In mature Cirripedes these ducts can be followed, in a slightly sinuous course, along the muscles on each side within the peduncle, till they expand into two small organs, which I have called cement-glands. These glands are found with great difficulty, except in Conchoderma aurita, where they are placed on each side under the inner layer of corium, at the bottom of the sack, so as to be just above the top of the peduncle; they resemble in shape a retort, ([Pl. IX], [fig. 3.]). In [Pollicipes mitella] and polymerus they lie half way down the peduncle, close together, and apparently enclosed within a common membrane; in these two species the broad end of the gland is bent towards the neck of the retort. In Scalpellum the position is the same, but the shape is more globular. In Ibla the structure is more simple, namely, a tube slightly enlarged, running downwards, bent a little upwards, and then resuming its former downward course, the lower portion forming the duct. The gland contains a strongly coherent, pulpy, opaque, cellular mass, like that in the cement-ducts; but in some instances, presently to be mentioned, this cellular mass becomes converted within either the ducts or gland, or within both, into transparent, yellow, tough cement. Generally in Conchoderma, Pollicipes, and Scalpellum, two ovarian tubes, but in one specimen of [Conchoderma aurita], three tubes, and in Ibla one tube could be seen running into or forming the gland; of the nature of the tubes there could not be the least doubt, for at a little distance from the glands they gave out branches ([Pl. IX], [fig. 3]), containing ova in every state of development. In some specimens as in that figured of [Conchoderma aurita], the ovarian tube on one side of the gland is larger than on the other, and has rather the appearance of being deeply embedded in the gland than of forming it; but, in other specimens, the two ovarian tubes first formed a little pouch, into which their cellular contents could be clearly seen to enter; and then this pouch expanded into the gland; thus quite removing a doubt which I had sometimes felt, whether the ovarian tube was not simply attached to or embedded in the gland, without any further connection. By dissection the multiple external coats of the gland and ovarian tubes could be seen to be continuous. The cellular contents of the tubes passed into the more opaque cellular contents of the gland, by a layer of transparent, pulpy, pale, yellowish substance. There appeared in several instances to be a relation, between the state of fulness and condition of the contents of the gland, and of the immediately adjoining portions of the ovarian tubes. In one specimen of [Pollicipes mitella] it was clear that the altered, tough, yellow, transparent, non-cellular contents of the two glands and ducts, had actually invaded for some little distance, the two ovarian tubes which ran into them, thus showing the continuity of the whole. From these facts I conclude, without hesitation, that the gland itself is a part of an ovarian tube specially modified; and further, that the cellular matter, which in the ovarian tubes serves for the development of the ova, is, by the special action of the walls of the gland, changed into the opaquer cellular matter in the ducts, and this again subsequently into that tissue or substance, which cements the Cirripede to its surface of attachment.

As the individuals grow and increase in size, so do the glands and cement-ducts; but it seems often to happen, that when a specimen is immovably attached, the cementing apparatus ceases to act, and the cellular contents of the duct become converted into a thread of transparent tough cement; the investing membrane, also, of the ducts, in Conchoderma sometimes becomes hard and mamillated. I have already alluded to the case of a Pollicipes, in which both glands and ducts, and even a small portion of the two adjoining ovarian tubes, had become thus filled up. As in sessile Cirripedes, at every fresh period of growth a new cement gland is formed, it has occurred to me, that possibly in Pollicipes something similar may take place. In sessile Cirripedes, the old cement-glands are all preserved in a functionless condition, adhering to the membranous or calcareous basis, each new larger one attached to that last formed, and each giving out cement-ducts, which, bifurcating in the most complicated manner, pass outside the shell and thus attach it to some foreign body.

The cement, removed from the outside of a Cirripede, consists of a thin layer of very tough, bright-brown, transparent, laminated substance, exhibiting no structure under the highest powers, or at most a very fine dotted appearance, like a mezzotinto drawing. It is of the nature of chitine; but boiling caustic potash has rather more effect on it than on true chitine; and I think boiling nitric acid rather less effect. In one single instance, namely, in Coronula, the cement comes out of the four orifices of the two bifurcating ducts, in the shape of distinct cells, which, between the whale’s skin and the basal membrane, arrange themselves so as to make a circular, continuous slip of cement; then the cells blend together, and are converted into transparent, structureless cement. Cementing tissue or membrane would, perhaps, have been a more correct title than cement; but, in ordinary cases, its appearance is so little like that of an organised tissue, that I have for this reason, and for brevity-sake, preferred the simple term of Cement.

In the larva the cement always escapes through the prehensile antennæ; and it thus continues to do throughout life in most or all of the species of Lepas, Conchoderma, Dichelaspis and Ibla. In the first two of these genera, the cement escapes from the borders of the lower side of the disc or penultimate segment of the antennæ, and can be there seen radiating out like spokes, which at their ends divide into finer and finer branches, till a uniform sheet of cement is formed, fastening the antennæ and the adjoining part of the peduncle down to the surface of attachment. In [Dichelaspis Warwickii] and [Scalpellum Peronii], the cement, or part at least, comes out of the ultimate segment of the antennæ, in the shape of one tube, within another tube of considerable diameter and length. In [Scalpellum vulgare], and probably in some of the other species, which live attached to corallines, the cement soon ceases to debouch from the antennæ, but instead, bursts through a row of orifices on the rostral margin of the peduncle ([Pl. IX], [fig. 7]), by which means this margin is symmetrically fastened down to the delicate, horny branches of the zoophyte. In Pollicipes, the two cement-ducts, either together or separately ([Pl. IX], [fig. 2, 2 a´]), wind about the bottom of the peduncle in the most tortuous course, at each bend pouring out cement through a hole in the membrane of the peduncle. In Ibla the lower part of the peduncle is internally filled by cement, and thus rendered rigid. In [Lepas fascicularis] a vesicular ball of cement surrounding the peduncle is thus formed ([Pl. I], [fig. 6]), and serves as a float! All these curious, special adaptations are described under the respective genera. How the cement forces its way through the antennæ, and often through apertures in the thick membrane of the peduncle, I do not understand. I do not believe, though some appearances favoured the notion, that the duct itself debouches and divides, at least this is not the case in Coronula, but only that the internal chord of cellular matter thus acts and spreads itself out; nor do I understand how, when the antennæ and immediately adjoining parts are once cemented down, any more cement can escape; yet this must take place, as may be inferred from the breadth of the cemented, terminal portion of the peduncle in Lepas and Conchoderma; and from the often active condition in old individuals of the cementing organs.

I have entered on this subject at some length, (and I wish I had space for more illustrations,) from its offering, perhaps, the most curious point in the natural history of the Cirripedia. It is the one chief character of the Sub-class. I am well aware how extremely improbable it must appear, that part of an ovarian tube should be converted into a gland, in which cellular matter is modified, so that instead of aiding in the development of new beings, it forms itself into a tissue or substance, which leaves the body[15] in order to fasten it to a foreign support. But on no other view can the structure, clearly seen by me both in the mature Cirripede and in the larva, be explained, and I feel no hesitation in advancing it. I may here venture to quote the substance of a remark made by Professor Owen, when I communicated to him the foregoing facts, namely, that there was a new problem to solve,—new work to perform,—to attach permanently a crustacean to a foreign body; and that hence no one could, a priori, tell by what singular and novel means this would be effected.

[15] The protrusion of the egg-bearing pouches in Cyclops and its kindred genera, outside the body, offers a feeble analogy with what takes place in Cirripedes. Professor Allman (‘Annals of Natural History,’ vol. xx, p. 7,) who has attended to the subject, says that the external egg-bearing pouches are “a portion of the membrane of the true ovaries:” if the membrane of these pouches had been specially made adhesive, the analogy would have been closer.

Filamentary Appendages.—These have generally been considered to act as branchiæ; they occur at the bases of the first pair of cirri in Lepas, Alepas, Conchoderma, and in three species of Pollicipes: in Conchoderma there are similar appendages attached to the pedicels of the cirri ([Pl. IX], [fig. 4, g-k]); and in the above three species of Pollicipes there is a double row of them on the prosoma: their numbers differ in different species (in some there being none) of the same genus, and even in different individuals of the same species; they are entirely absent in the majority of the genera. These facts would indicate that they are not of high functional importance; and they seem so generally occupied by testes ([Pl. iv], [fig. 5]), that I suspect their function is quite as much to give room for the development of these glands, as to serve for respiratory purposes. With the exception of the four above-named genera, the mere surface of the body and of the sack must be sufficient for respiration: in Conchoderma aurita the two great expansions of surface, afforded by the folded, tubular, ear-like projections, aid, as I believe, towards this end.

The shape of the body varies, owing to the greater or less development of the lower part of the prosoma, the greater or less distance of the first from the second pair of cirri, and of the mouth from the adductor scutorum muscle, ([Pl. IX], [fig. 4], and [Pl. IV], [8 a´]). In all the genera, the body is much flattened. I may here mention a few particulars about the muscular system. One of the largest muscular masses is formed by the adductor scutorum, and by the muscles which surround in a double layer (the fasciæ being oblique to each other) the whole of the upper part of the prosoma. From under the adductor, a pair of delicate muscles runs to the basal edge of the labrum, so as to retract the whole mouth, and two other pair to the integument between the mouth and the adductor, so as to fold it: again, there are other delicate muscles in some (for instance in [Lepas Hillii]) if not in all the Lepadidæ, crossing each other in the most singular loops, and serving apparently to fold the membrane between the occludent edges of the scuta. Within the prosoma there is a strong adductor muscle, running straight from side to side, for the purpose, as it appears, of flattening the body. The thorax, on the dorsal and ventral surfaces, is well furnished with straight and oblique muscles (without striæ), which straighten and curl up this part of the body. The muscles running into the pedicels of the cirri, cross each other on the ventral surface of the thorax; the muscles within the rami are attached to the upper segments of the pedicels. Finally, I may remark that the whole of the body and the cirri are capable of many diversified movements.

Mouth.—This is prominent, and almost probosciformed ([Pl. IX], [fig. 4 b]), and in the abnormal Anelasma ([Pl. IV], [fig. 2 d]), quite probosciformed,—such, also, was its character in the larval condition. In outline, it is either sub-triangular, or oval with the longer axis transverse; the whole is capable, as well as the separate organs, of considerable movement, as I have seen in living sessile Cirripedes. It is composed ([Tab. V], [fig. 2]) of a labrum, swollen or bullate, often to such an extent as to equal in its longitudinal axis the rest of the mouth; of palpi soldered to the labrum; of mandibles, maxillæ, and outer maxillæ, the latter serving as a lower lip. These organs have only their upper segments free, but there are traces, clearly seen in the mandibles ([Pl. X], [fig. 1, a, b]), of their being formed of three segments. The two lower segments are laterally united, and open into each other, the prominence of the mouth being thus caused: this condition appears to me curious, and is, to a certain limited extent, intermediate between those articulated animals which have their trophi soldered into a proboscis, and those furnished with entirely free masticatory or prehensile organs. The palpi adhere to the corners of the labrum; and I call them palpi only from seeing that they spring laterally from above the upper articulation of the mandibles. The prominence of the mouth, measured from the basal fold by which the whole is separated from the body, is much greater on the half formed by the labrum and mandibles, than on the other half facing the cirri. The trophi surround a cavity—the supra-œsophageal cavity—in the middle of which, between the mandibles is seated the orifice of the œsophagus. The œsophagus is surrounded by long, fine, muscular fasciæ, radiating in all directions, opposing the constrictor muscles, and is capable of violent swallowing movements,—constriction after constriction being seen to run down its whole course: there are also some fine muscles attached to the membrane forming the supra-œsophageal cavity. The trophi serve merely for the prehension of prey, and not for mastication.

The Labrum, as stated, is always bullate or swollen; and sometimes the upper exterior part forms, as in Ibla ([Pl. IV], [fig. 8 a], [c]), and Dichelaspis, an overhanging blunt point. The object, I suspect, of this bullate form is to give, in the upper part, attachment to longer muscles running to the lateral surfaces of the mandibles, and lower down to the œsophagus. The crest close over the supra-œsophageal cavity, is generally furnished with small, often bead-like teeth. The Palpi are small, their apices never actually touching each other; they are more or less blunt, not differing much in shape in the different genera ([Pl. X], [figs. 6 to 8]), and clothed with spines. They are not capable of movement; their function seems to be to prevent prey, brought by the cirri, escaping over the labrum; I infer this from finding in Anelasma and in the male of Ibla, which have the cirri functionless, that the palpi are rudimentary.