (above t), indicating a tendency to an additional bifurcation. In the later-formed glands, the ducts proceed only from the outer sides and form the ends of the glands furthest from the centre; but in the earlier-formed and smaller glands of the same individual, other ducts proceed from the inner sides, where in the older glands the spurs (m) are situated: moreover, in the younger glands, all the ducts bifurcate much oftener (how often I was not able to ascertain), before entering the circumferential duct; many of the branches, however, terminating in spur-like points. Now if we imagine twenty or thirty repetitions of the ducts given in fig. [4 a], (independently of the greater complication of the ducts of the younger glands), each a very little smaller than the other, and placed, with the main branches parallel, one over and within the other, we shall gain some insight into the wonderfully complicated structure of the cementing apparatus in this and many other species of [Balanus].
[72] It should be observed that fig. [4 b] ought to have been drawn with its present upper end downwards, to make it correspond in position with fig. [4 a].
I have as yet only alluded to the circumferential duct (i, i, Pl. [28], fig. [4 a]): we have not hitherto met with this duct, but I suspect that the branches which in [Chelonobia] inosculate, and which seem to run nearly parallel to the circumference of the basal membrane, answer the same purpose of connecting the ducts together, and are, perhaps, strictly homologous. In this, and some other species of [Balanus], the last-formed circumferential duct runs round the margin of the upper lamina of the basal plate, close to the basal edges of the walls; and as these latter have projecting longitudinal ribs, the duct curves a little round each rib; so that the whole duct is formed by as many short inwardly curved portions as the walls have ribs, or longitudinal septa. Between the basal extremities of these parietal, longitudinal septa, the extremities of the radiating septa of the basis project and enter; and along the crests of the latter, little branch-ducts (i′), proceeding from the circumferential duct, extend. In the basis, beneath the tubes formed by the just-mentioned radiating septa, there is a cancellated shelly mass (which, in fig. [4 a], was of unusual thickness), and along the crests of the branching ridges forming this cancellated mass, the sub-branches of the above branch-ducts (i′) run; these soon become so minute as not to be distinguished by the highest powers, and thus form a sheet of cement, which attaches the last-formed zone of the shelly basis to the supporting surface. At what point the membrane forming any one duct ceases, the cement-tissue being alone left, I was not able to ascertain; but the lower parts of the reticulated slip (z, z, fig. [4 a]) closely resembled the cement-tissue which surrounds the disc-segment of the pupal antennæ in Lepas australis. The circumferential duct, here and there, forms little loops, as may be seen in fig. [4 a]: and often two branches, running along the crests of two adjoining basal septa, proceed from a common point of the circumferential duct. The cement itself, under different parts of the basis, appears as little separate discs, as threads, globules, and as a fine network, but most commonly as simple layers. As each thick zone of shelly matter is added round the basis, the exterior branches of the ducts, between the circumferential duct and the new layer of cement beneath, are fairly imbedded in shell, and are for ever hidden, without, indeed, acid be used for the dissolution of the calcareous matter: so, also, the pre-existing ducts and glands, and the main trunk, would all have been hidden, if the layer of calcareous matter, which, I believe, is thrown down at each period of growth over the entire surface, had not been of excessive tenuity.
I cursorily examined the cementing apparatus in [Balanus galeatus], [improvisus] and [crenatus], which have all calcareous bases, and belong to different sections of the genus; and the structure seemed to be essentially the same. In [Bal. galeatus], I found the cement-ducts varying in diameter from 1/4000th to 1/10,000th of an inch in diameter. In [B. improvisus], the cement-glands do not differ much from those of [B. tintinnabulum]; but the cement-ducts bifurcate often before entering the circumferential duct; and the little branches, which proceed from the latter, are very short, and almost immediately, owing to the thinness of the basis, blend into a slip of cement.
I hope to be excused for describing at such length, the apparatus by which sessile cirripedes are permanently attached to a supporting surface; for this is the great leading character of the sub-class, not hitherto observed in any other Crustacean.[73] It is not easy to overstate the singularity and complexity of the appearance of the basal membrane of a [Balanus] or [Coronula]: and when we consider the homological nature of the apparatus, the subject becomes still more curious: I feel an entire conviction, from what I have repeatedly seen in several genera of the Lepadidæ, both in their mature and pupal condition, and from what I have seen in [Proteolepas], that the cement-glands and ducts are continuous with and actually a part of an ovarian tube, in a modified condition; and that the cellular matter which, in one part, goes to the formation of ova or new beings, in the other and modified part, goes to the formation of the cementing tissue. To conclude with an hypothesis,—those naturalists who believe that all gaps in the chain of nature would be filled up, if the structure of every extinct and existing creature were known, will readily admit, that Cirripedes were once separated by scarcely sensible intervals from some other, now unknown, Crustaceans. Should these intervening forms ever be discovered, I imagine they would prove to be Crustaceans, of not very low rank, with their oviducts opening at or near their second pair of antennæ, and that their ova escaped, at a period of exuviation, invested with an adhesive substance or tissue, which served to cement them, together, probably, with the exuviæ of the parent, to a supporting surface. In Cirripedes, we may suppose the cementing apparatus to have been retained; the parent herself, instead of the exuviæ, being cemented down, whereas the ova have come to escape by a new and anomalous course.
[73] Rathke has described (‘Acta Nova,’ 1839, p. 147), in some siphonostomatous crustaceans, a pair of curious organs, which serve to secrete a substance that holds the eggs attached together in a mass to the parent’s body: these organs Rathke has designated by a similar name to that which I have used, namely, the cementing organs or receptacles; they are distinct from the oviducts, but enter them near their external orifices. As in Cirripedes, the cement-glands and ducts are certainly continuous with an ovarian tube; and as they occupy a quite different position in the animal’s body, these organs of Rathke, though in some degree analogous in function, must be homologically distinct.