A Monograph on the Sub-class Cirripedia · Charles Darwin
TABLE OF CONTENTS.
When the shell of a Balanus is broken open, the second and third pairs of cirri are repeatedly clasped over the mouth with a convulsive movement, in a manner indicating, I think, that their chief function is to seize and carry to the mouth any object entangled by the sweeping movement of the three posterior pairs. The first pair is also well adapted to aid in this seizing action; but I suspect that the long anterior ramus likewise acts as an organ of touch, warning the animal of danger. The mouth being itself moveable as a whole,--the outer maxillæ being capable of a backward and forward sweeping action, and being furnished with orifices apparently olfactory,--the inner maxillæ having more diversified movements,--the toothed mandibles overhanging the œsophagus,--and the œsophagus itself possessing a powerful swallowing movement, are all admirably adapted to secure any prey, when once entangled by the cirri. _Mouth._
The mouth, in the sub-family Chthamalinæ, cannot be distinguished from that of the Lepadidæ, which has been pretty fully described in my former volume. In the Balaninæ, however, the labrum differs considerably in not being swollen; that is, in its outer and inner fold of membrane being close together, and in having a central notch: the palpi are also larger, and the lower teeth on the mandibles, are laterally (Pl. 26, fig. 5) double, as will be more particularly stated under these two sub-families. I have given a drawing (Pl. 26, fig. 1) of the mouth, seen from above, of _Balanus perforatus_, with the right-hand palpus (_d′_) and outer maxilla (_a′_) cut off, in order that the labrum (_e_), mandibles (_c_), and inner maxilla (_b_) might be better shown; the cut-off bases (_x_, _x_) of the first cirrus on each side are also shown. In fig. 2 we have the deep supra-œsophageal cavity in _Bal. improvisus_ torn open and laid flat, with the inner surfaces of the labrum (_c_) and outer maxillæ (_a_) exhibited, the palpi, mandibles, and inner maxillæ having been removed. Figs. 3 and 4 will presently be referred to; they are parts of the mouth, with the muscles, &c. removed, of Coronula.
The mouth differs extremely little in the different genera and species of the Balanidæ, much less than amongst the Lepadidæ. In the Balaninæ, the crest of the labrum is sometimes hairy, instead of having, as is usual, from two to six teeth on each side of the central notch: in _Balanus improvisus_ (Pl. 26, fig. 2) and _eburneus_, and in _Chelonobia_, the crest on each side of the central notch (_e′_) is furnished with a row of finely graduated teeth. A sub-triangular portion of the inner fold of membrane of the labrum, which overhangs the œsophagus, is always thickened and yellowish; it is also often punctured in patterns (Pl. 26, fig. 2, _f_), which, I believe, give attachment to little muscles that serve to open the upper end of the œsophagus. Opposite to this thickened, sub-triangular portion of membrane, the thin membrane forming the supra-œsophageal cavity (or the cavity surrounded by the gnathites) is strengthened by a pair of curved ribs (_h_, fig. 2) of thickened yellowish membrane, running down from the inner bases (_a′′_) of the bilobed outer maxillæ to the opening of the œsophagus (_g_): a broad branch from each of these ribs supports the sides of the orifice of the œsophagus; and this branch almost joins on to a slightly thickened rim or bar (_f′_), which branches off from the upper part of the sub-triangular (_f_) inner fold of the labrum. This structure, in _Bal. improvisus_, is represented in Pl. 26, fig. 2, as well as it could be, considering that the deep supra-œsophageal cavity has to be torn open; and then laid flat.
The _Palpi_ (Pl. 26, fig. 6) differ little, except in size, in the different genera, being squarish, more or less elongated, or even approaching to club-shaped: in most of the Balaninæ they are larger even than the mandibles, of which they normally form a part. Their upper margins, especially towards their free extremities, are always thickly clothed with spines; and there is generally a single row, either short (_r_) or long, of spines of greater length, which arise from a little above, and stand almost in a parallel line to, the basal margin. On the internal surface, there is sometimes a row (_t_) of very short little spines, which overhang the crest of the labrum. The _Mandibles_ (Pl. 26, fig. 5) have from three to five teeth; the lower point or angle is generally pectinated. In Coronula and its close allies, there are some small teeth intermediate between the four or five main teeth; and in these genera, though members of the sub-family Balaninæ, the lower teeth exhibit only rudiments of being laterally double.[33] The _Maxillæ_ sometimes have a notch under the upper large pair of spines, and in _Octomeris brunnea_ there is a double notch: in many species of Balanus, the inferior corner stands up like a step (Pl. 26, fig. 7, _a_): in many other genera and species, the whole edge is straight. In all, or almost all cases, the row of spines on the middle portion is double. The _Outer Maxillæ_ are always bilobed on their inner faces, and are clothed with bristles. On all the gnathites, the bristles are often doubly serrated.
I have described pretty accurately the muscles of the mandibles in my former volume, and there given a drawing (Pl. 10, fig. 1) of them. There are four muscles: first, the depressor muscle, which is the largest, and is attached, at its upper end, to ligamentous apodemes under the free toothed portion of the jaw; and at its lower end, spreading considerably out, is attached to a concavity close above the basal margin of the labrum; to understand the action of these muscles, it should be borne in mind that the mandible almost faces the labrum. In some genera, as in Coronula,[34] the swelling near the basal margin of the labrum (Pl. 26, fig. 3, _k_), caused by the internal concavity for the above muscle, is conspicuous. The depressor muscle is opposed by a small elevator, attached to the mandible close by the depressor; thence it runs upwards, and is united at its upper end to the base of the palpus, at the point where the latter adheres to the labrum: I have ventured to call this muscle the elevator, from being apparently so well fitted for this purpose; but I feel some little doubt, from having observed an apparent slight movement in the palpi of living Balani; and this is the only muscle entering those organs.
The free part of the mandible is articulated on a square, thickened piece of membrane, forming part of the side of the mouth (Pl. 26, figs. 3, 4, _c^1_; and Pl. 10, fig. 1, _a_, _b_, in my volume on the Lepadidæ); to this square piece of membrane, two short muscles are attached, one above the other, and which ought, in the Plate in my former volume, to have been represented crossing the depressor muscle at nearly right angles; at their further ends they are attached to about the middle of the labrum, where, at least in Coronula (Pl. 26, fig. 3, _i_), a slight concavity can be detected. The action of these two muscles must be to draw the whole mandible against the labrum; and the depressor muscle might, at the same time, draw the toothed edge downwards, and thus force any prey into the œsophagus. [34] This is figured by Burmeister in his 'Beiträge zur Naturgeschichte der Rankenfüsser,' Tab. 2, fig. 6.
The inner maxillæ are likewise furnished with four muscles, very nearly as figured in my former volume (Pl. 10, fig. 10); namely, two muscles, one inside and the other outside the curious apodeme, which in the Balanidæ (Pl. 26, fig. 7, _b′_) is as invariably present as in the Lepadidæ: these two muscles are attached at their lower ends to the outer membrane of the mouth, close to its basal articulation: the outer one of these two muscles would, I presume, act as an elevator, and the inner one as a depressor; the free part of the organ working on the top of the apodeme, like an axe, on a hinge, on the top of a pole. But there is also a larger depressor muscle, in an analogous position with that (_i. e._ the first-mentioned muscle) of the mandibles; and a fourth muscle, crossing the latter depressor at nearly right angles, and attached (as far as I could make out) on the side of the orifice of the œsophagus, close under the mandibles: the action of this latter muscle would be to draw the whole organ towards the labrum.
I must not conclude my description of the mouth, without drawing particular attention to its peculiar compounded nature. It is prominent, and is capable, as a whole, of movement; it is separated from the body by a fold or articulation, which can be traced all round. It is, as we have seen, composed of a broad labrum and three pairs of gnathites, which latter have only their terminal segments free; and these surround a conical hollow, at the bottom of which lies the opening of the œsophagus. The prominence of the whole mouth appears to result from the lateral fusion of the two basal segments of the three pairs of gnathites. I have examined the mouth of ordinary Crustaceans, and can see no trace of a structure like this. That there has been some union of the parts is indisputable; for the palpi, which in ordinary Crustaceans are quite free, are here firmly united to the upper and outer corners of the labrum; and indeed, at first appear to be more intimately connected with the labrum than with the mandibles.
The palpus on its upper and exterior surface, is in direct continuity with the square thickened piece of membrane, on which the mandible is articulated, and likewise with that side of the upper or free portion of the mandible which faces the labrum. This face of the mandible, beneath the toothed edge, is hollowed out or arched (Pl. 26, fig. 5, _p_), owing to the above-mentioned continuity of its membrane with that of the palpus. On the lower surface, the palpus is firmly united to the lateral corners of the labrum; or indeed the corners of the labrum may be almost said to be formed by the soft, swollen bases of the palpi: the point of union, when viewed from the outside, is seen to form a knob on the shoulder of the labrum, beneath the level of its crest, and at this knob (Pl. 26, fig. 3, close to _d′_) several thickened bands in the surrounding membrane unite. The free portion of the palpus stands out transversely behind (_i. e._ anteriorly to, in a homological sense) the labrum. I suspect that the palpus possibly may consist of two segments, of which the terminal one is free, and the lower one confluent with the labrum.
Before proceeding any further, I should observe that figs. 3 and 4, in Pl. 26, represent the membranes of the mouth of _Coronula diadema_, perfectly cleaned. In fig. 3, all the front part of the mouth has been removed, the mandible on one side, the labrum with the two palpi, and the œsophagus being alone left, and these are viewed from the inner side; the front part, however, of the supra-œsophageal cavity has been cut away. In fig. 4, the labrum, with the œsophagus, has been removed, whilst the two outer maxillæ, the right-hand inner maxilla and mandible (with the exterior and basal portions, _d_, _d′′_, of one palpus) are seen from the outside; but in order that these parts should all be shown, the whole of the right-hand side of the mouth has been spread out, for the teeth of the mandible should have stood in a vertical line between the two outer maxillæ. In the mandibles, the free upper part is separated, by a distinct articulation, from the square piece of thickened membrane (fig. 3, _c^1_) on which it is supported; and this latter is separated by a second articulation from a portion of thickened membrane (_c^2_), the basal edge of which forms the third and lowest articulation, separating the mouth from the body.
This basal, thickened portion of membrane curls round and inwards, towards the outer maxillæ or front of the mouth, and its terminal points sometimes even penetrate a little way within the muscles, like apodemes: it is not distinctly separated by any line or suture from the membrane, which forms the whole broad labrum; so that I at first concluded that the labrum dipped under the mandibles, and thus afforded a support on which they were articulated; but this appears so opposed to all analogy, that it is more probable that the above basal thickened portion of membrane is truly the basal segment of the mandibles, completely confluent with the labrum; and it is, I think, not very improbable that even a large portion of what in appearance belongs to the labrum, namely, those concavities to which the muscles of the mandibles are attached, may, also, be part of the basal segment of the mandibles. Whether or no there really are two segments beneath the upper free portion of the mandibles, which have become laterally confluent with other parts, I must think that the square thickened piece of membrane (_c^1_) represents at least one segment. I may here observe, that Prof. Milne Edwards seems to consider the mandible of the higher Crustaceans as answering homologically to the haunch of the leg; but, according to M. Brullé,[35] there ought to be two basal segments (sous-maxillaire and maxillaire) bearing the proper mandible, and giving rise, on the outer side, to the palpus,--a structure which perfectly corresponds with my view of the mandible and palpus in Cirripedes.
_Outer Maxillæ_: the upper free segment has a spinose lobe (_a′′_, Pl. 26, figs. 2 and 4), on its inner face, which may indicate a lower and second, almost free segment. Passing over this, we have, on the outside of the mouth, beneath the free, upper segments, an expanse of membrane, which, on the side, close to the inner maxillæ, is perforated (Pl. 26, fig. 4, _n_) by orifices which I believe are olfactory. In some species, as in _Bal. eburneus_ and _improvisus_, there is a longitudinal medial suture in this expanse of membrane, which I suppose indicates the lateral confluence of the middle segments of the two outer maxillæ. A short, transverse articulation or fold separates this middle segment (fig. 4, _a^1_) of each maxilla from the third or basal segment; and this latter (_a^2_) is separated from the body by a very distinct fold, which (at least amongst the Lepadidæ) sends inwards a short, medial, tongue-formed apodeme. Here, then, we apparently have, as in the mandibles, two segments under the upper free segment of each outer maxilla, laterally confluent with the adjoining organs. But I must state that, in old specimens, and only in old specimens of _Coronula diadema_, I have found under the outer maxilla an additional transverse ridge and fold, which plainly shows how easily a mere thickening of the membrane might be mistaken for an articulation. I can, however, hardly persuade myself that the articulated membrane, under the free part of the mandibles, which has now been figured and described, has no homological signification; and the fusion of the palpus and labrum seems too plain to be mistaken. Hence I must conclude that the mouth, in the Cirripedia, does truly exhibit a compounded structure of a very peculiar nature.
_Cirri._ There are always six pairs; each biramous and multiarticulated, supported on a pedicel formed of two segments. A shield-like swelling at the exterior bases of these pedicels often appears like another segment; but such, I believe, is not its nature. The five posterior pairs answer to the five pairs of ambulatory legs in the higher Crustaceans; and as in the case of the latter, the three, or the four hindermost pairs almost invariably resemble each other. The first pair, which is homologous with the outer maxillipeds of ordinary Crustaceans, is separated by an interval from the second pair;--though this is not the case with the legs of the pupa, from which the cirri are metamorphosed. These anterior cirri are attached to the lateral edges of the mouth, namely to the thickened rim of membrane, forming the supposed basal segment of the mandibles. They are capable of more diversified movements than the other cirri: the anterior ramus is always elongated, with the terminal segments more or less tapering, and is directed beyond (or anteriorly to) the mouth: the shorter ramus closely resembles in structure the rami of the second pair.
In the Chthamalinæ the second pair, and in the Balaninæ the second and third pairs (as will be more particularly described under these sub-families) differ in structure from the posterior pairs, from which they are separated by a slight interval. The number of segments on the posterior cirri is often great, amounting in Chelonobia even to fifty. Each segment normally is furnished on its inner face, which is usually somewhat protuberant, with from two to rarely eight or ten pairs of long spines or bristles, placed in a double row; the two spines in the lower pairs stand nearer to each other, and are shorter than the spines in the upper pairs. Between each pair of spines there is either a single, very thin bristle, or often a tuft of such. The pairs are directed somewhat upwards, and they diverge when the cirri are uncurled; their function is obviously to entangle the prey. On the dorsal or exterior surface of each segment, close to its upper margin, there is a tuft of spines, often composed of thicker and thinner spines; these, I believe, serve to prevent any creature intruding within the sack.
On both sides of the upper margin of each segment, there is generally a row of short, blunt, excessively minute spines, which only deserve notice, inasmuch as it is by their increase in number and size, and by the spreading out of the dorsal tufts, and, lastly, by the increase of the little tuft intermediate between the pairs of spines situated in front, that the segments on the two or three anterior pairs of cirri become covered, like brushes, with bristles. The bristles or spines on the second and third cirri are often, especially in Tetraclita, doubly and coarsely pectinated. The bristles on the pedicels follow the same arrangement as on the rami; namely, being in regular pairs on the posterior cirri, and crowded thickly, like a brush, on the anterior cirri. The segments in the shorter ramus of the first cirrus, and in both rami of the second, and often of the third cirrus, are broader than the segments of the posterior cirri; they are, also, especially in the genus Balanus, frequently produced in their upper, ante-lateral corners, into remarkable prolongations (see Pl. 29, fig. 4, of the third cirrus of _Bal. perforatus_), clothed on their inner surfaces, and at their extremities, by numerous bristles. The number of the segments in each cirrus is in some degree variable, and increases with age; this is likewise the case, to a certain extent, with the number of the spines borne on each segment.
As compared with ordinary Crustaceans, I presume the two rami answer to the "_tige_" and "_palpe_" of Milne Edwards; and the pedicel (as I have called it) to the two basal segments of the leg.[36] The "_fouet_" or flabellum does not appear to be developed in any Cirripede; for though the filamentary appendages in certain genera of Lepadidæ, might at first be thought to be of this nature, yet their usual position _beneath_ the basal articulation of the first pair of cirri, and the occasional presence of more than one, proves, I think, that such is not the case. [36] According to this author's new nomenclature, the pedicel would consist of the coxopodite and basipodite; the tige would be the ischiopodite and following segments; and the palpe would be the exopodite; the epipodite or flabellum being absent. ('Annales des Sciences Naturelles,' tom. xviii, 1852.)
Though the structure of the cirri is very uniform, yet we meet with some peculiarities. In Chelonobia, the segments of the posterior cirri bear only two pairs of main spines; whereas in some varieties of _Balanus balanoides_, they carry as many as ten pairs in a longitudinal row; but in this latter species, the number of these spines varies, in a singular manner, from six to ten pairs. In Tubicinella, the pairs of spines on the segments of the posterior cirri are arranged so closely one under the other, that they appear almost like a single transverse row. Considering the whole family, the third pair of cirri differs most in structure in the different genera. Thus, in _Chthamalus antennatus_, the anterior (or outer) ramus (Pl. 29, fig. 3) is thicker and much longer than the posterior (or inner) ramus; the number of the segments in one instance being, in the two rami, 53 and 18; in the longer ramus, the spines are arranged abnormally, tending to form a little circle round each segment; and the whole ramus may be said to be antenniformed, and I believe acts as an organ of touch: the relative number of the segments, I may add, in the two rami and the arrangement of their spines varies greatly in this species.
In two other species of the same genus Chthamalus, we have _occasionally_ the anterior ramus in some degree antenniformed, so that this whole structure is variable. In the allied _Chamæsipho columna_, it is the posterior or inner ramus which is antenniformed, but this peculiar development is more plainly marked in the case of the second pair of cirri than in that of the third pair. In _Tetraclita porosa_ it is, also, the posterior ramus of the third pair which is antenniformed; in this third pair, and indeed in the other cirri, the relative numbers of the segments vary extremely. A similar structure in the third pair, but in a lesser and variable degree, may be observed in some of the other species of Tetraclita. In _Balanus vestitus_, also, we have, in the third pair, an analogous structure. It is scarcely possible to believe that the circumstance of the second pair of legs, which answer to the third pair of cirri, being antenniformed in certain decapod Crustaceans, is an accidental coincidence; it must be owing to some special affinity in the two groups. In Chelonobia, the third pair of cirri is of unusual length compared with the second pair, but does not otherwise differ from the type of its sub-family: in Coronula and its allies, on the other hand, the third pair is very short and broad, as may be seen (Pl. 29, fig. 5) in Xenobalanus: in this latter genus, the front surfaces of the segments of the pedicels (fig. 6) of the posterior cirri, are extremely protuberant, almost as in _Scalpellum vulgare_.
The last peculiarity in the cirri at all worth mentioning, is in the sub-genus Acasta, in which, differently from in all other known Cirripedes, the anterior ramus of the fourth pair does not absolutely resemble the rami of the fifth and sixth pairs; in most of the species, the spines on this anterior ramus are more crowded together, are larger, and are mingled with some short thick points; and the spines in the dorsal tufts are also longer than in the two posterior pairs of cirri; but in _A. sulcata_ (Pl. 29, fig. 2), and in a lesser degree in _A. cyathus_ and _A. purpurata_, the front margins of the lower segments of this anterior ramus, and of the upper segment of the pedicel, are developed into strong, downwardly curved teeth: it is very remarkable that so beautiful a structure should be extremely variable, as it certainly is in _Acasta sulcata_. _Caudal Appendages._ With extremely few exceptions, these are present in all the Lepadidæ and Verrucidæ; whereas amongst the Balanidæ they occur only in the two species of Pachylasma, and in one species of Catophragmus; these being the genera most closely allied to the Lepadidæ, and where, consequently, their presence might have been anticipated. These appendages are seated close together over the anus; they are multiarticulate, each segment being sub-cylindrical, with a few small bristles round its upper edge.
Ideas discussed in this chapter
Text from Project Gutenberg, public domain in the United States.