[61] Quoted by Dana, ‘Crustacea of United States Exploring Expedition,’ p. 1264.
Antennæ.—These, from their present position, and from standing, in their earlier stages whilst within their envelopes or horns, exteriorly to the small medial pair (since aborted), I believe to be the second pair; and this is Mr. Dana’s opinion. In my former description of these very singular and important organs (Pl. [30], figs. [4] and [8]), I have fallen into some considerable mistakes: the two plates or segments (fig. [4], N), of which the posterior margins are inflected as apodemes (n), carrying the eyes, are certainly, as may be clearly seen in the pupa of [Alcippe], Pl. [23], fig. [16], and as affirmed by Burmeister,[62] the basal segments of the antennæ. The second or main segment (formerly called by me the basal segment) has in some species an upper portion of the membrane of which it is composed, next to the body, excessively thin, and separated from the rest of the membrane composing the segment, by an oblique line (fig. [8], o), which I mistook for its articulation with the body.[63] We then come to the disc or third segment; and lastly to the fourth, or ultimate segment. This ultimate segment, generally, has its external corner projecting up, as a step; and this sometimes, as in Dichelaspis Warwickii, gives the appearance of its consisting of two segments; but a careful examination of this part in Ibla, in which the step-like structure is carried to an extreme, makes me believe that there is only one segment.[64] The prehensile antennæ, therefore, like the natatory legs, are formed of four consecutive segments, of which the basal segments give rise to the singular apodemes, presently to be noticed (fig. [7]), that carry the great compound eyes. This basal segment, in all Cirripedes, is moulted with the eyes, the three other segments invariably remaining cemented to the surface of attachment.
[62] ‘Beiträge zur Naturgeschichte der Rankenfüsser,’ p. 19. In tab. 1 of this work there are good drawings of the general structure of the pupa of a species of Lepas, probably L. australis. I believe this author was the first who made out the structure of the abdomen of the pupa.
[63] In the table of measurements of the antennæ of the several genera and species of the Lepadidæ (p. 286) of my former volume, the articulation, called by me basal, I now know to be really the articulation between the basal and second segment. In the fourth column, headed “Length from end of the disc to the inner margin of the basal articulation,” the term inner margin really applies to the oblique curved line separating the thin and scarcely visible membrane from the thicker membrane of that segment. These corrections do not in the least affect the object for which the table was given.
[64] In a sketch, sent me by Mr. Dana, of this organ in the pupa of a Lepas from the Antarctic Ocean, I observe that he divides my ultimate segment into two segments.
In the Southern Atlantic I took some specimens of the pupa of Lepas australis, not yet attached, and therefore with the muscles of the antennæ, not having suffered any of that absorption, which they undergo, as soon as the pupa is permanently cemented to some floating object. In my former volume I noticed a pair of strong muscles, attached to the tips of the middle forks (Pl. [30], fig. [7]) of the apodemes, and I now find two pairs attached to the bases of the two outer forks, and directed dorso-anteriorly; and two other pairs, also attached to their bases, but directed dorso-posteriorly, so that altogether there are five pairs of muscles attached to the apodemes; their chief function, I should think, was to draw the antennæ posteriorly and upwards within the carapace; but as the apodemes cannot be moved without the great compound eyes being likewise moved, the muscles probably serve a double purpose. When the pupæ were alive, I noticed that their eyes were constantly kept in a state of vibratory movement. Flexor and extensor muscles are attached at one end to the posterior margin of the basal segment, and at the other end to the second or main segment; other powerful muscles attached to this latter segment, are prolonged by ligaments into the disc. In [Cryptophialus] I observed that the disc-segment had a movement almost like that of the wrist. Whether any muscles enter the small terminal segment, I know not.
The drawing in Pl. [30], fig. [8], of part of the second segment, of the third or disc segment, and of the fourth or ultimate segment, in Lepas australis, is, I think, very accurate. The second segment articulates on the upper or dorsal surface of the disc, and has the articulation on one side constricted and formed of thin and flexible membrane; the little terminal segment, which is turned outwards at right angles, also, articulates on the disc. That the disc forms a true segment is shown clearly in [Cryptophialus] (Pl. [24], fig. [18]), where the articulation with the second segment is not in such close contact. The disc is either circular, as in Lepas, or hoof-shaped, as in Ibla: in [B. balanoides] the disc is rather hollowed out on the inner side. It has the power of adhering even to so smooth a substance as glass, placed vertically. It is surrounded by a rim of transparent membrane. On the hinder margin some spines arise from the central and more opaque part: in Lepas australis, there are no less than seven of these spines (fig. [8]): in Conchoderma virgata there are only four, in Scalpellum and Ibla only one. When the disc is placed on the surface of attachment, these spines lie parallel to it. The middle part of the disc is, almost always, nearly opaque; and in tracing the cement-ducts from within the body of the pupa, or of the young Cirripede, I in many cases traced them as far as this point, but here lost them. From this same obscure central part of the disc, in most, if not in all species of the Lepadidæ, spokes radiate, which sometimes are branched, and are not regular, not always even resembling each other on the opposite sides of the same individual. Round the proper membranous border of the disc, a second one may be observed (fig. [8], p), which differs in shape and extent in different specimens: under favorable circumstances, and very high powers, it may be seen to have a reticulated structure, and to be of a very pale brown colour; towards the exterior margin, the reticulations become finer, and are blended together and lost; on the inner margin, the substance forming this membrane may be seen to come out of the spokes. This substance is the cement, which has the power of adhering to whatever substance it grows against; and thus the disc of each antenna becomes cemented down, and soon both the antennæ are surrounded by a common border of cement, which gradually increases, after the metamorphosis, in extent. Occasionally the cement forms little projections, like short spines, on the edges opposite to the orifices of the spokes.
The small terminal segment usually bears on its truncated extremity six spines, some of which are occasionally hooked; in Scalpellum, two spines, rather longer than the others, are borne on a step some way down on the inner side of this segment; but in Lepas, two spines (fig. [8]), very much longer than the others, arise from the outer corner of the extremity. These two are very different from the other four borne by this segment, or indeed any other spines on the body; for they are quite flexible, and are furnished with a double row of very long, straight, excessively fine hairs, which seem to be articulated on them—the whole presenting a very beautiful appearance. These spines are of considerable length, and in Conchoderma virgata they even equal in length the whole antenna. I can hardly doubt that these beautiful, plumose, flexible spines, into the thick bases of which the coloured corium could sometimes be seen to enter, serve as feelers. Owing to the facts immediately to be mentioned, I erroneously stated, in my former volume, that there were three long spines.
In three species of Lepas, in Dichelaspis Warwickii, and in Scalpellum Peronii, after having torn the lately-cemented antennæ from the surface of attachment, I observed proceeding from the end of the terminal segment, from between the above two groups of spines, what appeared to be a long narrow ribbon with its end torn off; and which, in the case of Lepas australis, I fancied was one of the plumose, long, flexible spines ripped open. But now that I have examined some of the pupæ of this species before their attachment, I find (as represented in Pl. [30], fig. [8], v′) a flattened tube, ending in a blunt point, and having a peculiar ringed structure. I have noticed similar appendages to the antennæ of specimens just attached of Lepas anatifera. In the Dichelaspis and the Scalpellum, the tube was very long, and seemed, from its torn appearance, to have been firmly attached to the supporting surface. In both these cases, the tube came out from within another slightly larger tube, which had been broken off close to the extremity of the terminal segment of the antenna. In the case of the Lepas anatifera, the tube expanded a little after leaving the antenna. In the Dichelaspis, it had exactly the same diameter as the cement-duct, which could be clearly distinguished within the two lower segments. From these several facts, and from the peculiar appearance of the tube itself, I believe it to be a tube of cement-tissue which thus, sometimes even before the pupa is attached, independently grows outwards. That the cement-tissue can grow outwards and assume definite forms, we know from the singular case of Lepas fascicularis, in which the cement proceeding from several apertures, forms a vesicular float round the peduncle of not only a single individual, but often of a group of specimens: we shall presently find a somewhat analogous fact in the case of [Coronula]. It is possible that this tube, proceeding from the extremity of the antenna, may be the channel through which cement continues to be poured forth during the continued growth of the above Cirripedes; and the manner in which this is effected, considering how firmly the end of the peduncle is cemented down, has always appeared to me a difficulty. In those pupæ of Lepas australis, which I caught swimming about unattached, it is surprising that the disc should have been edged with cement, and that a tube, similarly formed, should have grown out of the ultimate segment: it shows, I presume, that the cement-tissue will grow out, whether or no the pupa has succeeded in finding a proper object for attachment. Lastly, I have felt some surprise, in two or three instances in observing some dark purple pigment-cells, like those in the corium, within the terminal tube of cement; and likewise within the spokes of cement in the disc: this is the only fact which causes me the least doubt, whether I have rightly determined the nature of the terminal tube, as wholly formed of cement tissue; or whether it may not be covered by an outer integument, itself lined by true corium, coloured purple.