A Monograph on the Sub-class Cirripedia · Charles Darwin
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_Basis_, slightly concave or saucer-shaped; the circumference is produced into long spikes, corresponding with those on the basal margin of the parietes: these projections equal half the semi-diameter of the shell. The internal surface of the basis has slightly prominent, rounded ridges; and the external surface has extraordinarily prominent, sharp ridges, radiating from the centre; the edges of the external ridges are irregular, notched, and knobbed. I have seen in no other species external ridges on the basis or surface of attachment; and what is more remarkable, the interspaces between the ridges are penetrated by small rounded apertures, of irregular shape and unequal sizes; and these are generally arranged in an irregular double row, and externally are closed by the membrane, which clothes the basis. In the sub-genus Acasta, the basal cup is sometimes penetrated by similar holes, but these seem never to extend over the whole basis, and are very variable; nevertheless, in some specimens of _Acasta spongites_ from the Cape of Good Hope, portions of the basis closely resembled, except in the absence of the radiating ridges, the structure here described, but the holes were not arranged in any definite order. The internal surface of the parietes in _Acasta sporillus_ presents a somewhat analogous appearance, but the pits do not penetrate through the walls. This species, I have no doubt, is closely allied to the sub-genus Acasta, and to _Balanus navicula_ with its allies, and, but much less closely, to _B. allium_ with its allies. Indeed, had _B. terebratus_ inhabited a sponge, I should have been compelled to have ranked it in the sub-genus Acasta.
38. BALANUS VESTITUS. Pl. 8, fig. 3 _a_-3 _b_. _Shell pinkish-purple or white, clothed by an orange-coloured membrane; radii represented by mere fissures; basis solid. Scutum, with a sharp, curved adductor ridge; with crests for the lateral depressor muscle: tergum, with the spur short, truncated, one third of width of valve._ _Hab._--New Zealand, New South Wales, Mus. Brit. and Stutchbury; attached to shells. _General Appearance._--Shape conical, often steeply conical; orifice small; radii not developed, represented by mere fissures. The walls are smooth, or slightly, or strongly ribbed longitudinally. The shell itself is of a fine peach-blossom pink, or nearly white, but it is generally covered by a thick yellow or brownish-orange epidermis. Opercular valves pinkish, but similarly covered. Basal diameter of largest specimen .7 of an inch.
_Scuta_, with the lines of growth closely approximate; surface somewhat convex. Internally, the articular ridge is very little prominent, but runs far down the tergal margin; in some specimens, however, it is shorter and more prominent. The adductor ridge is strongly prominent, is curved towards the rostral angle, and runs down nearly to the basal margin. The rostral depressor muscle is lodged in a small cavity, formed, as usual, by the overlapping of the occludent margin; within this cavity there are either tolerably distinct little crests, or merely traces of them, for the attachment of the muscle. The lateral depressor muscle is attached to several quite distinct crests, seated in a concavity beneath the adductor ridge. _Tergum_ rather narrow, with the apex produced or beaked; the beak is purplish and flat. There is a slight rounded longitudinal furrow, or depression. The spur is fully one third of the width of the valve: it is short, with the end truncated, and placed close to the basi-scutal angle; the basal margin on the carinal side slopes gently towards the spur. Internally, the scutal margin is scarcely at all inflected, and the articular ridge is very little prominent: the crests for the tergal depressores are pretty well developed. From the points here enumerated, it is clear that the opercular valves are articulated together much less strongly than is usual with most species, excepting _B. allium_ and its allies. It is remarkable that in this species the terga are united to the sheath, not, as is usual, by a single opercular membrane, but by five or six, one above the other, the upper membranes not having been exuviated as each new lower one was formed. The minute spines on the membrane lining the sheath are rather larger and more numerous than is usual, and to the base of each spine a tubulus of unusual diameter runs, imbedded in the shell.
The _Walls_, internally, have unusually numerous, narrow, approximate, strongly prominent, longitudinal ribs, denticulated at their bases, and inserted into the furrows on the borders of the basis: in old specimens these internal ribs die out in the upper part of the walls. The _Radii_ are not developed in any of the many specimens which I have seen, and the edges of the compartments on both sides of each suture are equally marked by slight, irregular ridges or knobs, answering to the septa and their recipient furrows, in the species with ordinarily developed radii. There is very little diametric growth, the orifice being gradually enlarged by the disintegration of the upper ends of the walls; the _alæ_, however, in some specimens, do grow a little along their lateral or sutural edges, so that some little diametric growth must be effected. The summits of the alæ are very oblique; their sutural edges are plainly crenated. The sheath descends about half way down the walls. The _Basis_ is flat, not permeated by pores, but deeply furrowed in lines radiating from the centre.
_Mouth_: labrum sometimes with no teeth, sometimes with four minute teeth; mandibles with four teeth, of which the third is blunt and rather large; the fourth is a mere knob. Maxillæ; there is, as usual, an upper pair of large spines (beneath which there is sometimes a small notch), but all the lower spines, instead of standing as usual in pairs, form a single row. _Cirri_; first pair with the rami remarkably unequal in length, one ramus having twenty-two segments, and being more than twice the length of the other, having only nine segments: these segments, and likewise those of the second and third pairs, have an inverted conical shape; and they are all less thickly clothed with spines than is usual. The second pair is short, about as long as the shorter ramus of the first pair, and has ten or twelve segments. The third pair is above twice as long as the second pair, and contains twenty-four segments: this very unusual length is owing to the presence of numerous thin tapering upper segments, unlike those generally found in _Balanus_, in the third pair of cirri, and apparently serving as feelers. These upper tapering segments are of an inverted conical shape, and support on their upper margins two very small tufts of spines, one behind and one in front: on the segments lower down these tufts increase in size, and the spines are more spread out, so that in the basal segments, the tufts in front form on the upper margin two or three crowded rows of bristles. The three posterior pairs of cirri have elongated segments, which bear on their upper half three pairs of spines; of these the lowest pair is minute, and the middle pair is only one third of the length of the upper pair. The sixth cirrus, in the same individual as before, contained twenty-seven segments in each ramus, that is only three more than in the third cirrus! I must observe, that the cirri in all the specimens were irregular, often distorted and monstrous; and therefore, probably, there is considerable variation in the proportional numbers of the segments in the cirri.
At the base of the penis there is a minute, knife-edged, triangular projection. The branchiæ are rather narrow, pointed, and not very large. _Affinities._--This is a very distinct species, as shown by the peculiarities in the cirri, by the absence of radii to the shell, and by the presence of crests for the attachment of the lateral scutal depressores. With the exception of this latter character, the opercular valves clearly show, that _B. vestitus_ is allied to _B. allium_, _cepa_, and _quadrivittatus_. In some respects this species manifests an affinity to _B. imperator_, which latter has its third pair of cirri nearly similar to those of _B. vestitus_. 39. BALANUS IMPERATOR. Pl. 8, fig. 4 _a_-4 _c_. _Shell internally imperial purple; parietes thick, with their internal basal edges rough with irregular points and ridges; radii narrow; basis very thin, solid. Scutum, with crests for the rostral and lateral depressor muscles; tergum, with the end of spur rounded._ _Hab._--New South Wales, Sydney, Port Stephens, Moreton Bay; attached to sandstone-rocks and shells, at low-water line; Mus. Brit., College of Surgeons, Cuming, Stutchbury. _Shell_ conical, very thick and very strong; longitudinally sulcated more or less strongly; whole thickness of shell beautifully coloured rich violet, or more strictly "imperial purple;" externally the surface, from disintegration, is generally whitish; internally the colour is best developed: the narrow radii and the thin basis are white. The largest specimen which I have seen was one and three-quarters of an inch in basal diameter, the walls close to the basis being, in this instance, actually .3 of an inch in thickness.
_Operculum_ thick and strong, covered by yellowish-brown epidermis; internally, the shelly substance is either all of the richest purple or yellowish-white, tinged, especially in the upper part, with purple. _Scuta_, with the apex beaked and somewhat reflexed; articular ridge very thick, little prominent; articular furrow very narrow, the impression made by the adductor muscle is seated very high up the valve; there is hardly an adductor ridge, but the surface of the valve is angularly prominent in a curved line, running from the articular ridge to near the rostral angle of the valve. At the rostral angle, the occludent margin is not folded inwards, as is generally the case, but the surface is flat, and is marked by four or five crests for the attachment of the rostral depressor muscle. There are other crests for the lateral depressor muscle. _Tergum_, with the apex somewhat produced and beaked, but blunt; longitudinal furrow shallow; spur of moderate breadth, with its lower end rounded; the basal margin on the carinal side of the spur slopes towards it. Internally, articular ridge moderately prominent. Crests for the tergal depressor well developed.
_Parietes_, solid, thick, with the basal internal edge (4 _c_) formed of short ridges, or flattened and irregular points, which in very old specimens are but little prominent; inner surface, finely, closely, and irregularly ribbed longitudinally, but in some specimens nearly smooth. The _radii_ are nearly white; they are narrow, sometimes hardly at all developed, and have their summits very oblique and jagged; exteriorly, they are sulcated in a transverse direction, and sometimes form oblique steps, from having been formed layer over layer: their sutural edges are formed of irregularly branching crests or septa. The _alæ_ have their summits very oblique; their sutural edges are thick and crenated: the part added during diametric growth on the inner surface is smooth, and has a different appearance from the transversely ribbed portions of the sheath, of which the alæ form a portion. The lower edge of the sheath is hollow beneath. The carino-lateral compartments are very narrow. _Basis_ calcareous, thin, white, sometimes opalescent, apparently formed by an aggregation of very minute calcareous beads, with no trace of furrows radiating from the centre. _Mouth_: labrum hairy, with apparently some very minute teeth; mandibles, with the fourth and fifth teeth small and rudimentary; maxillæ rather broad, with a narrow and rather deep notch under the two great upper spines: outer maxillæ with the lower lobe very large.
_Cirri_: first pair, with the rami unequal by several segments: second pair, with the rami unequal in length by about six segments: third pair elongated, with the segments very numerous, almost equalling those in the sixth cirrus; upper segments of both rami much elongated, each with only a circle of spines; segments in the above first three pairs of cirri only slightly protuberant. Posterior cirri elongated, with the upper segments bearing three pairs, and the lower segments four pairs of main spines, between which there is a small intermediate tuft. _Affinities._--This noble Balanus, in all the characters derived from its opercular valves, and from its cirri, is closely allied to the last species: in the structure, however, of the shell and of the basis, it comes closer to the following, _B. flosculus_. The crests on the under side of the scutum, for the lateral depressores, are confined to these three species; and the crests for the rostral depressores are confined to _B. imperator_ and _vestitus_, but they are generally rudimentary in the latter. The internal basal structure of the parietes is singularly like that of _Chelonobia caretta_, though there is no other special affinity to that genus. In the nature of basis; in the structure, to a certain limited extent, of the walls of the shell; in the narrowness of the carino-lateral compartments; in the elongation of the third pair of cirri; in the crests for the rostral and lateral scutal depressores, _B. imperator_ comes nearer to the genus _Tetraclita_ than does any other species of Balanus.
40. BALANUS FLOSCULUS. Pl. 8, fig. 5 _a_-5 _f_. _Shell purple or dirty white, with the internal basal edges of the parietes rough with irregular points and ridges; radii narrow or absent; basis excessively thin, in appearance absent. Scutum with crests for the lateral depressor muscle; tergum very narrow, with the spur pointed._ _Var._ sordidus. (Pl. 8, fig. 5 _b_) _shell globulo-conical, dirty white, with numerous sharp, narrow, longitudinal folds or ridges_. _Hab._--Peru and Chile; generally attached to the _Concholepas Peruviana_, or to _Balanus psittacus_, and associated with _Chthamalus scabrosus_. _Var. sordidus_, inhabits Tierra del Fuego, attached to littoral shells, wood, and rock, associated with _Ch. scabrosus_. _General Appearance._--Shell either extremely much depressed and irregular, or globulo-conical, or more rarely cylindrical and elongated. Walls, either with a few rather broad, smooth, irregular, longitudinal folds, or, in _var. sordidus_, with numerous sharper and more prominent longitudinal ridges; basal margin very sinuous. Colour, either fine rich peach-blossom purple, or so pale as to be almost white; or in _var. sordidus_ dirty white, generally stained greenish from confervoid matter. Orifice small, oval, entire. Radii very narrow, white, or not at all developed, and with even the sutures not distinguishable. The purple coloured varieties, with the narrow white radii, the small oval orifice, and folded walls, have a very pretty appearance, which is far from the case with _var. sordidus_. The largest specimens attained a basal diameter of .6 of an inch, but this is an unusual size: I have seen a cylindrical specimen of _var. sordidus_ one inch in length.
The opercular valves are united to the sheath by unusually strong membrane: internally, their upper parts are stained purple. _Scuta_, these vary considerably in breadth, some being even broader than in Pl. 8, fig. 5 _c_, and others as much elongated as in fig. 5 _d_: these latter come from an elongated cylindrical shell. The valve externally is unusually convex: the apex, also, projects freely to an unusual degree. Internally, the articular ridge is moderately prominent: the adductor ridge is prominent and much curved: in the upper part it either lies close to, or at some little distance from the articular ridge. The lateral depressores are attached to several little crests, occupying a cavity, (often bordered above by a little ridge) close beneath the adductor ridge. _Tergum_, extraordinarily narrow and elongated; beak triangular, purple: a longitudinal furrow runs down the valve: spur narrow, long, bluntly pointed, lying close to the basi-scutal angle of the valve: the scutal margin is nearly straight and parallel to the spur. Internally, the articular ridge is prominent: the crests for the depressores are moderately well developed: the upper part of the valve is marked by a purple patch, bounded on one side by the articular ridge, and on the other side by a very slight special ridge. There is some variability in the narrowness of the whole valve, and in the sharpness of the spur.
_Parietes._--The under surface, in the more depressed varieties, is roughened in a remarkable manner nearly or quite up to the sheath, with very irregular, projecting, and branching ridges, and sometimes with depending points. These ridges and points are granulated on their under surfaces. The roughened surface in the more conical varieties is confined to the basal inner margin. This structure is nearly the same as that in _B. imperator_, represented (Pl. 8, fig. 4 _c_), but the little ridges are here more apt to be concentric, instead of radiating. The lateral edges of the compartments on the inside, especially the carinal edges, project inwards beyond the inner surface of the shell. The sheath is but little hollow on its under side. The diametric growth of the shell seems to be quite capricious; in the same group, some individuals thus increasing, and others not at all. When the radii are developed they are narrow and white, with their upper margins nearly if not quite parallel to the basis: their sutural edges are formed by large, irregular, branching teeth or septa. The _alæ_, also, have their sutural edges coarsely crenated; and when the shell increases by diametric growth, they are added to above the opercular membrane, so as to be nearly square at top.
_Basis._--When a shell is removed from the surface of attachment, and inspected even under a lens, there appears to be no basis whatever, either adhering to the shell, or to the supporting surface: but when a more careful examination is made, with a higher power, an excessively thin, translucent, calcareous, irregular layer, or rather film, can almost always be discovered. This would be more easily distinguished if the specimens had adhered to rock instead of to the rugged shells of molluscs. At one time I thought the basis was partially membranous, for I have certainly scraped off small fragments of membrane from the supporting surface; but these, when examined under the compound microscope, seemed always to consist of a thin sheet of the yellow cementing tissue; and in some instances, a brittle film of shell, representing the true basis of the cirripede, still adhered to the upper surface of the membranous bits of cement. Nevertheless, so imperfect is the calcareous basis, that I should not be surprised if portions of a true membranous basis did really in some cases exist. _Mouth_: labrum with the notch wide, generally with a few little teeth; mandibles with three teeth, and some inferior knobs; maxillæ notched. _Cirri_, first pair with one ramus shorter by three segments than the other ramus. Second and third pairs short, of nearly equal length, thickly clothed with spines; segments very little protuberant. Posterior cirri, having elongated segments, supporting seven pairs of spines.
_Var. sordidus._--This form is very common on the tidal shores of the Strait of Magellan, and of the southernmost parts of Tierra del Fuego, near Cape Horn: it lives attached to rocks, mytili, and logs of wood, and is associated with _Chthamalus scabrosus_. It almost certainly is the most antarctic form of the genus Balanus. If I were guided by external appearance alone, I should certainly separate this form specifically from _B. flosculus_, but, as will be seen in the following description, the differences consist only in _var. sordidus_ being much duller and rather differently coloured, in the longitudinal folds being sharper and more prominent, and in the whole shell being rather more globular, and on an average rather larger; but in the true _B. flosculus_ there is considerable variation in all these respects, as there likewise is in _var. sordidus_; thus some of the cylindrical varieties of the latter have less prominent ridges than even _B. flosculus_. In general appearance I have seen some _nearly_, but not exactly, intermediate forms; therefore, I do not feel positive that these forms may not be specifically distinct, but have failed, after careful examination, to find any sufficient diagnostic _characters_. Moreover, in the case of _Balanus lævis_, I was led to believe that there is an equal and somewhat analogous amount of variation in the specimens inhabiting Tierra del Fuego and northern Chile; and in this case I was enabled to show the existence of strictly intermediate forms in the intermediate districts.
The shell in _var. sordidus_ is generally globulo-conical, dirty white, frequently with a green tinge, from the growth of confervoid matter. Orifice small. The exterior surface is covered with numerous prominent, narrow, sharp ribs or folds, the basal margin being serrated with projecting points where the folds terminate. When the radii are not developed, the sutures are very often obscure. Internally, the shell is faintly tinted of a port-wine purple. In all points of structure this form is identical with the true _B. flosculus_. In some few specimens the whole exterior surface was disintegrated and smooth; and this is generally the case with the upper parts of the shell. Some other specimens, which had grown crowded together on wood, had become cylindrical, and consequently the orifice was as large in diameter as the shell, namely, half an inch: in some of these cylindrical varieties the sheath was entirely soldered to the walls. The largest specimens which I have seen were .6 of an inch in diameter; and above one inch in height. _Affinities._--This species, in its opercular valves, even in such trifling characters as the strength of the opercular membrane, and in its cirri, approaches closely to _B. cariosus_. We even see on the under side of the scutum, in that species, a single little ridge, foreshadowing, as it were, the crests for the lateral scutal depressores, so remarkable in our present species. In the structure of the shell and of the basis, _B. flosculus_ is much more closely related to the last species, or _B. imperator_. If it had been possible to have arranged the species in a single line, _B. flosculus_ ought undoubtedly to have been placed between _B. cariosus_ and _imperator_.
41. BALANUS BISULCATUS. Pl. 8, fig. 6 _a_-6 _c_. BALANUS SULCATINUS (?) _Nyst_, apud D'Omalius (sine descript. aut tabulâ), Geologie de Belgique, 1853.[100] [100] I am indebted to M. Bosquet for a specimen, bearing this name and reference, found in the 'Systeme Bolderien' of Dumont, (miocene according to Sir C. Lyell) at Bolderberg. The specimen consists of a rostrum, with a portion of the base attached; and as these parts are in some degree characteristic, I fully believe this specimen to be the _B. bisulcatus_. I hope hereafter to give in the Palæontographical Series fuller illustrations of this and the following fossil species. _Radii with their upper margins oblique and smooth; sutural edges smooth: basis permeated by large pores. Scutum narrow, with from two to four longitudinal furrows: tergum with the spur very short, broad as half the valve._ _Var._ plicatus, _with the walls deeply folded; radii narrow, with their upper margins very oblique_. _Fossil_ in Coralline Crag, Ramsholt, Gedgrave, Sutton; Mus. S. Wood, Bowerbank, J. de C. Sowerby. Rauville, dans le Cotentin, Mus. G. B. Sowerby. _Var. plicatus_, Coralline Crag, Sutton, Mus. S. Wood. Bolderberg, near Hasselt, Belgium, Mus. Bosquet. _General Appearance._--Shell conical or tubulo-conical, often rather globose; walls frequently thin, either very smooth, or deeply plicated longitudinally: occasionally the same specimen is smooth in the upper part, and strongly plicated in the lower. The Radii in the larger specimens are wide, and with their upper margins only slightly oblique; in the smaller, they are narrower and much more oblique, but in each case their upper margins are smooth and slightly bowed. Colour apparently originally nearly white, but with the alæ generally, in the smaller specimens, clouded with a dark tint: the radii are usually striped feebly in longitudinal lines. Basal diameter of largest specimen .8 of an inch; but this seems to have been an unusual size.
_Scuta_: narrow, with the basal margin forming an unusually small angle with the occludent margin; surface slightly convex, with lines of growth approximate, moderately prominent; on the tergal half of the valve, two distinct rather broad furrows, with sometimes a third, and even a fourth, nearer to the occludent margin (Pl. 8, fig. 6 _a_), extend from the apex down the valve, and give it a very peculiar appearance: the furrows near the tergal margin are the deepest. Internally, the upper part of the valve is roughened with small points: the articular furrow is unusually wide: the articular ridge is very prominent and but little reflexed, with the lower end almost abruptly cut off: the adductor ridge is prominent, but short: there are small deepish pits for the rostral and lateral depressores. _Terga_ (6 _b_), broad, flat, with a slight narrow prominent rim along the scutal margin, which margin is slightly bowed. The basal margin on the carinal side of the spur slopes so gradually towards the spur, that the latter is barely distinct, and is very short, not depending nearly half its own width beneath the basi-scutal angle: it is broad, namely, measured across the upper part, as broad as half the valve; its basal end is obliquely rounded off on the carinal side; it is placed close to the basi-scutal angle. The carinal margin of the valve is just perceptibly bowed, and is formed by rectangularly upturned lines of growth. Internally, the upper part of the valve is rough; the articular ridge is prominent; the crests for the tergal depressor muscles are moderately well-developed.
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