A Monograph on the Sub-class Cirripedia · Charles Darwin
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4. ELMINIUS SIMPLEX. Pl. 12, fig. 3. _Shell ribbed longitudinally, dirty white; radii extremely narrow, smooth-edged; scutum having an adductor ridge._ _Hab._--New South Wales (Sydney and Twofold Bay); Van Diemen's Land; tidal rocks, often attached to other Cirripedes, and associated with _Balanus nigrescens_, _Tetraclita purpurascens_, _Catophragmus polymerus_; Mus. Brit., Cuming, Stutchbury, and Darwin. This species, of which I have seen specimens from the above three localities, all exactly agreeing with each other, is perhaps the Australian representative of _E. plicatus_, which seems to be confined to New Zealand.[113] In all essential points it comes so near that species, that I shall make the greater part of my description comparative. [113] I am bound to state that I have seen two specimens of _E. plicatus_ marked Sydney, and one marked Moreton Bay, but in both cases the collectors had visited New Zealand, so that a wrong habitat by mistake might easily have got attached to the specimens in question. _General Appearance._--In external appearance there is considerable difference from _E. plicatus_, for _E. simplex_ is generally of a regular conical shape, of a dirty-white colour, with the surface well preserved, having moderately wide, not very prominent longitudinal ribs. The orifice is rather small and pentagonal. The radii are extremely narrow or linear, with quite smooth edges; the sutures, however, are always very distinct, and in the upper part, the alæ are generally rather widely exposed, as viewed from the outside. The largest specimen which I have seen was .7 of an inch in basal diameter.
The _opercular valves_ are closely similar to those of _E. plicatus_, but the _scutum_ is generally a little more elongated, and the articular furrow not so deep: in accordance with this last fact, the articular ridge in the _tergum_ is not so prominent as in _E. plicatus_; but we have seen that these several characters are highly variable in _E. plicatus_. The slope of the basal margin of the tergum towards the spur varies in the present species, in a strictly analogous manner, as it does in _Tetraclita porosa_. _Structure of the Parietes and Radii._--The parietes are not so thick as in _E. plicatus_; internally they are tinted pale purple; when broken transversely, a row of microscopically minute orange-coloured dots can generally be distinguished between the outer and inner laminæ; and these evidently represent the orange-coloured layer in _E. plicatus_. The sheath also exhibits a faint tinge of orange. The radii are very narrow, and are quite smooth-edged, differently from in _E. plicatus_. The edges of the alæ barely exhibit a trace of being crenated. In the body I could perceive no difference from _E. plicatus_, excepting that in the third pair of cirri the two rami are like each other, and do not support any coarsely pectinated, only serrated, spines; but after what we have seen on the variability of these very same characters in _Tetraclita porosa_, I dare not trust to them. The three posterior pairs of cirri, also, seem here to be more elongated in proportion to the others, than in _E. plicatus_.
_Affinities._--It is certain that this species is most closely allied to _E. plicatus_; but as I have seen many specimens of the latter brought by different persons from New Zealand, and as I have observed in them no approach to the characters of _E. simplex_, which, in specimens from three localities, also appear to be constant, I have considered the two forms as specifically distinct. The present species differs from _E. plicatus_, in its white, conical, moderately ribbed, well preserved, smaller shell; and more especially in the orange-coloured intermediate lamina of _E. plicatus_ being here represented only by microscopically minute dots. But the radii being smooth-edged, is the most important differential character, though in _E. plicatus_, during its earliest growth, whilst still immature and colourless, the radii are likewise smooth-edged. 5. _Genus_--PYRGOMA. PYRGOMA. _Leach._ Journal de Physique, tom. 85, 1817. BOSCIA. _Ferussac._ Dict. Classique d'Hist. Naturelle, 1822. SAVIGNIUM. _Leach._ Zoological Journal, vol. 2, July, 1825. MEGATREMA. _Ib._ Ib. ADNA. _Ib._ Ib. DARACIA. _J. E. Gray._ Annals of Phil. (new series), August, 1825. CREUSIA. _De Blainville._ Dict. Sc. Nat., Pl. 116, 1816-30. NOBIA. _G. B. Sowerby, junr._ Conchological Manual,[114] 1839. [114] The name, Nobia, is given in this work on the authority of Leach, but this must be a mistake, probably caused by some MS. name, (that fertile source of error in nomenclature), having been used.
_Shell formed of a single piece; basis cup-formed, or sub-cylindrical, attached to corals._ _Distribution_, imbedded in corals, chiefly in the tropical seas, round the world. I feel no hesitation in including the above several genera in one genus. In external appearance the _P. monticulariæ_ (Pl. 13, fig. 5 _a_), which forms the genus Daracia of Gray, is the most distinct, but it is so intimately allied to the two, indeed to the three, foregoing species, that it cannot be separated from them. Of the first five species, _P. grande_, _conjugatum_, and _cancellatum_, form a graduated series, but with the steps very distinct, the chief difference being in the length of the spur of the tergum; for the fact of the scutum and tergum being calcified together in _P. grande_ and _conjugatum_, and distinct in _P. cancellatum_, is certainly unimportant, as may be inferred from what we shall see in comparing together the last four species of the genus, and from what we shall see in _Creusia spinulosa_. The three above-mentioned graduated species are connected with the last four species of the genus, by several points of resemblance between _P. grande_ and _crenatum_. The first two species, namely, _P. Anglicum_ and _Stokesii_, are the most closely related together, and may indeed possibly be identical; these two, in all the characters derived from the opercular valves, resemble Balanus and other ordinary forms, and for this very reason they have some claims to be generically separated from the other species of Pyrgoma; for in these latter, the opercular valves seem to have broken loose from all law, presenting a greater diversity in character than do all the other species of Balaninæ and Chthamalinæ taken together.
_General Appearance._--The shell consists of a single piece, generally without any suture, even on the internal surface; and this is the case, at least in _P. Anglicum_, in extremely young colourless specimens: nevertheless, in some specimens of this very species, and of _P. conjugatum_, there were traces of two, but only two, sutures on the sheath, one on each side towards its carinal end. The shell is much depressed or actually flat; and I have seen specimens even slightly concave; in _P. Anglicum_, however, the shell is steeply conical. The outline is generally oval; but in _P. monticulariæ_ it is extremely irregular. The surface is generally furnished with more or less prominent ridges, radiating from the orifice, which is oval and small; sometimes, as in _P. monticulariæ_, excessively small. The colour is white, or pinkish-purple. Most of the species are small, but I have seen specimens of _P. grande_ three quarters of an inch in diameter in the longer axis, and, including the deep, almost tubular, basis, more than three inches in length or depth.
_Opercular Valves._--In three species, viz., _P. conjugatum_, (Pl. 12, fig. 7 _c_), _grande_, and _monticulariæ_, the scuta and terga, on each side, are calcified together so perfectly, that there is no trace of a suture or line of junction: in _P. milleporæ_, these valves are generally slightly calcified together, but with the suture distinct. The _Scuta_ differ so much in shape in the different species, that little can be said of them in common: in _P. Anglicum_ and _Stokesii_ they resemble those of Balanus; but in the other species they are much more elongated than is usual, and this is carried to an extreme in the last four species; this elongation is due to a great increase in breadth, as may be inferred from the position of the apex of the valve, and from the direction of the lines of growth. But the two most remarkable characters are, first,--the extraordinary development of the adductor ridge, so that, in _P. conjugatum_, _cancellatum_ (Pl. 12, fig. 5 _c_), _grande_, and _crenatum_, it extends considerably beneath the basal margin, being produced, in the first two species, at the rostral angle, into a point; at the tergal end of the valve, the adductor ridge, when thus much developed, blends into the articular ridge.
The second very remarkable character is the addition of a special ledge along the occludent margin of the scutum, and along the carinal margin of the tergum, which I will call the occludent ledges (_limbus occludens_), and which serve to close the orifice leading into the sack. The occludent ledge is small in _P. grande_, and is clothed with thick yellow spines, giving it a brush-like appearance: in _P. crenatum_ and _dentatum_ it is largely developed, the ledge on the scutum being articulated with that on the tergum, as shown in Pl. 13, fig. 4 _a_, 4 _b_,--the ledges being here and elsewhere marked by little bristly points. In _P. monticulariæ_, however, this ledge arrives at its maximum development (Pl. 13, fig. 5 _f_), the rest of the valve (the scuta and terga being here, as in _P. grande_, calcified together) being reduced to a mere basal edge or border. Excepting for the adductor muscle, the depressions or crests for the other muscles, both on the scuta and terga, are hardly developed.
_Terga_: these, as in the case of the scuta, differ so much in shape in the several species, that little can be said of them in common. In _P. Anglicum_ and _Stokesii_, they are of the normal shape; in _P. cancellatum_ (Pl. 12, fig. 5 _d_) this, to a certain extent, is likewise the case, but the spur is produced to a quite extraordinary length. In _P. grande_ (Pl. 13, fig. 1 _b_) there is no distinct spur, and the whole valve is square. In _P. milleporæ_ (2 _f_) there is no spur, and the valve is arched and triangular. In _P. crenatum_ the spur is broad, rounded, and depressed (fig. 4 _b_), with the carino-basal portion of the valve reduced to a mere border, barely distinguishable from the great occludent ledge. In _P. monticulariæ_ there is no spur, and the whole valve forms a mere border to the occludent ledge; and, lastly, in _P. dentatum_, the valve is extremely variable in shape (fig. 3 _c_, 3 _d_, 3 _f_), and on its internal surface (fig. 3 _g_) there is an inwardly projecting, most singular and anomalous, tooth. Hence we see how wonderfully variable the terga are in this genus.
_Structure of the Walls._--The shell consisting, as has been stated, of a single piece, is generally thick. From the close alliance between this genus and _Creusia_, it is probable that the shell, if examined immediately after the metamorphosis, would be found to show traces of being formed of four compartments. The walls are either solid or porose; their basal margin is formed by strong crenated ridges, answering to the longitudinal septa in Balanus; but these in _P. monticulariæ_ are modified into a very irregular surface. The internal surface of the shell is generally smooth, or only slightly ribbed. The sheath has its lower edge free in _P. Anglicum_ and _Stokesii_, and in a slight degree in _P. milleporæ_, but in the other species it is closely attached to the walls. In _P. monticulariæ_ the sheath might easily be overlooked. In _P. Anglicum_, _grande_, and _crenatum_, it descends almost to the basal margin of the depressed shell, and as the opercular valves and membrane are attached to the lower edge of the sheath, the animal's body necessarily comes to be almost exclusively lodged in the cup-formed basis. In _P. grande_ and _conjugatum_, the lines of growth in the sheath are bent upwards on each side, at points corresponding with the line of union between the scutum and tergum, in a manner I have not seen in any other cirripede; and this sometimes gives the appearance of two lateral sutures. I may here remark, that the manner of growth in Pyrgoma is almost the converse of that in Balanus, Tetraclita, and other allied genera; for in these latter, the basis increases in diameter, and the shell chiefly in height; whilst in Pyrgoma, the shell, from being so flat, increases almost exclusively in diameter, whereas it is the basal cup which is added to in height or depth.
_Basis._--This in all the species is more or less regularly cup-formed or sub-cylindrical. In _P. grande_ it penetrates the coral to a surprising depth; in _P. monticulariæ_ it is irregular in outline, corresponding with the shell. The basis is generally almost wholly imbedded in the coral; but this is not the case with _P. Anglicum_, in which the basis is generally exserted, as it is in a slight degree in _P. grande_. The shelly layer forming the basis, in most of the species, is very thin, and is finely plicated owing to its edge folding between the ridges or septa that form the basal edges of the shell; this is very conspicuous in _P. cancellatum_. The basis is not permeated by pores, except in _P. Anglicum_. In some sessile cirripedes a cleft, covered only by membrane, may be observed all round between the lower edge of the shell and the basis; a cleft of this nature is rather conspicuous in _P. crenatum_, so that small portions of the septa on the internal surface of the walls can be seen from the outside. In _P. monticulariæ_, an analogous structure, developed to an extreme degree, presents a very different and unique appearance; the shell is nearly flat, and the smooth outer lamina does not nearly reach to the circumference, a wide border being thus left exposed, which is roughened (Pl. 13, fig. 5 _a_, 5 _c_, 5 _d_, 5 _e_) by the exposure of the irregular septa. I have not seen a fresh specimen, but there cannot be a doubt that this border is properly covered by membrane.
_Animal's Body._--From some cause, perhaps from the corals in which the species of Pyrgoma are imbedded, long remaining damp, the internal organs are generally badly preserved. I have received, in spirits of wine, only _P. Anglicum_, but I have examined dry specimens, in a tolerable condition, of _P. milleporæ_ and _crenatum_. As neither the mouth nor cirri, in these three species, offer any noticeable characters, distinct from those in Balanus or Acasta, my ignorance of these organs in the other six species is not important. In the above three species the labrum is deeply notched, with about three teeth on each side of the notch, except in _P. milleporæ_, in which the number is six. In all, the mandibles have five teeth, the two lower ones being small: the maxillæ are not notched: the outer maxillæ are bilobed. In the _Cirri_, the rami in the first pair are very unequal in length, the segments being slightly protuberant in the shorter ramus. On the segments of the posterior cirri there are four pairs of spines in _P. Anglicum_, and three pairs (of which the second and third are short) in _P. milleporæ_. At the dorsal basis of the penis there is a small straight projecting point.
_Affinities._--The species (with the exception of the first two) are much more distinct from each other, and more easily determined than is usual with sessile cirripedes; it is, however, quite useless to attempt naming the species without disarticulating and cleaning the opercular valves. Although these valves differ so greatly in some of the species from those of Balanus and the allied forms, the genus itself, as a whole, does not differ much, except in the shell not being divided into compartments, and in the basis being cup-formed and not generally permeated by pores,--these latter characters being in common with Acasta. With respect to the absence of separate compartments, it should be remembered, that in the same species of Tetraclita we have individuals with the four compartments distinct and furnished with radii, and other individuals without any trace of a suture externally,--the outer lamina of shell (though not the inner) having become completely confluent all round. At the commencement of this description, when giving my reasons for uniting the several proposed genera into one genus, I gave a sketch of the affinities of the species: I have only to add, that the following sub-genus Creusia is closely, perhaps too closely, allied to Pyrgoma.
_Geographical and Geological Distribution._--Most of the species are inhabitants of the hot coral-growing zones, in both the eastern and western hemispheres, but more especially, as it would appear, in the East Indian Archipelago. From the habits of the corals, most of the species must be inhabitants of shallow water. _Pyrgoma Anglicum_, however, is an inhabitant of deep water on the southern shores of England, whereas at St. Jago, in the Cape de Verde Islands, I myself collected it adhering to a Caryophyllia, within the tidal limits. This same species existed on the shores of England during the Coralline Crag period; and at this epoch it attained a larger size than at present. Two species of the genus, according to Sismonda, are found in the tertiary beds of Piedmont. Mr. Stutchbury, who is so well acquainted with recent cirripedes, informs me that he has for many years examined fossil Secondary corals, in the expectation of finding imbedded, species of this genus or of some allied form, but without success. The same species of Pyrgoma is by no means always confined to the same coral: I have seen _P. crenatum_ on four or five different corals, and _P. Anglicum_ on at least three kinds: on the other hand, I have seen _P. milleporæ_ only on the _Millepora complanata_ (a member, as I believe, of the vegetable kingdom), and _P. monticulariæ_ on a _Monticularia_ from near Singapore.
1. PYRGOMA ANGLICUM. Pl. 12, fig. 4 _a_-4 _c_. PYRGOMA ANGLICA. _G. B. Sowerby._ (sine descript.) Genera of Recent and Fossil Shells, fig. 7, No. 18, Sept. 1823. MEGATREMA (ADNA) ANGLICA. _J. E. Gray._ Annals of Philosoph. (new series), vol. x, Aug. 1825. PYRGOMA SULCATUM. _Philippi._ Enumeratio Molluscorum Siciliæ, Tab. 12, fig. 24, (1836). ---- ANGLICA. _Brown._ Illustrations of Conchology, (2d edit., 1844), Tab. 53, fig. 27-29. _Shell steeply conical, purplish red: orifice oval, narrow: basis permeated by pores, generally exserted out of the coral: scutum and tergum sub-triangular._ _Hab._--South coast of England and of Ireland, (12 to 45 fathoms, Forbes and MacAndrew); Sicily; Madeira; St. Jago, Cape de Verde Islands; generally attached to the edge of the cup of a Caryophyllia, in deep water, but at St. Jago within the tidal limits; Mus. Brit., Cuming, Lowe, &c. _Fossil_ in the Coralline Crag, Ramsholt; Mus. S. Wood. _General Appearance._--Shell steeply conical, slightly compressed, the lower part with rounded, approximate, radiating ribs: colour dull purplish-red: orifice oval, small, and narrow. The basis is not deeply conical, and occasionally is even flat. Generally it stands exserted; but in the Coralline Crag specimens, it is almost wholly imbedded. Externally it is furnished with ribs corresponding with those on the shell. The largest recent specimen which I have seen, from St. Jago, was .22 of an inch in basal diameter; but some few of the British specimens are nearly as large, and one of the fossils from the Coralline Crag a little larger.
The _Scuta_ and _Terga_ are of the ordinary shape of these valves in Balanus and its allies. _Scuta_ triangular, with the basal margin a little curved and protuberant: adductor and articular ridges distinct from each other, moderately prominent: there is a small hollow for the lateral depressor muscle. _Terga_ triangular, with the spur rather narrow, moderately long, placed near, but not confluent with the basi-scutal angle of the valve. The basal margin forms an angle rather above a right angle with the spur. Internally, the articular ridge and crests for the depressor muscles, feebly developed. _Internal Structure of the Shell and Basis._--Internally, the shell is ribbed more or less prominently. The lower edge of the sheath, which is reddish, and extends far down the walls, seems always to project freely. In several specimens there were on each side, at the carinal end of the shell, a trace of a suture, which could be perceived only on the sheath. The basis appears always to be permeated by minute tubes or pores, though these are sometimes rather difficult to be seen. 2. PYRGOMA STOKESII. Pl. 12, fig. 6. MEGATREMA STOKESII. _J. E. Gray._ (sine descript. aut figurâ) Annals of Philosophy, (new series), vol. 10, Aug. 1825. _Shell moderately conical, pale-purplish red; orifice oval: basis not permeated by pores, deeply imbedded in the coral: scutum and tergum sub-triangular._
_Hab._--Imbedded in the Mycedia (Agaricia) agaricites; therefore from the West Indies;[115] Brit. Mus. and Stutchbury. [115] I am greatly indebted to Mr. Dana for having named for me the coral in which this species was imbedded, and informing me that it is a West Indian species. This species comes so close to the last, that I am not sure that I have acted rightly in retaining it, but I think that it is distinct; and in this case, it is the representative, on the other side of the Atlantic, of _P. Anglicum_ of our own side. It will be sufficient to point out the few points of difference. The shell is much more depressed, with the orifice oval, larger, and not so narrow. It is apparently of a paler red, and the radiating ribs perhaps not so prominent. The basis offers the most important difference, being deeply imbedded in the coral; and there is not the least appearance of the thin shelly layer, of which it is composed, being permeated by pores, as, we have seen, is always the case with _P. Anglicum_. As in this latter species, the sheath here depends freely. The opercular valves are closely similar; but in the scutum, the adductor ridge occupies a rather more central position; and in the tergum, the basal margin is more inclined towards, or forms a greater angle with, the spur: these differences, by themselves, I consider quite insufficient to characterise a species; but conjoined with the flatter shell, the larger orifice, the more deeply imbedded and non-porose basis, they may, I think, be admitted as specific. In dimensions, this species seems to attain a slightly larger size than _P. Anglicum_, for several specimens were .22 of an inch in diameter.
3. PYRGOMA CANCELLATUM. Pl. 12, fig. 5 _a_-5 _f_. PYRGOMA CANCELLATUM. _Leach_ (!). Encyclop. Brit., Supplement, vol. 3, Pl. 57, 1824. ---- LOBATA. _J. E. Gray_ (!). Annals of Philosophy, (new series), vol. 10, 1825. CREUSIA RAYONNANTE. _De Blainville._ Dict. Sc. Nat. (sine descript.), Pl. 116, fig. 7, _a_, _b_. _Shell with the circumference generally lobed: scutum elongated, with the adductor ridge descending far below the basal margin, and produced at the rostral end into a square point; tergum with the spur four times as long as the upper part of the valve._ _Hab._--Imbedded in a Gemmipora, probably from the West Indies;[116] Mus. Brit. and Cuming. [116] In this case I am again indebted to Mr. Dana for naming the coral; he informs me that the genus is found in the Pacific and East Indies; the specimen sent he believes is the _G. cinerascens_.
_Appearance and Structure of Shell._--Shell nearly flat, sometimes tinted dull dirty purple, with the surface marked by slight, broad, approximate ridges, the ends of which form considerable projections, giving to the shell, when not too much encrusted by coral, a lobed border. In young specimens (.15 of an inch in diameter) some of the points projected half as much as the semi-diameter of the shell, giving it a radiating appearance. The orifice is oval and rather small, but of variable size. The shell is thick, and, near the outer lamina, is penetrated by pores: the internal surface is smooth. The sheath (fig. 5 _b_) extends down but a short distance from the orifice; it is closely attached to the walls: the lines of growth, at a point on each side, bend a little downwards (instead of upwards, as in the two following species), and hence the lower edge of the sheath irregularly projects downwards on each side. The basal cup, internally, is plicated, the hollows corresponding with the projecting, longitudinal, parietal septa, which form the lobed border of the shell. The largest specimen which I have seen, was rather above .4 of an inch in diameter.
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