[146] I may venture to remark that I succeeded in every attempt, which I made, in seeing plainly the œsophagus, and the acoustic and olfactory orifices and sacks, which, according to all analogy, would be of much smaller size, and far more difficult to discover, than the rectum and anus. I may mention that, according to Mr. Newport (‘Annals of Nat. Hist.,’ 1849, p. 277), the larvæ of certain parasitic Hymenoptera have a stomach without any anus. No crustacean, according to Milne Edwards, is destitute of this orifice.
The stomach, in [Alcippe], is much corrugated, so as to be deeply pitted; but there are no regular cæca. The enveloping hepatic layer is thick, brownish, pulpy, and formed of pellets of cellular matter, not distinctly arranged in lines as is general; there is the usual delicate muscular layer. The stomach was in every case empty, and I did not notice the separated epithelial coat, so generally found in other cirripedes.
Organs of Sense.—I failed in discovering the eye, which I have no doubt exits, as it is conspicuous in the pupa and in the male. The olfactory pouches are seated rather laterally under the maxillæ, as in Ibla. As in this same genus, the acoustic sack is seated remarkably low down (fig. [5]), at a very considerable distance beneath the basal articulation of the first cirrus: the orifice is seated on a slight prominence: the acoustic vesicle, I believe, is sub-cylindrical, with irregular projections. I did not make out anything distinctly on the nervous system.
Female Generative System.—The animal we have thus far described is exclusively female: when a longitudinal section of the thorax is made, and the stomach removed, it can be most plainly seen that there are no vesiculæ seminales or testes. Mr. Hancock has remarked on the absence of the usual probosciformed penis. The male of [Alcippe] will be subsequently described in detail. The female organs differ in no respect from those of other members of the family, excepting in so far that the layer or mass formed by the ovarian cæca (c) does not lie transversely to the longitudinal axis of the whole animal, but longitudinally under the horny disc. The ovigerous fræna are largely developed, but serve, as previously stated, as branchiæ, and not for their proper function of giving attachment to the ovigerous lamellæ. The ovigerous lamella is single, and nearly corresponds, in size and shape (as would ensue from the manner of its formation) to the under side of the horny disc. The ova are broadly oval, and rather above 1/100th of an inch in length.
Metamorphoses.—The larva in the first stage has been fully described and figured by Mr. Hancock: it differs in no essential respect from other larvæ of the family. Mr. Hancock overlooked the inferior minute antennæ. With respect to the larvæ in the last stage, or pupa, I obtained several specimens attached to the disc of the female, and which were on the point of being developed into males; and another specimen identical in all respects, but attached independently to the shell of the mollusc, and which, therefore, I have every reason to suppose, would have been developed into a female. In any case these pupæ may be conveniently here described (Pl. [23], fig. [16].) They are .025 of an inch in length; they are of the usual shape, with the anterior end not very blunt and the postero-ventral surface somewhat produced. The whole carapace or shell is very thin and smooth. There are six pairs of thoracic natatory legs, situated far back towards the posterior end of the body; each leg has the usual articulations, and the two rami their usual long but not plumose spines; the presence of the legs deserves notice, considering the rudimentary and modified state of their homologues in the mature animal. The abdomen differs considerably from the same part, as far as I have seen, in other pupæ; it consists of only a single almost globular (fig. [17], q) segment, instead of three segments; and the two caudal appendages (r) are very long, and are composed each of only a single segment (instead of two), carrying at its tip two short spines. There are two purple eyes, 4/3000ths of an inch in diameter, which, after having been dried and then soaked, could be seen to be compound; they are fixed in the usual manner to two rather short apodemes, which latter have their usual origin. But the pupa has a very unusual appearance owing to the presence of a single dark purple eye, half the diameter of the two larger eyes, situated behind and above the latter, and quite disconnected with the apodemes; this is the eye of the mature animal, which, for some reason, is here developed earlier than usual. The prehensile antennæ are remarkable from being seated very close to the anterior extremity: owing to this, the articulation of the second or main segment with the basal segment, is hardly at all oblique. The whole pupa is of exactly the same length as the pupa of Ibla quadrivalvis, and so are the antennæ, (see p. 286 of my volume on the Lepadidæ), viz., 32/6000th of an inch, but the second segment is narrower, (being only 8/6000ths in breadth in the broadest part), and is longer in proportion, for the disc which forms part of the total length is only 4/6000ths in length, whereas in Ibla it was 8/6000ths; the disc is here hoof-shaped, as in Ibla. The ultimate segment is remarkably short and narrow, (being only 3/20000ths in width, and less than half the size of that in Ibla); it carries (I believe) three terminal spines, and is not notched. Altogether the antennæ more nearly resemble those of Ibla than of any other genus in the family. From the position of the antennæ, and from the length of the second segment, the pupa, when cemented by the disc or third segment, to the supporting surface, adheres, with its posterior end almost vertically upwards. With respect to the young cirripede within the pupa, I could only observe that its anterior end was formed into a blunt point.
Powers of Excavation; Inorganic Deposit of Calcareous Matter; Attachment.—[Alcippe], according to Mr. Hancock, attacks only dead shells of the Fusus and Buccinum, and always on their inner sides, especially on the columella. The excavations, in the specimen which I examined, were so numerous as almost to touch, and sometimes to run into each other, the included animal being thus rendered distorted. The orifices are directed with respect to the shell indifferently upwards or downwards. From the shape and size of the cavity corresponding to that of the included animal, there can be no doubt, as stated by Mr. Hancock, that [Alcippe] forms its own cavity. That the action is mechanical I think may safely be inferred from the whole outer membrane being studded with minute, star-headed points of hard chitine, which rise from halo-like little discs of thickened membrane, which latter are well adapted to allow the underlying adherent muscular layer to act on the points, and thus on the surrounding shell. Consequently the points generally show signs of severe attrition, but they are periodically and often replaced, at each exuviation, by new and much sharper points. There are no points on the permanently attached layers of the horny disc, but it particularly deserves attention, that the renewable membrane always extends beyond the circumference of the disc, and is there most thickly studded with the points. We have met, in Lithotrya, with a precisely analogous fact in the extension of the periodically moulted membrane of the peduncle, furnished with star-headed points of chitine, and in addition with minute calcareous beads (which, however, seem soon worn away), beyond the calcareous discs, by which this cirripede is attached in its cavity. We need not feel much surprise at points of chitine being hard enough to wear away shell, when we consider what work the jaws of insects, likewise formed of chitine, will effect.
With respect to the first commencement of the excavation, the pupa, owing to the position of its prehensile antennæ, fixes itself with its posterior end almost vertically upwards; and the young cirripede, after its metamorphosis, from the greater length of the ventral integuments formed round the eye-apodemes, must be thrown backwards into nearly the position represented in Pl. [22], fig. [12], b. I have not seen a young female at this early age, but I have traced the development of several males, and have found that the lower end of the peduncle, (i. e. what was the anterior end of the pupa), grows at quite a remarkable rate, so as very soon to form a great bag extending beyond the attached prehensile antennæ. Now if we suppose an analogous structure in the female or ordinary [Alcippe], and the supposition is quite allowable, we shall almost immediately have the anterior or lower end of the young cirripede, just in advance of its antennæ, pressing against the surface of the shell of the mollusc; and if armed with triturating points, as we have every reason to believe it is, it would wear for itself a cavity. The horny disc on the ventral surface of this protuberant anterior end of the young animal will, we may assume, soon become cemented to the near side of the cavity just supposed to have been excavated. And the whole animal, by further slight changes in direction, namely, by working down more and more obliquely, will take, as shown at (c), its final position. As the whole surface of the animal, with the exception of the horny disc, is provided with triturating points, the animal, when once imbedded, can and does increase its cavity at both ends in length, in depth, and all round the edges of the horny disc,—in short, in every direction excepting directly over the horny disc. I believe, as already explained, that the young [Alcippe], (b, diagram), first bores obliquely into the shell; and whatever amount of downward extension the horny disc attains before the young cirripede assumes its proper position, with its ventral surface upwards and parallel to the inner surface of the shell of the mollusc, that amount determines the thickness of the plate of shell hereafter to be left unabraded over the horny disc, as the latter continues to extend in circumference. This plate of shell over the horny disc is so thin, that, as mentioned at the commencement, the colour of the ovaria is seen through; and until I reflected on the following considerations, I was much surprised how the instinct of the animal could so neatly guide it not to grind too deeply, and yet to grind till only a very thin plate of shell was left over its horny disc: these considerations are, that whatever thickness was first given to this plate of shell, when the animal was very young and first assumed its ultimate position, that thickness would in most cases be always retained, owing to the flatness of the disc, and to the membrane armed with triturating points protruding very slightly beyond and above the horny disc, only just enough to wear away the surrounding shell to the thickness necessary to allow of the formation of each new zone of disc; as the disc itself is not armed, it subsequently has no power of wearing away the plate of shell above it. Thus the horny disc, besides giving support and attachment to the peduncle, is of this peculiar service that it seems to guide, (somewhat like the wood-part in a plane), the rasping powers of the lower extreme margin of the peduncle.
I may here observe that certain radiating and often punctured lines, mentioned and figured by Mr. Hancock, which help to render the thin plate of shell over the peduncle conspicuous (fig. [3]), are formed by the burrows of an excessively minute annelid, the punctures being apparently the exit orifices: I imagine that these annelids find it difficult to commence their burrows on the smooth surface of the shell, and that they congregate at these particular spots and thence burrow in radiating lines, owing to their having taken advantage of the little cliff-like edges, at the narrow and disused ends of the fissures leading into the cavities occupied by the [Alcippe], where alone they would not be disturbed by the action of the cirri, when first they commenced making their little burrows in the shell.
The fissure leading into the cavity is required to be broad at the posterior end, in order that the cirri may be there freely exserted out of the sack; and narrow in other parts, to prevent, as it would appear, anything injurious getting in between the animal’s body and the cavity in the shell of the mollusc. As the fissure is increased in length by attrition at the broad posterior end, which end during growth becomes broader and broader, the lower part of the fissure has to be narrowed, and this is effected in a very singular manner, namely, by advantage being taken of the strong tendency, which triturated shell with animal matter, has to set into a solid shelly mass, although constantly agitated.[147] Mr. Hancock noticed this edging of hard shelly matter, and naturally thought it was a secretion. Lines of deposition (Pl. [22], fig. [4], b), parallel to the edges of the furrow can often be perceived in it: its thickness and extension vary much: I have seen it on one side alone of the orifice: it is, of course, never found at the broad end where the process of enlargement goes on. The peculiar worn surface with which it irregularly thins away downwards, on the sides of the cavity, made me (together with the apparent impossibility of such a secretion proceeding from an animal wholly invested by a chitine membrane) suspect it to be inorganic; and this view is certainly correct, for when a fragment is dissolved in acid, a considerable residuum is left of bits of membrane, rubbish, and, in one instance, even of the remnants of a foreign animal, apparently an annelid. We have here all the circumstances favorable for inorganic deposits of this nature, namely, finely triturated shell and chitine or animal matter, produced by the excavation of the chamber, sea-water, and movement.
[147] I have given some remarkable cases in my volume on ‘Volcanic Islands,’ (p. 49), in which limestone, having almost the hardness and specific gravity of marble, has been thus deposited. Almost every coral-reef offers similar examples. The curious substance described by Mr. Horner and Sir David Brewster, (‘Philosoph. Transact.,’ 1836, p. 65), which is formed during the manufactory of cloth, offers another example of the strong tendency which lime and animal matter have to unite. Lately, Dr. Horsford, in ‘Silliman’s North American Journal,’ Jan. 1853, has discussed the chemical theory in an analogous case on the coast of Florida; he attributes the aggregation to the formation of a hydrate of lime through the action of the animal matter. Mr. G. B. Sowerby, Junr., has described a case very analogous to that of [Alcippe], (‘Proceedings of Zoolog. Soc., Mollusca,’ Pl. 5, fig. 4, p. 162, 1850), namely, that of Pholas calva, in which a tube is formed of inorganic calcareous matter, serving to narrow the entrance.