A Monograph on the Sub-class Cirripedia · Charles Darwin
ORDER II.--ABDOMINALIA.
The only function which I can assign to these appendages, is that of aiding the retention of the ova within the sack: for the ova, when first produced, are aggregated round them; at this period I several times observed long, somewhat curled, very thin fibres, not tapering like hairs, adhering to both appendages, the nature of which fibres I cannot explain. In very many cirripedes there seems a strong tendency to the production of tapering, filamentary appendages, somewhat like the two (_i_, _k_) here described,--namely, at the bottom of the sack in some Balaninæ, at the bases of the anterior cirri in Lepas and in some other genera, and on the dorsal surface of the prosoma in certain species of Pollicipes; in this latter case some of these appendages were covered by scales; and the prosoma whence they arose answers to the third segment of the body in Cryptophialus, or that supporting the lower and larger appendage. Appendages of this nature, in several cirripedes, serve for the lodgment of the testes, but in some cases they are of no apparent use, excepting, perhaps, in aiding respiration by the expansion of corium thus exposed, and this partially may be their function here, for there are no proper branchiæ.
_Cirri._--There are three pairs, together forming a nearly straight brush, of considerable length, projecting in a line with the last thoracic segment. Each cirrus is biramous. In a moderately large specimen there were twenty segments in one of the longest rami. Each segment (fig. 14, _a_) is strengthened by a dorsal or posterior shield of thickened yellowish membrane, from the upper edge of which a single smooth spine projects; the anterior surface is likewise strengthened on the two edges by thickened membrane, and supports two pairs of long spines, which are plumose, or rather hirsute, on both sides. In the lower segments of both rami of the several cirri, the inner spine of each pair is considerably shorter than the outer spine,--evidently in relation to the little power of divergence of the two rami. All the cirri resemble each other, excepting that the rami of the anterior pair, are rather shorter than those of the other cirri, but the dorsal spines on their segments are longer. In all ordinary cirripedes the two rami are equally supported by the upper segment of the pedicel, which latter is very short compared with the lower segment, and is separated from it by a transverse articulation. Here (fig. 13) the exterior or anterior ramus is articulated on the pedicel, almost like a branch, in front of the other ramus, which seems more directly continuous with the pedicel. The upper segment of the latter is separated from the lower segment, both being of about the same size, by a very oblique articulation. On the front surfaces of the pedicels there are two or three pairs of spines.
_Abdomen._--The three cirri on one side are separated from their opposite pairs by a prominent, longitudinal fold, formed of thickened yellowish membrane, which, when foreshortened by being viewed dorsally, looks like a style projecting immediately beneath the anus. The transverse folds separating the three pairs, are exceedingly slight. The inner basal edges of the pedicels of the cirri project slightly inwards as apodemes, giving, I believe, attachment to some muscles. The anus lies between the posterior pair of cirri, and deeply indents the last thoracic segment. The transverse folds separating the three pairs of cirri, little developed as they are, must, according to all analogy, be considered as representing three segments of the body, and as we have accounted for seven cephalic and seven thoracic segments, we must conclude that these are three abdominal segments. We know, moreover, that the abdomen in the pupæ of the Thoracica, with a single exception, does actually consist of three segments.
_Movements of the Mouth, Thorax, and Cirri, &c._--Judging from the fact of the lancet-shaped appendage of the labrum being more or less exserted in dead specimens out of the shell-cavity, and from the analogy of other cirripedes, I do not doubt that the whole mouth can be considerably raised and depressed; we have seen, also, that the rostral end of the toothed orifice of the sack can be pulled down, which would aid in exposing the mouth and labrum. The well-articulated thorax, by the aid of the muscles attached to every segment, can certainly be doubled up and contracted, so that the cirri with their pedicels (coloured purple like the exposed labrum) can be wholly protruded out of the sack and shell-cavity. The three cirri no doubt can be separated a little from each other, both transversely and longitudinally; and according to analogy, the two rami of each cirrus can likewise be separated: there are, also, muscles for moving the two segments of the pedicel of each cirrus; and other muscles run up the many segments of the rami. We have seen that the great lancet-formed appendage of the labrum, laterally fringed with fine hairs, can be erected; and I do not doubt that the prey when entangled by the expanded cirri, is borne against this appendage, and is then, by the retraction of the thorax, dragged down its smooth surface to the mouth, where it is seized by the mandibles and maxillæ, which lie like a trap at the bottom of an inclined and moveable plane.
_Alimentary Canal._--The œsophagus is long; it runs backwards from the mouth and then downwards; at its lower end, where it enters the stomach, the part, which in other cirripedes is expanded and bell-shaped, is modified in a most singular and quite peculiar manner; for the lower part of the œsophagus, after widening a little, becomes converted into what appears at first like a square box, 8/1000ths of an inch across. This box is really deeply folded (see diagram, Pl. 24, fig. 11) into six longitudinal ridges and hollows: two of these hollows, facing each other, are wider than the others, and when the organ is dissected out of the body, it generally lies (fig. 10) with one of these faces exactly over the other, the narrower lateral folds being thus hidden, and the whole consequently appearing like a simple square box with concave sides. But when a section is made, or the lower open end is turned upwards, we see that the organ is six-rayed and elongated, with the longer axis standing parallel to the flattened sides of the animal's body. The edges of the folds are formed of yellowish, thickened membrane, with a sinuous or beaded outline, which serve to strengthen the organ.
Internally, the two broad concave sides are armed, in their upper inwardly prominent (fig. 12) part, each with a disc, 2/1000ths of an inch in diameter, crowded with short, thick, brownish, inwardly projecting teeth. The two discs stand exactly opposed to each other. The bases of the teeth, seen from the outside (Pl. 24, fig. 10), seem like brown little circles, with a smaller circle within. The disc obscurely appears to be formed by the confluence of two smaller discs which lie, I believe, at a very small angle to each other: beneath each of these half discs there is a longitudinal band of very fine hairs, the two bands uniting into one, lower down within the organ. The internal longitudinal edges, also, of the four lateral smaller folds are likewise clothed with fine hairs; hence we have six parallel longitudinal rows of very fine but stiff hairs, or eight, if the united bands on the two broader faces under the discs be counted each as two. These bands of hairs, and the opposed discs, armed with very strong teeth, can be separated and brought together with force, by the action of strong constrictor and diverging muscles. Hence any prey carried down the œsophagus, before entering the stomach, would have to pass, as it were through a mill, and be subjected to a severe trituration by the discs of teeth, and immediately beneath to a brushing by the six longitudinal bands of hairs. This curious and unique structure answers, I believe, the same purpose as the four convex, hardish, crenated buttons on the posterior thoracic cirri in Alcippe, which are likewise unique in that genus. I observed that in some specimens the teeth had been worn quite blunt, but the teeth and hairs are periodically moulted and renewed, together with the whole œsophagus.
The stomach is broadest at the upper end, and extends from a little beneath the mouth down to the fifth segment of the body, where it becomes narrow. It presented an irregularly contracted appearance, and was covered by a pulpy hepatic layer. The rectum is of rather large diameter; it extends from the middle of the fifth segment to the end of the eighth segment of the body, or seventh of the thorax, where the large anus is situated, lying between the posterior abdominal cirri, and partly hollowed out in this seventh segment. The rectum, as in all other cirripedes, is periodically moulted. The food is of a bright green colour, as if of a confervoid nature; the triturating and brushing action of the œsophagus seems to roll this matter into pellets, which apparently retain this form until finally expelled as excrement: certainly the excrement is in pellets, and I have several times seen pellets within the stomach.
_Organs of Generation._--The specimens as yet described are exclusively female, there being certainly no testes or vesiculæ seminales. As in every specimen collected (early in January) there were within the sack either nearly mature eggs, or young larvæ, it was the worst period for seeing the ovarian cæca, and I have failed to discover them in the specimens now long kept in spirits of wine; but I cannot doubt they would be found, between the inner and outer tunics of the carapace or general covering, near the disc. I have stated in my original notes, made when the specimens were alive, that the ova are at first perfectly detached; but some appearances make me believe that I overlooked (as might easily happen) the often _excessively_ fine membrane which in other cirripedes unites the ova together, and so forms the ovigerous lamellæ. The ova are much less numerous than in other genera, varying from only nineteen to about sixty. In the same individual all the eggs were always in the same state of development.
_Metamorphoses._--The true ova, in their earliest condition, when in the sack, are ovate (Pl. 24, fig. 15), orange-coloured, quite smooth, and barely 10/1000th of an inch in their longer axes. They soon become broader at one end than at the other; and by degrees the narrow posterior pointed end becomes developed into a slightly club-shaped, almost transparent (fig. 16) horn, and the broader anterior end, into two rather longer horns. The length of the oval part, not measuring the horns, is nearly the same as in the primary true egg condition. There is as yet no trace of internal organs, the whole contents consisting of pulpy granular matter. How far the above changes are effected by moulting, either of the whole or of part of the integuments of the egg-like body, I cannot say; but the pulpy matter within the ovum, even in its earliest stage, was included within an inner envelope or case.
In the next distinct stage (there being, however, slighter intermediate changes) the posterior horn has shrunk, and become converted into a bluntly-pointed conical termination for the whole body (fig. 17), whereas the two anterior horns have approached each other on the future ventral surface, and have increased considerably in length and thickness, and contain within them the prehensile antennæ, which can be externally seen, and which I dissected out of these horn-like cases. The oval part of the egg-like larva (for I hardly know what to call it) is now very slightly shrunk, being hardly more than 9/1000ths of an inch in length. At this stage, these bodies adhere by the tips of their anterior horns, containing the antennæ in process of formation, to the inner tunic of the sack, and likewise in little groups one to another: as the included prehensile antennæ ultimately become attached by cement (proceeding, no doubt, as usual, from a modified portion of the ovarian tubes), it seems probable that some cement may at this early period be excreted, but I could not make out the exact means of attachment. The egg-like larvæ are, also, aggregated round the tapering curled dorsal appendages of the second and third segments of the body, and it is possible that at this, or at an earlier period, these appendages may act like the ovigerous fræna in the Lepadidæ, and serve to retain the egg-like larvæ within the sack.
We come now to the last larval, or pupal condition, before the final metamorphosis into the mature animal; the changes above described have been, at least to a great degree, if not absolutely gradual; but the pupa suddenly appears perfectly developed, from the moulting of the last-described horned, egg-like larva. It is now a free animal crawling about the sack of its female parent. It has increased a little in length, as compared with the oval part of the egg-like larva in its second stage, namely, from a little above 9/1000ths to 16/1000ths of an inch: from the position of the prehensile antennæ in the two states, I have no doubt that this increase of size is entirely due to the anterior part of the pupa being doubled up whilst within the egg-like larval envelope. The pupa in shape (fig. 18) somewhat resembles a coffin, and is far less laterally compressed than other pupæ, and hence can easily be placed either on its dorsal or ventral surface. The prehensile antennæ are of large size: when the animal was alive, they were concealed under and partially included within, the front part of the carapace or shell, which in this condition was not so much truncated as in the drawing given (fig. 18) of a specimen lying on its back, with its antennæ protruded. Some specimens formerly examined for me by Dr. Hooker, had their antennæ and whole ventral surface forced outwards, apparently from the endosmose of the spirits of wine. The whole dorsal surface, and the overlapping sides of the carapace are elegantly punctured, and are formed of a rather brittle substance, here and there supporting, especially at the front end, some fine and rather long bristles,--which latter I have not seen on the pupæ of other cirripedes. The ventral surface is very narrow towards the posterior end of the animal; it is formed of thin, structureless membrane. On this surface, close to the posterior end, there is a minute orifice, through which three pairs of bristles are protruded, attached to (as I believe) the rudimentary abdomen; the bristles apparently cannot be withdrawn.
The antennæ, (fig. 18) as stated, are of large size compared to the whole animal: they resemble, in all essential respects, the same organ in other cirripedes. The ultimate segment is unusually thick;[152] it is terminated by five bristles, one of which is longer than the others, and stands rather separated from them. The disc-segment is large, nearly circular, with the broad edges transparent and membranous; on its posterior edge there is a single small spine. The second or main segment, counting from the base, has a single spine on its upper margin, close beneath the spine on the disc; it is articulated to the disc-segment, a little way from the disc itself,--which is a peculiarity I have not elsewhere noticed. The basal segment is thick and not so short as usual. These organs are furnished with powerful muscles. They are generally protruded alternately; and by the adhesion of the sucker-like disc, the animal drags itself along. The sucker-disc has great play, and when observing specimens alive, I compared its action to that of a wrist-joint. The antennæ, when retracted within the carapace, lie parallel to each other. [152] As I have given, in my former volume on the Lepadidæ, p. 286, so many measurements of the antennæ, I may here add those of Cryptophialus,--the length from the end of the disc to the end of the second segment, (formerly called by me, erroneously, the basal,) is 26/6000ths of an inch; the greatest width of the second segment, 9/6000ths; the length of the little ultimate segment, 3/6000ths, and its width under 3/20000ths of an inch.
Posteriorly to the antenna, I distinctly saw the apodemes to which the eyes are attached: I was not able to distinguish any middle fork to the apodeme, which consequently does not resemble a UU, but U. The eyes are dark purple, and, as usual, compound: in one specimen I counted twelve ocelli within the common spherical envelope. I could not distinguish any thorax, and certainly there is no mouth; nor, from analogy, could the latter be expected, excepting as forming part of the young cirripede: there are no natatory legs, which the pupæ in all other cirripedes possess. Of the three postero-ventral pairs of bristles, the most posterior or dorsal pair, differs from the other two pairs in being considerably smaller, and in being mounted on elongated pedicels: the two anterior pairs of bristles are strong: the three pairs are articulated, one behind the other, on a small body, apparently enclosed in a minute sack, and certainly attached all round by membrane to the internal edges of the orifice, through which the bristles are protruded. These bristles, when the pupa was alive, were often moved, and served apparently to steady the body during the alternate protraction and retraction of the prehensile antennæ. From the fact of the pupa of other cirripedes having an abdomen, formed of three segments, placed exactly in the same position as the minute body here supporting the three pairs of spines, I believe this body to be the abdomen. In other cirripedes only the posterior segment of the abdomen bears spines, which are supported on little limbs or pedicels, namely, the caudal appendages, the other segments being naked. But as the mature Cryptophialus, unlike other cirripedes, has abdominal cirri, the presence of spines on the corresponding abdominal segments in the pupa, is explained and rendered probable: there can, I think, be little doubt that the small terminal pair of spines, supported on elongated pedicels or limbs, answers to the caudal appendages found in many cirripedes.
The whole course of the metamorphosis is very peculiar. The gradual changes in the egg-like larvæ (for I suppose they must be called larvæ) from a simple oval egg, to pointed oval, to oval with three horns, and lastly to oval with the two anterior horns larger, and the posterior horn reduced to a mere point, seems to me very curious; and offers, as far as I know, a unique case. It is interesting to reflect how perfect a series, in the development of an animal, we have, in different members of the Articulata,--from an ordinary egg, in which all the changes go on unperceived, and whence a perfect animal is matured,--to an egg-like larva which undergoes the changes just described, and which turns into a pupa that does not eat or increase in size,--to a larva which eats and increases in size, but undergoes only one great change, as in most insects,--to a larva undergoing several great changes, as in the case of ordinary cirripedes, before its final metamorphosis into the mature animal. The first larval condition of other cirripedes, in which there is a single eye, three pairs of thoracic limbs, and a much elongated pointed body, covered by a prolongation of the carapace, is here not fully developed or matured; but this stage is, I think, clearly and very curiously indicated by the posterior horn of the egg-like larva, which we may suppose represents the posterior pointed end of the body, for it disappears in the succeeding stage, just as it does in the second larval condition of other cirripedes. In the first stage of ordinary cirripedial larvæ, the anterior horns are always present, serving, as in the case of these egg-like bodies, to enclose and protect the antennæ during their formation. The second egg-like stage answers to the second larval condition of ordinary cirripedes, as described (and figured, Pl. 30, fig. 1) in the introduction to the Balanidæ. The third or pupal state is fully developed in all cases.
Finally, the pupa of Cryptophialus is peculiar in its punctured, hairy surface, and in its shape, which, in being so much more depressed than usual, retains an earlier larval condition; but its chief and highly remarkable character consists in the entire absence of natatory legs; and, in consequence, instead of there being a large sack within the carapace, with an elongated orifice on the ventral surface, there is only a quite minute orifice at the extreme posterior end of the animal, through which the bristles, borne apparently on all three segments of the minute abdomen, are protruded. The pupæ of the male and female are exactly alike in all their general characters, and probably in every point of detail; but my later and more minute observations were made only on pupæ, which, from their place of attachment, would certainly have turned into males. As these pupæ, without any further metamorphosis, were developed into males, we may, I think, safely infer that such is the case with the females: and, consequently, that the whole course of the metamorphosis has been, in this cirripede, seen and described. During this whole course, no food could possibly have been obtained, for the pupa is destitute of a mouth or organs of prehension, and the stock of cellular matter, enclosed within the ovum, has been sufficient for all the above changes, and for the final metamorphosis. We shall, moreover, immediately see, in the case of the male, that the stock of cellular matter has also sufficed for the development of testes, spermatozoa, and a wonderfully elongated probosciformed penis.
MALE. Pl. 24, fig. 19. By throwing pieces of the perforated shell of a Concholepas into acid, I examined several scores of specimens of the Cryptophialus, and on all, with the exception of a few young individuals, males were attached. They were attached by cement, proceeding in the usual manner from the prehensile antennæ, outside, to the edges of the upper half of the disc formed of the thicker not-moulted membrane, by which the female adheres in her chamber: hence the males are included in the upper part of the same cavity with the female, into which they must have crawled as pupæ. I found from one or two up, in one case, to seven males, attached to the same female; four or five being the most usual number. In the early part of January, when all my specimens were taken, many of the males had not shed their pupal integuments, and of those that had, the majority were immature, a few only having spermatozoa: all the females had within their sacks, either ova including almost perfect pupæ, or fully developed pupæ: we may, consequently, conclude that these young males were maturing in order to impregnate the next set of eggs.
The male, immediately after its metamorphosis from its pupal condition, which has been fully described, is almost globular, but slightly bilobed, and is formed of strong, structureless, transparent membrane, including a mass of cellular matter, apparently without any included organs: it is attached by about the middle, between the anterior and posterior lobes, by the not-moulted prehensile antennæ. When the male is mature, its greatest length, measured from the posterior end, where the orifice is seated, to the anterior and blunter end, is about 13/1000 of an inch, and therefore rather less than the pupa, which was 16/1000ths in length. Relatively to a full grown female, the male slightly exceeds half the diameter of the toothed orifice leading into her sack, see (_z_) fig. 1, Pl. 23. In the mature condition, (fig. 19), one lobe, namely, the upper or posterior, has become more pointed, and is terminated by a minute orifice, 8/6000ths of an inch in diameter. This orifice is formed by a rim of thickened brownish membrane, which, on what was the ventral surface, has a few very minute, but strong, sometimes bifid spines;--in this one character, the male resembling the female.
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