A Monograph on the Sub-class Cirripedia · Charles Darwin
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_Var._ (8).--_Hab._ Unknown, Mus. Cuming.--I have seen only a single specimen of this, and refer to it on account of _var._ 11. The shell is rather steeply conical, with distant and prominent ribs; the radii are narrow; the walls are not permeated by pores; the colour is pale purple. Altogether its external appearance is very different from that of the foregoing varieties; but the scuta are identical with those of _var._ 1, excepting that the rostral tooth is rather larger, being nearly as large as in the last, _var._ 7. The tergum precisely resembles that in some specimens of _var._ 2. Hence this variety differs from the first two varieties only in the shade of colour, the external shape, and the greater prominence of the external radiating ribs of its shell. All these characters are variable in the several foregoing varieties, and they have been found, as yet, insufficient to discriminate species in any genus of sessile Cirripedes. _Varieties With the Scuta and Terga Calcified Together._
_Var._ (9), Pl. 14, fig. 6 _s_ [_C. spinulosa_, of Leach (!)]--_Hab._ Unknown.--The shell is undistinguishable by a single character from many specimens of the first, third, and fourth varieties; it is not permeated by pores. The scutum and tergum, with the exception of the one striking difference of their being calcified together without any trace of a suture, are identical with those of _var._ 3, as may be seen by comparing the figures 6 _h_ and 6 _s_. Hence to separate this form specifically from _var._ 3, we should have to rely solely on the calcification or union of the scuta and terga; but we have seen this is a point which is variable in _Elminius Kingii_, _Pyrgoma milleporæ_, and in some species of Balanus. The serial affinities, moreover, in Pyrgoma, clearly show that this is a character of no great importance. I must add that in several specimens of several of the varieties, the scuta and terga were so closely joined, that until careful examination, I was unable to detect the suture separating them; such being the case it must be quite unimportant for any functional purpose, whether or not the valves are calcified together. I feel, consequently, hardly any doubt that I have acted right in treating the present form as a mere variety.
_Var._ (10) _an. spec._? fig. 6 _t_.--_Hab._ Unknown.--This variety bears nearly the same relation to _var._ 6, as the last variety did to _var._ 3. The shell is rather stronger than in _var._ 6, with the internal ribs not so prominent; and except in being tinted pale dull purple, it differs in no respect from the shell of _var._ 1. If we imagine the scutum and tergum in the third sub-variety of _var._ 6 (6 _p_, 6 _q_), in which the adductor ridge descends far beneath the true basal margin of the valve, to be calcified together, without any suture, we shall produce almost the identical valves of the present variety. The scutum, however, here is not quite so much elongated transversely, and the occludent margin is spinose and is furnished with large teeth; these two characters give the valve a somewhat different aspect, and hence I am more doubtful than in the foregoing case, whether this form may not be specifically distinct. I must, however, state that in _Tetraclita porosa_, I ascertained that the teeth on the occludent margin of the scuta were even more variable than here is supposed to be the case; and as for the shape of the valve we have seen what wonderful variation there is in _var._ 6. The tergum in this variety is about intermediate between the two common forms, in the sub-varieties of _var._ 6. As for the calcification of the two valves together, we have seen, under the last variety, how little important a character it is.
_Var._ (11) fig. 6 U, 6 _u_ [_Creusia grandis_, of Chenu, Tab. 1, fig. 2, but not fig. 2 _a_ and _b_].--_Hab._ Singapore, associated with _Pyrgoma monticulariæ_.--This variety is very closely related to the last. The shell, however, has a very peculiar aspect, which made me for some time think it must be specifically distinct. It is of a much brighter pink than in any of the foregoing varieties; the surface is marked with very prominent, distant ribs, and the radii are narrow, in which latter points, together with the tint (though here brighter), this variety cannot be distinguished from _var._ 7. The shell, however, is permeated by several rows of pores, in which respect it resembles the shell in _var._ 2, and some specimens of _vars._ 3 and 4. In the opercular valves there is a close general resemblance with those of the last _var._ 10; the tooth, however, near the rostral angle, is not so prominent; and in the tergum, the spur is more truncated, shorter and broader than in _var._ 10, and closely resembles that in _var._ 1 and 2. But I cannot consider any of the points here specified of much weight. The foregoing descriptions show how singularly the affinities of the several varieties interlock in the most complicated manner. Hereafter some one may, perhaps, succeed in grouping several of these forms as species; but I am sure he ought not to attempt it without possessing a very large suite of specimens, or without the great advantage of comparing some two or three of the forms, fresh in their native site.
_Species dubiæ._--Under Pyrgoma, I have stated that though Chenu, in his 'Illust. Conch.,' has given beautiful external figures of the shells, imbedded in the coral, yet from the want of details on the opercular valves and on the structure of the shell, I cannot recognise his species. So it, likewise, is with Creusia. Chenu gives the following new species; _C. radiata_, _multistriata_, _decorata_, and _striata_. The _C. madreporarum_, I suspect to be the _Pyrgoma milleporæ_ of this work, as there stated. The _C. grandis_ no doubt is the _P. grande_ of this work, the _Nobia grandis_ of Sowerby. The name _Creusia Childreni_ is given by Dr. Gray, without description or figure, in the 'Annals of Philosophy,' vol. 10, new series, 1825. 7. _Genus_--CHELONOBIA. Pl. 14: Pl. 15, fig. 1. CHELONOBIA. _Leach._ Journal de Physique, tom. 85, (1817). CORONULA. _Lamarck._ Animaux sans Vertèbres, 1818. ---- _Ranzani._ Memoire di Storia Naturale, 1820. ---- _De Blainville._ Dict. des Sciences Naturelles. ASTROLEPAS. _J. E. Gray._ (Klein) Annals of Philosophy, (new series), vol. 10, (1825). _Compartments extremely thick, six; but one of them, the rostrum, internally is composed of three rudimentary compartments, united together: basis membranous: scuta narrow, united to the terga by a horny articular ridge._ _Distribution_, throughout the tropical, and warmer temperate seas of the whole world; attached to turtles, crustacea, or smooth gasteropod molluscs.
This is a distinct and well-defined genus. Several authors have confounded it with Coronula, but this has been owing to an entire misapprehension of the structure of the shell in the two genera. In Coronula, the parietes are very thin, and are so deeply folded as to appear like rays or septa connecting the outside of the shell (the expanded ends of the folds) and the internal surface of the shell, but the open spaces between the folds are occupied by the epidermis of the Whale, and are external to the cirripede. In Chelonobia, the parietes are remarkably thick; hence the plates or septa, connecting the outer and inner lamina, are of unusual length; and the spaces between them, though of course internal with regard to the cirripede and occupied by the ovaria, have been compared to the spaces external to and between the folded walls of Coronula. There is but little special affinity between these genera; and I regret that they have come to be placed one after the other in this work: but the elongated opercular valves,--the thick and double opercular membrane,--the weak depressor muscles,--and the peculiar manner in which the scutum is articulated by the aid of a horny projection to the tergum, may indicate some real but slight affinity to Coronula; the many points of difference, however, in the structure of the shell and of the opercular valves, and especially in the cementing apparatus of the basal membrane, and in the branchiæ, all prove that the genera are very distinct. The singular structure of the rostrum, which, in fact, consists of three compartments externally blended together, and which three correspond in all essential respects to the rostrum and two rostro-lateral compartments in the Chthamalinæ, offers a very striking point of identity with that sub-family; but neither in the mouth, cirri, or other part, can I detect any other evidence of this relationship. Having so far discussed the affinities of the genus, I may add, that the three species, though decidedly distinct, are closely and nearly equally related to each other.
_General Appearance._--The shell is generally depressed, and broadly oval or almost circular; in _C. testudinaria_ and _caretta_, it has a massive appearance: the surface is generally smooth, or, when disintegrated, finely striated: the colour is white. The six compartments do not differ much in size: the rostrum is rather larger than the carina, and the lateral compartments, than the carino-lateral compartments. It is remarkable that in _C. caretta_ (Pl. 14, fig. 2), even in specimens which have not grown crowded together, the compartments are almost invariably placed rather unsymmetrically, the rostrum and carina not exactly facing each other.[121] The shell, though so thick and massive, yields easily along the lines of suture. The radii are moderately wide, or narrow, or not at all developed, being represented by mere sutures: in this latter case, in _C. caretta_, the orifice of the shell is enlarged, in the same manner as we have seen in some species of Balanus and Tetraclita, by the gradual wearing away of the upper part of the shell. In _C. testudinaria_, the radii have a singular notched structure (fig. 1 _a_), and the whole shell a star-like appearance. The orifice is not filled up by the elongated opercular valves,--a considerable extent of opercular membrane being visible on the two sides. The largest specimen which I have seen, namely, of _C. testudinaria_, was nearly two and a half inches in its longer diameter.
_Sheath._--The layer of shell surrounding the internal cavity (_e c e_, fig. 4, Pl. 14), and extending down to the basal membrane, I have called the sheath, owing to its being distinctly continuous with the innermost layer in the upper part of the shell, to which the opercular membrane is attached: this can be best seen by examining the alæ in the separated compartments of _C. patula_. The sheath is not only remarkable from thus descending to the basal membrane, but in _C. testudinaria_ and _patula_ from its lower edge being perforated by arched channels (under _c_, in fig. 4), allowing thick ribbons of corium to reach the interspaces between the radiating septa. There is one central arch or channel in the middle of the lower part of the sheath of each compartment, and one on each line of suture, the sheath being a little hollowed out on both sides of the sutures. As the rostrum, as far as its internal structure is concerned, consists of three compartments, we have altogether in the shell eight compartments and eight sutures, and consequently altogether sixteen arches through the lower part of the sheath, allowing sixteen thick ribbons of corium to penetrate the parietes, and thus likewise reach the radii. There is, however, sometimes a little variation in the number of these arched channels. The upper part of the sheath is transversely marked by zones of growth, to the lower one of which the opercular membrane is attached. The line of attachment is not low down the sheath.
_Radii and Alæ._--The radii, when the compartments are disarticulated, present a remarkable structure, from appearing to consist of a distinct inner and outer portion. The radius normally consists of an inner and outer lamina, united by septa parallel to the basis; but here the inner portion is formed by a central ridge (_a a_, fig. 5, Pl. 14), sending off on both sides little septa, often sub-branched; it is of nearly uniform width; and there is no distinct inner lamina. The outer portion (_b_), which often equals or exceeds in thickness the inner portion, is, in fact, the normal outer lamina, developed to an unparalleled degree. In most Cirripedes the edge of the radius is received in a slight furrow in the opposed compartment, the lid of which furrow is narrow, and matches the outer lamina of the radius; here the lid of the recipient furrow is very broad, and resembles the outer lamina of the radius in all its characters. In order to allow of growth between the _thick_ opposed edges of the outer lamina of the radius and the lid of the recipient furrow, the two surfaces are finely dentated (look in fig. 5, under the pits, marked _b_), almost like the crown of a molar tooth; thus allowing films of corium to enter.
The structure here described is common, in a greater or less degree, to all three species, but is best seen in _C. testudinaria_. In this species, moreover, (fig. 5, _b_), the outer lamina, instead of being smooth and of either equal or gradually increasing thickness from top to bottom, is generally, but not always, (fig. 1 _a_), deeply pitted or notched in transverse lines, the outer lamina being thus rendered alternately thicker and thinner, and so formed into transverse ridges and valleys. Hence the lines of suture become toothed, the points of the teeth facing each other, and not interlocking. In the transverse section, fig. 1, Pl. 15, of the same species, taken high up across the shell, (_f_) is the pitted outer lamina, and (_e_) the inner portion of the radius. Although the radii are thus specially added to in thickness, they are not so thick as the very thick walls, and hence the lines of suture form furrows more or less deep. The _alæ_ are of moderate thickness, and have their sutural edges crenated by fine transverse septa.
_Rostrum._--I have already alluded to the peculiar compounded structure of this compartment, unlike anything we have as yet seen.[122] The thin outer lamina is quite continuous, and shows no trace of the triple nature of the compartment; as may be seen by comparing the drawing (Pl. 14, fig. 1 _a_) of the shell of _C. testudinaria_, with the transverse section (Pl. 15, fig. 1) of the same species: in this latter figure, _a_ is the outer lamina, and B A B the three compartments of the rostrum. But when the outer lamina is worn away, as is always the case with the upper part of the walls in _C. caretta_ (Pl. 14, fig. 2), the two fissures separating the three compartments of the compounded rostrum, are plainly exhibited on the outside of the upper part of the shell. On the internal surface, the sutures separating the three compartments are always open, except in the upper part of the sheath, above the attachment of the opercular membrane, where they are partially obliterated by a thin continuous layer of shell.
That these three portions of the rostrum are in their essential nature compartments, is well shown in _C. patula_ and _testudinaria_, by the sheath or inner lamina having loopholes or channels (such as before described) in the middle of each, and on each line of suture. From the number of these channels, seven altogether, (the two between the compound rostrum and lateral compartments being counted,) the sheath of the compound rostrum is reduced to mere flattened pillars between the several channels. By slight violence, the rostrum breaks into the three portions; and the sutures between them are found to be marked on both sides by sinuous, slight, calcareous ridges, those on opposite sides locking into each; these represent the septa on the edges of the radii and alæ and their recipient furrows, in the ordinary compartments. The outline of the middle compartment of the three (best seen in section Pl. 15, fig. 1, A), much resembles that of the carina; in fact, if we suppose the growth of the two alæ of the carina to have been arrested, no essential difference can be pointed out: in this rudimentary compartment, therefore, we have a rostrum, characterised as in the sub-family of the Chthamalinæ. In the two little rostro-lateral compartments, moreover, (B B), a slight swelling on the side opposite to the large existing radius, shows that if the development of these compartments had not been prevented, each, probably, would have had, exactly as in the Chthamalinæ, a radius on both sides. In the introduction to the Balanidæ, I have argued, from the structure here described and from some other facts, that in the Balaninæ the rostrum is composed of the three anterior compartments, which we see in the Chthamalinæ, indissolubly united together; hence in Chelonobia the middle one of the three partially-blended compartments is properly the rostrum, and those on the sides the rostro-lateral compartments.
Beneath the basal and generally slightly sinuous exterior margin of the valve, that is, the margin to which the opercular membrane is attached, a slight ledge depends (fig. 1 _d_), which narrows off towards the rostral end of the valve. I should have thought that this had been a depending adductor ridge, as in several species of Pyrgoma, had there not been a nearly similar ledge beneath the middle part of the basal margin of the tergum. The _Tergum_ is mitre-formed, with the summit more or less truncated, and with the carinal margin generally more sloping than the scutal margin: near the carinal margin there is a slight furrow (fig. 1 _d_), sometimes difficult to be distinguished, with the lines of growth curving down to it on each side, and consequently with a very slight, but variable, corresponding projection on the basal margin. This furrow and slight projection, there can be hardly any doubt, represent the spur, though here placed close to the carinal instead of to the scutal margin, as in other genera. The tergum has a small articular ridge, against which the overlapping horny articular ridge of the scutum abuts. There are no crests for the tergal depressor muscles. Altogether the scuta and terga are very peculiar. A portion dissolved in acid exhibits no tubuli. On the opercular membrane there are no hairs. The rostral depressor muscles of the scuta are singularly feeble, each consisting of only one or two, or sometimes three or four, very thin ribbon-like fasciæ; the lateral depressores of the scutum are twice as strong as the rostral depressores; and the tergal depressores a little stronger than the lateral depressores. All these muscles are plainly marked with transverse striæ.
_Mouth._--The labrum is not in the least bullate, which character, as well as some others that follow, I specify on account of the apparent affinity of Chelonobia to the Chthamalinæ, as indicated by its rostrum. The crest of the labrum is notched, and on each side of the notch there is a long row of teeth, which, however, are sometimes worn away. The mandibles have five main teeth; all excepting the first being laterally double. The maxillæ are not notched. The outer maxillæ are bilobed on their inner sides. _Cirri._--The four posterior pairs, in proportion to the size of the animal, are much elongated, and are remarkable from the number of their short segments. The rami of the first cirrus are a little unequal in length. The second cirrus is moderately short, with its segments rather broad and protuberant, and thickly clothed with spines. The third cirrus is of unusual length, being but little shorter than the fourth pair; its segments, however, are broad, and are thickly clothed with spines, as are the two segments of its pedicel: hence there is no real approach to that important character of the Chthamalinæ, namely, the similarity of the third with the three posterior pairs of cirri. The numerous segments of the fourth, fifth, and sixth pairs of cirri each support only two pairs of main spines; between each of these pairs there is a little tuft of fine intermediate spines; the upper of the two tufts on each segment is the longest. In a specimen of _C. patula_, in which there were fifteen segments in one ramus of the second cirrus, there were fifty segments in either ramus of the sixth cirrus. At the exterior bases of the pedicels of some of the anterior cirri, there are large tufts of finely plumose, delicate hairs.
_Branchiæ._--These are of large size: they consist of a single fold, much plicated and sub-plicated. _Ovaria._--The ovarian tubes run into the parietes, and fill up the interspaces between the radiating septa. _Range, &c._--The three species seem to range together, over the tropical and warmer temperate seas of the whole world. _C. patula_ and _testudinaria_ are found in the Mediterranean, and the former at Charleston, in the United States; I have not heard of specimens from any point further north. _C. testudinaria_ and _caretta_ live attached to turtles; whilst _C. patula_ always adheres to crustacea, to large and smooth gasteropod mollusca, and, I believe, sometimes to ships' bottoms. I have not heard of the discovery of any fossil species. _Attachment._--_Chelonobia patula_ leaves no impression on the crabs and shells to which it is attached. I have seen only a few specimens of _C. testudinaria_ attached, and the carapaces of the turtle were not at all, or scarcely at all, indented by them. The case is very different with _C. caretta_, in which the shell, even of young specimens, is always, as far as I have seen, imbedded to some depth, and occasionally to a very great depth in the tortoise-shell. From the extreme hardness of the tortoise-shell, when dry, the imbedment appears more wonderful even than it really is. The younger shells have the appearance of having grown from within the carapace, and then of having burst through it, almost like little volcanos. I have seen only one very young shell (1/10th of an inch in external diameter) attached, and here there was nothing to countenance an idea which at one time occurred to me, namely, that the larva perhaps fixed itself in some little crack or cavity in the carapace, and there underwent its metamorphosis.
I believe that the imbedment is effected simply by the sharp, growing, basal edges of the walls of the shell indenting the surface, and finally rupturing the outer laminæ of the tortoise-shell, through that same force by which the tender radicle of a plant penetrates very hard ground. As soon as the surface was once ruptured, the shell of the Chelonobia, growing outwards and downwards, would easily, like a wedge, turn up the laminæ of the tortoise-shell; and their ragged ends would surround the Chelonobia, as is seen actually to be the case. In the genus Coronula and its allies, which are attached to Cetaceans, we shall presently see, that the epidermis immediately under the downward growing shell, and apparently in consequence of the pressure thus exerted, is thinner than in the surrounding parts. In two specimens of _Chelonobia caretta_, imbedded much more deeply than usual--in one of which half the basal edge of the shell had fairly cut through the carapace, and in the other was on the point of effecting this--the tortoise-shell manifestly thinned out towards the line of yielding; now this, I suppose, must be attributed either to absorption, or to the living tortoise-shell being actually stretched till rendered transparent and ready to burst or until bursted.
On the latter view of the tortoise-shell having been stretched, we must further suppose that the pressure has prevented fresh layers of tortoise-shell being deposited under the old and yielding laminæ. In one of the above two specimens, the walls of the Chelonobia were deeply folded, nevertheless the laminæ of the tortoise-shell followed every curvature, showing that, though now so rigid, during the slow imbedment of the Cirripede it must have been sufficiently pliant. A shell attached, as these two specimens were, could never be removed, and, whether dead or alive, would remain for ever imbedded in the tortoise-shell. Dr. R. Ball, of Dublin, informs me, that he has seen specimens in which the shell of the cirripede not only had penetrated the carapace, but likewise the underlying bone, and had even entered some way into the body of the turtle: it is well known that the tusk of a boar or the horn of a ruminant, when curving in abnormally, will sometimes penetrate deeply into the bones of the face or head; and this, I believe, is effected, not by the fracture of the bone, but by the absorption of the point pressed on: I conceive a similar process must have taken place in the curious specimens examined by Dr. Ball. 1. CHELONOBIA TESTUDINARIA. Pl. 14, fig. 1 _a_-1 _d_, fig. 5; Pl. 15, fig. 1.
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