A Monograph on the Sub-class Cirripedia · Charles Darwin
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The _Scuta_ and _Terga_ are not calcified together: they are both much elongated. _Scutum_ (fig. 5 _c_, 5 _e_).--For the first time in the genus, or indeed in the family, this valve presents a remarkable character in the adductor ridge being immensely developed, so as to project far below the ordinary basal margin. At the rostral end, it at first appears to project even more than it really does, for the toothed occludent margin is in fact a prolongation of the true valve, as distinct from the adductor plate. Excluding this very narrow, prolonged, occludent margin, the adductor plate projects for a length equalling the rest of the valve. Along the tergal margin, the adductor plate is united to the articular ridge; and at the rostral end, it is produced into a square tooth, whence a square-edged ridge extends on the surface of the plate upwards to the ordinary basal margin of the valve. The exact shape of this adductor plate varies a little, as does the degree to which it is closely attached to the ordinary basal edge of the valve. The valve, as distinct from the adductor plate, is narrow, with the basal margin regularly curved. The articular ridge is very prominent.
_Terga_ extremely narrow, linear, consisting in chief part of the spur, which is fully four fifths of the entire length. Externally (fig. 5 _d_) the valve is furrowed, with the edges more or less folded in along the spur. The upper or ordinary part of the valve is about one third wider than the spur. The basi-carinal angle is sharp, owing to the basal margin being a little hollowed out. A special plate of shell (fig. 5 _f_), hollow under its basal edge, runs from the carinal margin to the articular ridge, which latter is situated in the middle of the valve, and against which the articular ridge of the scutum abuts. The spur is central; its end is bluntly pointed. The total length of the tergum rather exceeds that of the scutum, the produced adductor ridge being included in the latter. _Affinities._--Under _P. grande_ I shall make a few remarks, showing that in several characters _P. cancellatum_ and _grande_ are at opposite ends of a short series, with _P. conjugatum_ intermediate between them. 4. PYRGOMA CONJUGATUM.[117] Pl. 12, fig. 7 _a_-7 _c_. [117] Dr. Gray thinks this is the _Pyrgoma stellata_, of Chenu, ('Illust. Conch.'); it may be so; but the figure given of the shell will do equally well or rather better for the _Pyrgomum dentatum_ of this work, and for some varieties of _P. crenatum_. Without a careful description of the opercular valves, it is really impossible to recognise, with any approach to certainty, the species of this genus.
_Shell nearly flat with approximate radiating ridges: scutum and tergum calcified together without any suture: scutum with the adductor ridge descending below the basal margin, and produced at the rostral end into a point: tergum with the spur about as large as the upper part of valve._ _Hab._--Red Sea; Brit. Mus. and Cuming. _Appearance and Structure of Shell._--Shell white, or with a tinge of purple; nearly flat, with moderately prominent, narrow, approximate ridges, radiating from the orifice, which is oval, rather narrow, and not very small. In the largest specimens the ridges are less prominent than in the figure given. The walls are thick, and not at all porose: the sheath extends down almost to the base of the shell, and its lower edge is closely attached to the walls: on each side, towards the carina, there is a trace of a suture, and the lines of growth on the sheath are here a little upturned. The basis is deeply imbedded and internally furrowed; the calcareous layer forming it, is thin. The largest specimen was .4 of an inch in the longer diameter.
The _Scuta_ and _Terga_ (fig. 7 _b_, 7 _c_), in all the specimens which I have seen, are calcified together, with no trace of a suture. There is, however, a slight furrow, which, I believe, marks the normal line of separation between the two valves; and in the following description this is assumed to be the case. The _Scutum_ has the adductor ridge greatly developed, so as to project below the ordinary basal margin to a distance as great as the height of the valve. At the rostral end, this adductor ridge or plate is produced into a point; and at the tergal end, it is blended with the articular ridge, and united to the inner face of the tergum. That portion of the scutum which corresponds with the valve in ordinary cases, and alone is externally visible as long as the operculum is united by the opercular membrane to the sheath, is narrow, with the basal margin considerably hollowed out: the occludent edge is formed into thick teeth. The _Tergum_ is elongated, rather exceeding in length the scutum, the latter being measured from the apex to the rostral projection of the adductor plate. The surface of the valve is depressed in the line of the spur, with the basal end of the latter bluntly pointed. A very slight flexure (fig. 7 _c_) on the basal margin indicates where we may believe the spur to commence, showing that it rather exceeds in length the whole upper part of the valve. The lines of growth obscurely indicate a tendency to the formation of a slight "occludent ledge" along the carinal margin. Traces are just visible of crests for the attachment of the tergal depressor muscles.
5. PYRGOMA GRANDE. Pl. 13, fig. 1 _a_-1 _d_. NOBIA GRANDIS. _G. B. Sowerby, junr._ (sine descript.) Conchological Manual, fig. 29, 1839.[118] CREUSIA GRANDIS. _Chenu._ Illust. Conch., Tab. 1, fig. 2 _a_, sed non, fig. 2. [118] It is quite possible that this may be the _Balanus duploconus_ of Lamarck, but with such a character as the following, who can recognise a species? "_B. testæ parte supremâ univalvi, indivisâ, convexâ: inferiore turbinatâ, non clausâ: aperturâ ellipticâ._ L'exemplaire est sans opercule et incomplet." _Shell moderately convex, nearly smooth: scutum and tergum calcified together without any suture: scutum furnished with a small occludent ledge, with the adductor ridge descending below the basal margin: tergum square without a spur._ _Hab._--Singapore and East Indian Archipelago; Mus. Brit., Cuming, Stutchbury; imbedded in two kinds of coral.
_Appearance and Structure of Shell and Basis._--The shell is conical, though to a variable degree, and sometimes is much depressed. The surface is smooth, with only traces of narrow approximate ridges. The colour is white, often with a tinge of dark purple. The orifice is oval, and moderately large. The shell and a small portion of the basis usually stand exserted above the coral. The walls are of variable thickness; when thick, the pores, by which they are permeated, are but little apparent; sometimes there is more than a single row of pores. The points of the septa on the basal edge of the shell are small. The internal surface of the shell is smooth. The sheath is closely attached to the walls, and descends nearly to the basis; on each side its lines of growth are slightly upturned. The basis is deeply cup-formed or cylindrical, and in section oval like the shell; it penetrates the coral to a very remarkable depth,--in one instance to three inches. The shelly layer forming it, is thin, finely furrowed, and not permeated by pores. This is the largest species in the genus; one specimen was three quarters of an inch in its longer diameter, and above three inches in length.
The _Scuta_ and _Terga_ are calcified together, without any trace of a suture; the line of junction can be inferred only from the analogy of _P. conjugatum_, in which species the valves have a more normal character, and are separated by a slight furrow. It may be seen in the figure (Pl. 13, fig. 1 _d_) of the right and left hand opercular valves, viewed from vertically above in their proper relative positions, how abnormal their appearance is, which is partly caused by the spinose occludent ledges, presently to be described, but chiefly from the carinal margins of the two terga not being straight and parallel, as in all other cirripedes, and therefore not meeting each other, as is usual. In other genera, only the upper part of the carinal margin of the two terga can be opened for the exsertion of the cirri, the lower portion being united by membrane; but here, I have little doubt, from the position of attachment of the adductor muscle (fig. 1 _c_), so close to and almost on the terga, that the whole length of the carinal margin of the two terga is free or disunited for the protrusion of the cirri.
This opening between the two terga evidently cannot be closed, but is probably filled up, and the animal thus protected, by the dorsal surfaces of the curled-up cirri; such, I believe, being likewise the case with some pedunculated cirripedes, as with Conchoderma. The _scutum_ has a large adductor plate, which extends some little way (namely, about one quarter of the height of the valve), below the ordinary basal margin. This latter margin is slightly sinuous, and a little hollowed out towards the tergal corner of the valve. I believe that the ridge, which runs down to the basi-scutal corner of the tergum, though appearing to be part of the scutum, really belongs to the tergum. The adductor scutorum plate is not, as in the last two species, produced into a point at the rostral angle; at the tergal end it blends into the under surface of the tergum. The occludent margin is coarsely toothed. Rather on the under side of this margin and in the upper part, there is a narrow occludent ledge, which extends up beyond the apex of the valve, and thence runs a little way along the carinal margin of the tergum. This ledge is thickly clothed with strong, yellowish-brown spines, and hence appears like a brush. It is remarkable that the cavity for the adductor scutorum muscle is situated almost on the tergum.
The _tergum_ is of large size, and nearly square; it is, in appearance, separated from the scutum by a ridge running up to the apex. The basal margin forms a right angle with the carinal margin, along which latter margin the lines of growth are upturned, and blend into the occludent ledge, which is common to the two valves. There cannot be said to exist any spur, the whole basal margin being almost straight; nevertheless, on close examination, the ridge which in appearance separate the scutum and tergum, may, I think, be safely considered as one side of the spur (which, it should be remembered, has in all ordinary cases a longitudinal furrow or depression), and the other side of the spur is, apparently, very feebly indicated by a slight flexure in the middle of the basal margin. Hence, if the spur had been developed, it would probably have been half as wide as the valve. There are no crests for the tergal depressor muscles. _Affinities._--The present species, with the last two, form an interesting series. _Pyrgoma grande_ and _conjugatum_, however, are more closely allied to each other than to _P. cancellatum_. In the scutum, the whole valve is least elongated, with the adductor plate least developed, in _P. grande_, and most elongated, with the adductor plate most developed, in _P. cancellatum_. In the outline of the tergum the range of shape is quite remarkable; in _P. conjugatum_, which stands between the other two species, the spur is rather long, whereas in _P. grande_ there is no spur at all--a very unusual circumstance--and in _P. cancellatum_, at the other end of this short series, the spur attains a length wholly unparalleled in any other cirripede.
6. PYRGOMA MILLEPORÆ.[119] Pl. 13, fig. 2 _a_-2 _f_. [119] From external aspect I suspect this species to be the _Creusia madreporarum_, Leach (?), as given in Chenu, 'Illust. Conch.,' Tab. 1, fig. 6. But I feel sure that Leach has nowhere published this name; and it may be observed that Chenu gives it with a mark of doubt. The shell in its imbedded state is only figured; the opercular valves are not given; and no descriptive details are added. Under these circumstances I have not adopted this name; I have, perhaps, been in some degree influenced by the fact that this species, judging from the many specimens examined by me, is never imbedded in madrepores, but exclusively in millepores, so that Chenu or Leach's specific name of _Madreporarum_ is singularly inappropriate. _Shell with the orifice narrowly ovate: sheath dark purple: Scutum much elongated: tergum triangular, convex, without a spur._ _Hab._--Philippine Archipelago (Mindoro Island), Mus. Cuming. Mus. Brit., Stutchbury, &c. Imbedded in _Millepora complanata_, sometimes associated with _Balanus Ajax_.
_Appearance and Structure of Shell._--Shell oval, flat, coloured pale dull purple, or white, with slight and narrow ridges, radiating, from the orifice, which is not quite central, but is placed rather nearer to the carinal than to the rostral end of the shell. The orifice is small and narrow; the carinal end being rounded, and the rostral end narrower and sharper,--this being the exact reverse of the usual shape of the orifice in the Balanidæ. The walls are thick, and are formed of large square tubes. The internal surface of the shell is smooth. The sheath (fig. 2 _b_) is much more elongated than the shell, for at the rostral end it extends to the basal margin, and at the carinal end nearly to it, whereas on the two sides it is some way distant from the base. The orifice is considerably out of the centre of the sheath, being placed nearest to the carinal end. The sheath has its lower edge slightly prominent or free; the lines of growth are neither turned up nor down on the two sides, as in the last three species. When nearly full grown the sheath is coloured dark purple, but when young it is white, hence the upper part is white, surrounded by an oblong purple ring, and this is surrounded by the white shell. The basal cup is deep, and internally nearly smooth. The largest specimens were .3 of an inch in their longer diameter. Great masses of the Millepora are absolutely studded with this Pyrgoma, with usually more specimens on one side of the plate or branch than on the other. They stand in approximately parallel positions, the broad or carinal ends of the orifices pointing upwards.
_Scuta._--The scuta and terga are closely united, and _are_ often (perhaps always) in some slight, though variable degree, calcified together; and hence they often break, rather than separate, at the line of articulation. The external fissure or line of junction between them (fig. 2 _c_) is oblique to the longer axis of the scutum; in the uppermost part of the valve it is sometimes almost obliterated. The two valves together are nearly as long as the sheath, and consequently much longer than the orifice of the shell. The scutum is much elongated, being fully four times as broad as high. The valve narrows towards the rostral end, but in a variable degree: the basal margin is hollowed out a little (but to a variable amount), close to the basi-tergal corner. Along the occludent margin a slip of the valve, widening downwards, is a little bent inwards, and this inflected portion is separated from the rest of the surface by a slightly angular ridge, running from the apex to near the rostral angle. Internally, at the basi-tergal corner, a slight ridge, parallel and close to the basal margin (and which can be seen only when the basal edge of the valve is held upwards), represents the adductor ridge, which we have seen so largely developed in the last three species, and shall again see in _P. crenatum_. The articular ridge (fig. 2 _d_) is extremely prominent, consisting of a more or less rectangular shoulder.
_Terga_: these are rather small compared to the scuta: they are triangular and much arched: there is no trace of a spur. Internally (fig. 2 _f_), the articular ridge is central: there are some vestiges of crests for the depressor muscle. _Affinities._--This well-marked species, in the tendency of the opercular valves to be soldered together, and in the remarkable absence of a trace of a spur to the tergum, is allied to _P. grande_, but it is more closely allied to the three following species. 7. PYRGOMA DENTATUM. Pl. 13, fig. 3 _a_-3 _g_. _Scutum much elongated, with a tooth-like articular projection: tergum convex, irregularly triangular, sometimes with an imperfect spur, and on the internal surface with an inwardly projecting tooth; scutum and tergum furnished with an occludent ledge._ _Var._ (1), 3 _c_, 3 _g_: _tergum, with a sharp internal tooth, projecting rectangularly inwards_. _Var._ (2), 3 _d_: _tergum, with a broad blunt internal tooth, depending beneath the spur-like portion of the valve_. _Var._ (3), 3 _f_: _tergum, with the basi-carinal end of the valve truncated, with a small blunt internal tooth projecting rectangularly inwards_. _Hab._--Red Sea; Mus. Brit. and Cuming. Also associated with _Pyrgoma crenatum_, and attached to _Meandrina spongiosa_.[120] [120] Mr. Dana informs me that he believes that this coral comes from the West Indies; though the specimens originally described by him had no label. If this be so, both _Pyrgoma dentatum_ and _crenatum_ have very wide ranges.
_Appearance and Structure of Shell._--Shell nearly flat, oval, white or pink, with rather distant prominent ridges radiating from the moderately large (for the genus) oval orifice. The ridges are often obscured, and apparently sometimes almost obliterated by the encrusting coral. Shell permeated near the outer lamina by short imperfect pores: internal surface smooth: sheath inconspicuous, descending rather more than half way down the walls; lower edge closely attached to the walls. Basis deep. Diameter of largest specimen .3 of an inch. _Scuta_: these are elongated, but to a very variable degree, some specimens being quite three times, and some barely twice as broad as high. I observed this same variability in two sets of specimens, differing, as we shall presently see, in the form of their terga: it depends in part, but not wholly, on the varying width of the occludent ledge, which is sometimes only a fourth, and sometimes half as high as the rest of the valve. This is the first species in the genus in which the occludent ledge--a structure peculiar to the genus--has been amply developed. The basal margin of the valve is slightly sinuous, and a very little hollowed out near the basi-tergal corner; it is also very slightly reflexed, the reflexed portion being separated from the upper part of the valve by a very slight depression or even furrow. I notice this slightly reflexed portion, simply as indicating a well marked feature in the basal margin of the following closely allied species. Internally (3 _g_), the adductor ridge is thick and slightly prominent, but does not descend beneath the basal margin: it blends into the articular ridge, which here projects in a remarkable manner and degree (3 _b_, 3 _e_, 3 _g_), like a rounded tooth. This tooth is in part a development of the occludent ledge; it varies much in shape. The line of junction between the scutum and tergum is nearly straight, and nearly at right angles to their longer axes. In some specimens the scuta and terga are partially calcified together.
_Terga_: in three sets of specimens the terga differed considerably, but as in every other respect there was the closest resemblance, I do not doubt that these are merely varieties. In all three, the valve is rather small, irregularly sub-triangular in shape, and externally somewhat convex; in all three, there is an occludent ledge, of variable width as in the scuta; and in all three, there is an internal tooth-like projection, of variable form, unlike anything I have seen in any other cirripede. In the _first variety_ (Pl. 13, fig. 3 _c_, 3 _g_), the basi-carinal corner of the valve is bluntly pointed, and a slight flexure separates this portion of the valve from the other and scutal half, which latter thus exhibits some tendency to be converted into a spur: on the internal surface (3 _g_) of this spur-like portion of the valve, there is a rather long, sharp tooth, which projects rectangularly inwards; it is flattened in a plane at right angles to the longer axis of the scutum and tergum together: it cannot be seen from the outside. In the _second variety_, the shape of the valve is not very different (Pl.
13, fig. 3_d_), excepting that the flexure, separating the basi-carinal corner of the valve from the spur, is deeper; but on the internal face of the spur, the tooth is far broader than in the first variety, and is flattened quite differently, viz., in a plane nearly parallel to the surface of the valve, and instead of projecting rectangularly inwards, it depends beneath the basal edge of the so-called spur, and can be seen from the outside. In the _third variety_ (fig. 3 _f_), the whole carinal end of the valve is cut off, and there can hardly be said to be any trace of a spur, yet a slight furrow apparently marks the line of separation between the basi-carinal portion of the valve, here become very narrow, and the broad, irregular part, which would have formed the spur had such been developed: on the internal surface of the latter portion of the valve, there is a very small, blunt, slightly flattened tooth, projecting inwards, and more resembling that in the first than that in the second variety. _Affinities_: under _P. crenatum_ I will point out the diagnosis and relationship between this and that species. 8. PYRGOMA CRENATUM. Pl. 13, fig. 4 _a_, 4 _b_. PYRGOMA CRENATUM. _G. B. Sowerby._ Genera of Recent and Fossil Shells, (No. 218, Sept. 1823), fig. 1 to 6.
_Scutum much elongated, with the adductor ridge descending below the reflexed basal margin: tergum with a broad depressed spur: scutum and tergum furnished with a wide occludent ledge._ _Hab._--Philippine Archipelago; Singapore; Mus. Brit., Cuming, Stutchbury; sometimes associated with _Creusia spinulosa_. _Appearance and Structure of Shell._--Shell not distinguishable from that of _P. dentatum_; nearly flat, oval, white, sometimes pale pink, with rather distant prominent ridges radiating from the moderately large oval orifice. Shell solid, or permeated near the outer lamina by short imperfect pores: internal surface smooth. The sheath descends nearly to the base of the walls; it is but little conspicuous, and its lower edge is closely attached to the inner surface of the shell. Basis deep. Diameter of largest specimen under .3 of an inch. _Scuta_: in this species the scutum is more abnormal than in any other Cirripede in the whole family: this is owing both to the adductor ridge descending far beneath the basal margin, and to the great development of the occludent ledge; hence the middle and very narrow portion of the valve alone answers to the scutum, as seen in other genera. The whole valve, including the adductor ridge and the occludent ledge, is narrow, being more than twice as broad as high; but the proportional width varies, owing chiefly (as in the last species) to the varying width of the occludent ledge. This ledge commences a little way from the rostral point of the valve, and gradually widening, extends to the apex, where it is either as high or twice as high as the rest of the valve. It is articulated by a convex surface, and by a hollow on its under side with the occludent ledge of tergum. The scutum cannot be said to have any tergal margin; without, indeed, the articular surface of the occludent ledge be thus called.
The basal margin is curved, and considerably reflexed, of which peculiarity we have seen a vestige in the last species: the reflexion is not well shown, owing to the foreshortening of the reflexed edge, in fig. 4 _a_: this reflexed edge not being shown, causes the lines of growth to appear as if they ran more transversely to the longer axis of the valve, than they really do; for they run nearly as in the scutum (fig. 3 _e_) of _P. dentatum_. The direction of these lines of growth is of importance, for they show that the elongation of the scuta is due to an inordinate increase in their breadth, as compared to the same valves in ordinary species. The adductor ridge, having a sinuous margin, runs from near the apex to near the rostral angle: it descends below the basal margin about as far as the height of the true valve, excluding the occludent ledge. Of course this adductor ridge or plate lies beneath the membrane connecting the opercular valves with the sheath, and is concealed by it, as long as these valves remain within the shell. The edge of the occludent ledge is straight, but the edge of the middle portion of the valve, that is of the true valve, is much bowed.
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