A Monograph on the Sub-class Cirripedia · Charles Darwin
LEPAS ET BALANUS AUCTORUM.
_Cirri._--These consist of the first, fifth, and sixth pairs: the other pairs are absent, except in one monstrous specimen, in which there was a fourth cirrus quite like the fifth. _First pair_, fig. 14, these are seated on each side of the mouth in the usual position. They are formed of very thin and flexible membrane. The pedicel, as usual, consists of two segments, the upper one is short and not very distinct; but when viewed on the inner side can be seen to have the ordinary structure: both segments are destitute of bristles. There are two short rami, being about one third of the length of the pedicel: they are directed either in the line of the pedicel, or more commonly posteriorly, that is towards the other cirri, and therefore in an unusual direction. The anterior ramus is generally rather longer and thinner (as is commonly the case with other Cirripedes) than the posterior ramus; but there is some variation in this respect. On neither ramus is there any trace of the ordinary articulations: both are thickly clothed with fine bristles, which are singular from being thickened in their lower parts, and plumose, like a feather. These cirri have some resemblance, as remarked by Mr. Hancock, to a pair of pincers; but they cannot act as such; they serve, I believe, as brushes. Delicate muscles, transversely striated, enter and are attached within both rami and within both segments of the pedicel, on the usual type, showing that these organs (if there had been any doubt) are truly cirri.
The _fifth_ and _sixth pairs_ of cirri (fig. 13, _n′_, _o′_) are almost exactly alike: they are of very small size: each cirrus consists of four segments: the lower or basal segment is broad, with a few minute bristles scattered on its inner surface: the second segment is also broad, but shorter, with a few, generally hooked bristles, in two short irregular rows, in the upper part: these two segments answer to the two segments of the pedicel of ordinary cirri. The third segment is thinner and longer than the second; it bears two or three longitudinal rows of bristles, most of which are neatly hooked at the point; its upper end is surrounded with a circle of bristles. The fourth and terminal segment is short, thin, and simple, with only a few bristles at the apex. These two upper segments are bent a little inwards; they answer to one of the two normal rami of ordinary cirri. The third segment does not stand exactly on the middle of the summit of the second segment,--the posterior corner of the latter being occupied by a very curious, convex, oblong, rather hard (especially in the lower part), protuberant cushion (as called by Mr. Hancock) or button (fig. 9, _c′_), transversely wrinkled by fine, distinctly crenated ridges.
This button presents a considerably different appearance according to the point of view, fig. 9, 10: on one of its sides it projects beyond the outline of the second segment, whence it arises; on the other side it is prolonged, as a smooth ridge, on the top of the second segment, embracing to a certain extent the base of the third segment. On the face opposite to that which has been drawn (fig. 9) as most characteristic, it is seen to be somewhat constricted round its base; this constriction, representing, I believe, an articulation. When viewed directly in front (fig. 10) its outline is oval, passing into shield-shaped. Its longitudinal axis is 3/1000ths of an inch in length; but it varies a little in shape and size. I shall presently assign my reasons for believing that these buttons are the posterior or inner rami of the fifth and sixth pairs of cirri in a rudimentary and much modified condition. _Caudal Appendages._--These (fig. 13, _p_) are placed close together, being articulated between the bases of the sixth pair of cirri, the lines of junction being internally marked by minute apodemes. They consist of four segments, resembling in every respect those forming the cirri, with the important exception that there is not a vestige of the button on the summit of the second segment; the segments are not so thick as those of the cirri, and the terminal segment is smaller.
_Muscles and Functions of the Cirri._--For their size, the cirri and caudal appendages have voluntary muscles of remarkable strength, attached within their basal segments, and springing from the dorsal and ventral surfaces of the so-called third and fourth (_l_, _m_) thoracic segments. Other muscles, rising from within the basal segment of each limb, run to the second segment, and from that to the third segment. I could not distinctly make out whether any entered the terminal segment. I have seen no other instance of muscles entering the caudal appendages, but as in the pupa they are so furnished, we here have only an embryonic character preserved. I may remark that the fifth and sixth cirri, consisting of two large basal and two thinner terminal segments, is likewise an embryonic character. From the position of the cirri, the four hard protuberant buttons or cushions, tend to oppose each other at a common point; and the caudal appendages fill up a gap behind, between the cirri of the sixth pair. I at first thought, with Mr. Hancock, that these buttons served to catch the prey; but, reflecting on their convexity and hardness, they appear very badly adapted for this purpose; it would, in fact, be a marvellous feat to secure, in the dark, any moving object between four balls.
On the other hand, this very convexity, the hardness, and especially the crenated ridges, and the powerful muscles (which from the first surprised me), are all well explained, if we suppose the prey, being secured by the terminal segments, to be triturated between these four balls: any part which escaped upwards would, moreover, be retained in a sort of cage, formed by the inwardly inflected terminal segments with their hooked spines. This view of the very curious and unparalleled use made of a modified portion, not of the haunch, but of an upper part of the two posterior pairs of thoracic limbs, is in some degree confirmed by finding that Cryptophialus, which has apparently analogous habits, requires its food to be triturated, though in this case it is effected by very different means, namely, by four beautifully toothed discs, with brushes of hairs, developed within the lower end of the œsophagus.
The prey, when caught, would probably at once be carried by the movement of the articulated thorax to the mouth (itself moveable), and being there secured by the mouth in front, the caudal appendages behind, the tips of the cirri above, and the broad pedicels of the first pair on the two sides, it would be triturated by the four crenated buttons, and would then be forced down the œsophagus by the action of the simple jaws. I looked in vain in several specimens for any object within the stomach. I believe, that when the specimens are first taken, all half digested food is ejected by the mouth. Whether we may thus account for the extremely foul condition of the rami of the first cirri in all the many specimens examined by me, I know not; but that these rami, which are thickly clothed with fine plumose hairs, and are furnished with delicate muscles, act as brushes, so as to clean the orifice of the sack, I can hardly doubt. _Homologies._--I have as yet, to a certain extent, assumed that I have correctly named the different parts; and a few remarks on this head may be desirable, considering the absence of certain cirri, the singular condition of the others, the close general resemblance of the cirri and caudal appendages, and the fact of the latter being furnished with muscles.
The only cause for any doubt regarding the thoracic segments is the shield of thick membrane on that segment (_k_, fig. 5), which ought to have borne the second pair of cirri, causing two transverse wrinkles (not distinguishable, however, on the ventral surface), and sometimes making the segment appear as if it consisted of three segments: if it did consist of three, as there can be no doubt about the nature of the first pair of cirri, (not in a more rudimentary condition than in Anelasma) or about the segment whence this first pair arises, the two terminal oblique segments, with their appendages, would be abdominal instead of thoracic: but this is improbable, inasmuch as the abdomen is unusually little developed in the pupa (as presently to be shown), and more especially from the circumstance of a monstrous cirrus, identical in structure with the two succeeding pairs, having been borne on a segment (_m_), which, in any case must be considered as thoracic, for it is well known how very rarely thoracic and abdominal limbs resemble each other. I cannot myself feel hardly any doubt on the nature of these three pairs of appendages; for, in the first place, the posterior appendages are articulated on and between the bases of the adjoining pair, _exactly_ as the undoubted caudal appendages are articulated in all other members of the family on the sixth pair of cirri or terminal thoracic appendages. Secondly, we see in the male of the allied genus Ibla, the very same appendages preserved as in Alcippe, namely, the caudal, and the fifth and sixth pairs of cirri, which latter, moreover, are generally uniramous. Thirdly and lastly, in the likewise allied _Alepas cornuta_, we have the posterior rami of these same fifth and sixth pairs of cirri in a rudimentary condition, and resembling in every respect the caudal appendages.
Assuming, then, that the several appendages in Alcippe have been rightly denominated, we have to consider the nature of their segments: in all cirripedes, the pedicels of the cirri consist of two segments, of which the lower one (as here) is longer than the upper one, and both (as here) considerably thicker than the segments of the rami: in all cirripedial pupæ, likewise, the thick pedicels of the limbs consist of two segments, and each ramus, also (as here), of two segments: now, with these coincidences, and bearing in mind that in Alcippe the two upper segments do not arise from the exact middle of the summit of the second segment, but from rather its anterior side,--bearing, also, in mind the case just cited of _Alepas cornuta_ with the posterior rami of these very same cirri rudimentary,--we may, I think, safely conclude that here in Alcippe the two lower segments form the pedicel; the two upper segments, the anterior ramus; and that the button-like protuberance is the posterior ramus in a modified condition. As the caudal appendages in none of the Lepadidæ, either in the mature state or in the pupa, have two rami, we can satisfactorily understand the absence of any trace of the button-like protuberance on the top of the second segment.[145] [145] I almost wish I could persuade myself that I had taken an erroneous view of the thoracic segments, and therefore that the three pairs of terminal appendages were all abdominal, for then Alcippe would come into much closer relationship with Cryptophialus; though even in that case it would form a distinct family from it: but I cannot alter my opinion.
_Alimentary Canal._--The œsophagus runs down from the mouth, beneath and nearly parallel to the straight row of hairs on the two sides of the labrum: it is surrounded by the usual muscles: at the lower end it bends down, and expanding a little, like a bell, enters the stomach. The stomach is of considerable size and fills the main part of the body, bulging out under the mouth, and prolonged as far as about the middle of that segment (_l_), which ought to have borne the third pair of cirri; here the stomach terminates in a blunt rounded point. The tissue surrounding the stomach, and keeping it in its proper place, can be traced to the posterior end of the thorax, but there is no rectum or anus. I am prepared to assert positively that this is the case,[146] for I made repeated longitudinal sections of the whole thorax in two planes, and I subsequently cleaned the outer tissues with boiling potash, and then, when as transparent as a sheet of glass, I examined every part, and certainly there is no rectum (which in every case is formed of chitine, and so is not acted on by potash) nor an anal orifice. Singular as this fact is, it is not so improbable as it at first appears; inasmuch as I have shown, in my former volume, that the Lepadidæ can reject half digested food by their mouth, and secondly, that the final stage of digestion appears to take place in the upper part of the stomach. In the male of this very species, as we shall immediately see, there certainly is no mouth or stomach, and apparently no rectum or anus; so is it likewise with the males of _Scalpellum vulgare_ and _ornatum_: in Proteolepas, there is a mouth and an œsophagus, but no stomach, rectum, or anus. There are, I believe, no other known instances, in the whole great class of Crustacea, of the absence of an anus.
_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.
Ideas discussed in this chapter
Text from Project Gutenberg, public domain in the United States.