A Monograph on the Sub-class Cirripedia · Charles Darwin
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_Terga_ (fig. 4 _b_): these are of so irregular a shape that they can hardly be described; they may, however, be said to consist of two portions joined together, of which the lower portion is a little elongated transversely, of somewhat variable shape, with part of its surface considerably depressed (compared with the rest of the valve), sometimes being even concave: this concave portion apparently answers to the spur in other cirripedes. At the scutal corner of the valve there is a shoulder (perhaps answering to the inflected scutal margin in an ordinary tergum), which locks into a hollow on the under side of the occludent ledge of the scutum. The internal surface of the so-called spur is rounded and convex. The upper part of the tergum is in main part formed by a great occludent ledge; but this, on its lower side, is bordered by a narrow irregular slip, which, as shown by the lines of growth, represents the whole of the ordinary valve, excepting, of course, the spur already described. The occludent ledges of both valves support some fine spines.
_Affinities._--Observing how extraordinarily the terga varied in _P. dentatum_, and that the shells were identical in that and the present species, it occurred to me at first that they might, perhaps, be both extreme varieties of one form: but in the scutum of _P. crenatum_, the invariably great development of the adductor plate,--the marked manner in which its basal margin is reflexed,--the absence of a tooth-like articular projection,--and again in the tergum of _P. crenatum_, the invariably large size of the concave spur, without any internal tooth, altogether convince me that the two species must be considered as distinct. This species is allied to _P. grande_, in the scutum of that species having an occludent ledge, though small, and a great adductor plate. I have only further to remark, that the figure of the opercular valves, given in Sowerby's Genera of Recent and Fossil Shells, is so good, that there can not be the least doubt about the present identification. 9. PYRGOMA MONTICULARIÆ. Pl. 13, fig. 5 _a_-5 _f_. PYRGOMA (DARACIA) MONTICULARIÆ. _J. E. Gray_ (!). Zoological Miscellany, p. 6, 1831. _Shell of an irregular shape, with a roughened exterior border: orifice minute, circular: scutum and tergum both much elongated, calcified together without any suture, both furnished with a broad occludent ledge._ _Hab._--Singapore; Mus. Brit., Cuming, and Stutchbury. Sometimes associated with _Creusia spinulosa_.
_Appearance and Structure of Shell._--Shell dull white, very irregular in outline, sometimes rounded, more often unequally elongated, and frequently star-shaped,--the projections being quite irregular. Whole shell nearly flat, but with the central part saddle-backed, or formed into a more or less prominent ridge, extending in the line of the longitudinal axis of the animal's body: the circumferential portions of the shell not unfrequently are a little recurved upwards. Orifice extremely minute, circular. The outer lamina of shell, which is smooth, does not extend to the circumference, and consequently a rather broad, nearly equal border, which is rough, surrounds the whole shell. I have no doubt that, when the shell was alive, this border was covered by a membrane, which, in drying, has curled up and been lost, in the same manner as the strictly analogous but narrow open seam between the basal edges of the shell and the basis in some cirripedes (as in the last two species of _Pyrgoma_) is protected. The roughened border can sometimes be plainly seen to be formed of normal (Pl. 13, fig. 5 _e_) longitudinal septa having crenated edges, with shorter septa between the longer ones; but more often the septa are so irregular, and so much branched (5 _d_), that the whole resembles a mass of moss. Why the outer lamina of the shell in this one species does not nearly reach the circumference of the walls, I cannot conjecture. The extremely irregular, depressed shape of the shell, with the minute circular orifice, and the singular rough circumferential and often slightly reflexed border, together give to this species so peculiar an aspect, that until close examination I did not believe that it was a cirripede. The extreme irregularity of shape depends in great part upon the irregular growth of the Monticularia, in which it is imbedded.
Internally (fig. 5 _b_) the walls are smooth, but they are perforated by many quite irregular, small orifices, which have the appearance of having been formed by some boring animal, but really serve, as I believe, to admit threads of corium into certain irregular pores which penetrate the shell. The sheath descends but a very short distance from the orifice: it is closely attached to the walls, and might easily be overlooked. The basis is deep, of an irregular outline, like that of the shell, and is formed by a very thin shelly layer. The largest specimen which I have seen, measuring from the extreme projecting points, was .4 of an inch in diameter. The _Scuta_ and _Terga_ (fig. 5 _f_) are calcified together without any trace of a suture; together they form a bow with the two ends curled rather abruptly inwards; they are both extremely narrow, but furnished with an occludent ledge, twice or thrice as high as the proper valves themselves. This occludent ledge, which is finely hirsute, begins at about one third of the length of the scutum from the rostral angle, and runs to near the basi-carinal angle of the tergum. The scutum itself is curved, with a slip, along the true occludent margin (best seen at the rostral end), lying in a different plane from the rest of the valve, much in the same way as in the scutum of _P. milleporæ_. The adductor ridge descends a very little below the basal margin of the valve, and extends for nearly its entire length: this adductor ridge makes the proper valve even narrower than it at first appears. The _Tergum_ is extremely narrow, forming merely a border to the occludent ledge; but it is not short, being about two thirds of the length of the scutum. There is no trace of a spur; indeed, the valve is rather narrower where the spur should have stood, than it is at the basi-carinal end. The scuta and terga are calcified together by their apices.
_Affinities._--Although this species, as above stated, differs so remarkably in external appearance from the other species of the genus, and, indeed, of the whole family, yet the shell in no one essential point of structure materially differs from its congeners; and if we compare the opercular valves with those of the three last species, we shall be struck with their close, yet graduated, affinity: in _P. milleporæ_ the scuta and terga tend to become calcified together, and the rostro-occludent end of the scutum is bent into a different plane from the rest of the valve. In _P. dentatum_ we have an occludent ledge exactly as in the present species; but in that species the adductor plate is less developed than in _P. monticulariæ_; on the other hand, in _P. crenatum_, the adductor plate is more developed than in _P. monticulariæ_. If in _P. crenatum_ we were to remove the spur from the tergum (and it is much less developed in _P. dentatum_; and in _P. milleporæ_ it is entirely absent) this valve would be almost identical with that of _P. monticulariæ_. Under these circumstances I consider it impossible to separate the present species as a distinct genus. _Species Dubiæ._ The _Daracia Linnæi_ of J. E. Gray (Annals of Philosophy, new series, vol. 10, 1825), was published without description or figure.
The _Megatrema semicostata_ of G. B. Sowerby, junr. (Conch. Manual, fig. 33, 1839), is not described, and is very indifferently figured without the opercular valves, and therefore can never be recognised. There is an admirable figure of a Pyrgoma, without any specific name, in the great 'Description d'Egypte,' but from the want of some details, I cannot positively recognise the species; I am inclined to believe that it is the _P. dentatum_ of this work. M. Chenu, in his grand Illustrations Conchyliologiques, has given most beautiful figures of several species of Pyrgoma, and of Creusia, but unfortunately, from the opercular valves not having been figured, I find it impossible to recognise them. The new species are _Pyrgoma stellata_ (on which I have appended a note under _P. conjugatum_), and _P. spongiarum_ and _P. corymbosa_ of Valenciennes. 6. _Sub-Genus_--CREUSIA. Pl. 13, 14. CREUSIA. _Leach._ Journal de Physique, tom. 85, July, 1817. _Compartments four, furnished with radii; basis cup-formed: attached to corals._ _Distribution_, imbedded in corals throughout the tropical seas.
Creusia is closely allied to Pyrgoma; and had not this genus already been adopted by several authors, I should not, I think, myself have formed it; though, no doubt, it harmonises well with some of the other genera of the family, which are perhaps all too intimately related. Creusia differs from Pyrgoma only in the shell being separated by sutures, into four compartments, with well developed radii: in other respects, such as in general habit, in the cup-formed imbedded basis (not permeated by pores), in the opercular valves, in the characters derived from the mouth and cirri, there are no generic differences. This affinity is more particularly evident when Creusia is compared with the first few species of Pyrgoma: indeed, for a short time, I was inclined to consider _var._ 10 of Creusia as identical with _Pyrgoma conjugatum_. With respect to the species of Creusia, I confess I have been much perplexed in determining whether there be only one, or half a dozen.
The latter conclusion would almost certainly be arrived at if only a few specimens were examined; and it might naturally be thought that some of the species were extremely well marked; the difficulty of drawing any line between varieties and species, begins only when some hundreds of specimens, from various parts of the world, are disarticulated, cleaned, and carefully examined. Creusia, in this respect, offers a striking contrast with Pyrgoma, in which nearly all the species are strongly characterised. The shell differs very little in the several varieties or species of Creusia; and the most marked difference, namely, whether the walls are permeated by irregular pores or not, seems certainly quite variable. It is in the opercular valves, which in other genera offer by far the most reliable character, that we encounter the chief cause of perplexity; for the characters thus derived, though at first appearing very distinct, blend into each other, and are not accompanied by any well marked differences in the shell. Only a few of my specimens have any habitat; but the geographical range, as far as it does go, throws no light on the question which forms to regard as species and which as varieties.
As is generally the case with cirripedes, the variations are local, so that the greater number of specimens imbedded in the same coral resemble each other. Under these circumstances I have thought it best, after repeated examinations of a very large suite of specimens, to describe separately each variety, without attaching any name to it; but I will first make a few general remarks on the structure of the shell. If I do not thus throw much light on the subject, I shall at least not burden it with error. I believe that the species will be definitely made out only by persons resident in the coral-bearing zones. I have given copious illustrations of the opercular valves; for, if my view be correct, this genus offers a curious and striking case of variation; if, on the other hand, I am wrong, the drawings, I hope, will aid others in coming to a more correct conclusion. 1. CREUSIA SPINULOSA. Pl. 13, fig. 6 _a_-6 _h_: Pl. 14, 6 _i_-6 _u_, 6 U. CREUSIA SPINULOSA. _Leach_ (!). Encyclop. Brit. Suppl., vol. 3, Pl. 57, 1824. CREUSIA SPINULEUSE. _De Blainville._ Dict. Sc. Nat., Pl. 116, fig. 6. CREUSIA GREGARIA. _G. B. Sowerby_ (!). Genera of Recent and Fossil Shells, No. 18, Sept. 1823. ---- GRANDIS. _Chenu._ Illust. Conch. Tab. 1, fig. 2, sed non fig. 2 _a_ and _b_.
_Hab._--Philippine Archipelago, China, Singapore, Java, Red Sea, West Indies; imbedded in various corals; Mus. Brit., Cuming, Stutchbury, Dunker, &c. _General Appearance._--The shell is oval, generally flat, sometimes conical, with narrow and approximate ridges radiating from the orifice (fig. 6 _a_). The ridges, however, are sometimes distant from each other, and considerably prominent, projecting round the basal border. The orifice is either neatly diamond-shaped or oval. The four compartments are quite distinct; the radii are generally white, of considerable width, and with their summits not oblique. The colour is either white, or pale pinkish-purple; but in _var._ 11, bright pink. Even in the white specimens, when well preserved, the sheath is generally, but not always, either pale or dark purple. The largest specimen which I have seen, from the West Indies, was above half an inch in diameter; but from .3 to .4 of an inch is the more usual full size. I believe that the size, as well as the great variability of the present species, is partly determined by the rate of growth of the various zoophytes in which the specimens are imbedded, for the shell has to keep on nearly a level with the surface of the coral.
_Structure of Shell and Basis._--The walls are internally ribbed; the ribs being usually prominent, sometimes to such a degree as to deserve to be called plates. The outer lamina is of variable thickness, and the prominence of the internal ribs appears in considerable part to depend on the extent to which the outer lamina has been thickened from within. In many specimens, instead of the interspaces between these internal ribs or longitudinal septa being solidly filled up, separate and successive laminæ have been deposited, by which the shell is rendered cancellated or porose; but the pores are very irregular; and sometimes they form two or three irregular rows one behind the other: this structure seems eminently variable. The edges of the radii are formed by crenated, and occasionally branched, septa. That part of the alæ, which is added during the diametric growth of the shell, is very thin. The lower edge of the sheath seems always to be free. The shelly layer, forming the basis, which is deeply cup-formed, is thin, more or less finely furrowed in radiating lines, and not permeated by pores. The opercular valves will be best described under the following eleven varieties.
_Var._ (1), Pl. 13, fig. 6 _a_, 6 _b_, 6 _c_, 6 _d_.--_Hab._ Java, and probably several other districts.--I will first describe a typical sub-variety, by which I mean a sub-variety not presenting any extreme character. The scutum is of a sub-triangular shape, with the basi-tergal corner much rounded off (6 _d_), and generally but not always hollowed out in a rather remarkable manner. The adductor ridge is considerably prominent, and extends high up, parallel to the articular ridge, which latter is rounded and prominent, but to a variable degree. Near the rostral angle there is sometimes a small tooth, or only a trace of one, depending beneath the basal margin; this tooth we shall hereafter see much more developed. The _Tergum_ is about two thirds of the width of the scutum. It is often slightly beaked, but this is more conspicuous in some of the following varieties. The spur is about half the width of the valve, and its basal end is truncated, and nearly parallel to the basal margin of the valve, but the truncated form passes insensibly into a rounded outline. The shell in this variety is generally thick and is not permeated by pores; the orifice is diamond-shaped. But in another sub-variety the walls of the shell are always, or nearly always, permeated by pores, and the tergum is very much narrower, with the spur sharper, so that at first I concluded that these two sub-varieties were specifically distinct: we shall, however, soon see in _var._ 2 and in _var._ 4, that no confidence whatever can be placed in the exact breadth of the tergum, or in the porosity of the walls; hence I have been driven to consider the two varieties just mentioned as merely sub-varieties.
_Var._ (2), fig. 6 _e_, 6 _f_, 6 _g_.--_Hab._ China; Red Sea.--The shell is almost invariably permeated by pores, sometimes arranged in two or three very irregular rows. In some specimens the scutum exactly resembles that in _var._ 1, but with the tooth near the rostral angle often rather larger: in other specimens the scutum is much more elongated transversely (fig. 6 _e_), with the adductor ridge more medial, and the basal margin not at all hollowed out at the basi-tergal corner of the valve. The tergum, here, is the remarkable feature, being sometimes excessively narrow, with a long sharp spur, which often, but not always, terminates in a needle-like point. In other specimens, from the same coral and certainly belonging to this same variety, the valve is not so narrow (6 _g_), and the spur not so pointed; consequently (as in several analogous cases in other cirripedes) it is impossible to draw any line of distinction between the specimens with the narrow and broad terga. _Var._ (3), fig. 6 _h_ [_Creusia gregaria of G. B. Sowerby!_]--_Hab._ Unknown.--The scutum presents here exactly the same considerable range of variation as in _var._ 2. The tergum is broad, as in _var._ 1, but the spur is rounded, and from not being placed so immediately close to the basi-scutal angle of the valve, gives to it a rather different aspect. The breadth of the spur varies; an extreme variety is given in fig. 6 _h_.
_Var._ (4), Pl. 14, fig. 6 _i_, 6 _k_, 6 _l_.--_Hab._ Philippine Archipelago; West Indies.--The _scutum_ here presents the same sub-varieties as heretofore, excepting that I have not seen any so much elongated transversely. The shell is covered either with slight, closely approximate ribs, as in the foregoing varieties, or with more distant and more prominent ribs. In specimens taken out of the same branch of coral the walls were either porose or solid. Sometimes the sheath is bright purplish-pink. It is the _tergum_, again, which presents a remarkable range of difference; for the longitudinal depression or furrow which in the former varieties was quite open, here has its edges more or less folded inwards, and is sometimes quite closed. This same variation has been commonly observed in many species of Balanus, in which it appears to be dependent on the age of the individual; but this does not appear to be the case in the present genus. As a consequence of the greater or less folding in of the two sides of the furrow, the spur is rendered more or less narrow and pointed, and thus becomes removed to a greater or less distance from the basi-scutal angle of the valve. Further, as a consequence of this folding in, the internal surface of the valve along the line of the external furrow, is raised into a longitudinal ridge. The length of the spur varies considerably. In some very young individuals, the basal margin descends lower on the scutal than on the carinal side of the spur. In one set of specimens (fig. 6 _l_), a plate extended from the carinal margin to near the central longitudinal ridge just mentioned: a similar structure was described under _Pyrgoma cancellatum_.
_Var._ (5), Pl. 14, fig. 6 _m_.--_Hab._ Unknown.--We have seen in _vars._ 2 and 3 that the scutum varies considerably in shape: here it is unusually narrow, with the adductor ridge almost touching the articular ridge. There is no little tooth near the rostral angle, and the basi-scutal corner is not hollowed out. The tergum also varies; in some individuals it is truncated and like that figured of _var._ 1, but rather more rounded; in other specimens (from the same branch of coral) the basal margin so blends into the spur that the latter can hardly be discriminated (fig. 6 _m_); in other respects the outline resembles pretty closely that of one of the _sub-vars._ (fig. 6 _f_) of _var._ 2. The shell is not porose; it is thick, with strong internal ribs, and resembles that of _var._ 1; but it is of a pale purplish colour.
_Var._ (6), _an. spec._? fig. 6 _n_-6 _q_.--_Hab._ Philippine Archipelago.--This is a very remarkable variety; we have, imbedded in the same coral, and with shells absolutely identical, specimens with the scutum having three distinct but graduated forms. Firstly, a scutum transversely elongated, in all external respects like some of the varieties mentioned under _vars._ 2 and 3, with no rostral tooth, and not hollowed out at the basi-tergal corner, but with the adductor ridge more prominent. Secondly, a scutum of the same general shape, but with the adductor ridge so much developed (fig. 6 _n_) as to descend slightly beneath the basal margin, and to be seen when the valve is viewed externally; there is a very slight tooth near the rostral angle (as in some former sub-varieties), and which can be here rather more clearly seen than hitherto, to be formed by the adductor ridge (closely united to the external surface of the valve) extending thus far, and being here produced a little downwards. Thirdly (6 _p_), we have the adductor ridge immensely developed, descending far below the basal margin of the ordinary valve; and the basal margin at the basi-tergal corner is angularly and deeply hollowed out.
The appearance of the valve is widely different from that in the first sub-variety, yet it is impossible to separate the first and second sub-varieties, and it is almost equally certain that the third sub-variety is only an exaggeration of the second. The lower edge of the adductor ridge, in the third sub-variety, varies a little in outline; it is deeply sinuous, and is produced at the rostral angle into a point, of which we have heretofore seen only a feeble representation. It would appear as if the great development of the adductor plate had caused the exterior ordinary surface of the valve to shrink or be less developed. There is a striking resemblance in the structure here described with that in _Pyrgoma cancellatum_ and _conjugatum_. The terga belonging to the above scuta, also, vary; the spur being sometimes square (6 _o_), and sometimes bluntly pointed: when the spur runs in the same exact line (6 _q_) with the scutal margin of the valve, a peculiar aspect is given to it, but this is by no means always the case. Both opercular valves are often partially coloured pinkish-purple. The shell is not porose; it is thin, with remarkably prominent internal plates; it is apparently always of small size, which I attribute to this variety inhabiting a hard thin plate-like coral.
The sheath is bright pinkish-purple, of which we have had instances in some of the other varieties; and the shell itself is sometimes pinkish. Taking the scutum of the first sub-variety, together with the commonest accompanying variety of tergum, I find it quite impossible to assign to it a specific character; if, on the other hand, we consider the scutum of the third sub-variety by itself, nothing can appear more distinct; but I must repeat, there can be hardly a shadow of doubt that the three sub-varieties of scutum here described, graduate into each other, and are specifically identical. _Var._ (7), fig. 6 _r_.--_Hab._ Probably Philippine Archipelago, associated with _Balanus quadrivittatus_.--There can be hardly any question of this being specifically identical with the last variety. It inhabits a different coral. All the specimens were of small size. The walls are not so thin, and the internal ribs not so prominent as in _var._ 6. The sheath is either white or dull purple; I can, in short, point out no difference in the shell from the typical _var._ 1. The scutum is not so much elongated transversely as in _var._ 6, and the basi-tergal corner is more cut off,--in which respect it resembles the common varieties. The adductor ridge is largely developed, so as to be just visible when the valve is viewed from the outside, in a degree between the first and second sub-varieties of _var._ 6: but the most singular character is the larger development of the tooth near the rostral angle, and this was the case in the same degree in all the specimens which I examined. The tergum resembles that rather unusual sub-variety of _var._ 6 (fig. 6 _q_), which has the scutal margin and the one side of the spur forming a straight line. It appears to me that it would be absurd to consider these slight differences, in parts unquestionably subject to much variation, as specific, when we are almost forced to admit that the much greater differences in the three sub-varieties of _var._ 6, are not of specific value.
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