A Monograph on the Sub-class Cirripedia · Charles Darwin
SECOND SECTION OF THE SUB-FAMILY OF BALANINÆ.
_Geographical Distribution._--I have received only four specimens with certain localities attached to them: namely, the Arctic Seas of Scandinavia; the coast of the United States, and of Britain; and the Gulf-Stream. There is also a specimen in the British Museum, sent by Mr. Stephenson, mingled with shells of Mollusca from New Zealand; but a Coronula, procured from a floating whale in the early part of the outward voyage, might so easily be sent home with specimens subsequently collected in another county, that I do not as yet fully admit that this species is an inhabitant of the Southern Pacific Ocean: I am less willing to admit this, from suspecting that _C. reginæ_ in the Pacific, replaces the _C. diadema_ of our Northern Seas. 3. CORONULA REGINÆ. Pl. 15, fig. 5; Pl. 16, fig. 4. _Shell globulo-conical or depressed, with longitudinal, much flattened ribs, having their edges crenated, and their surfaces striated and granulated; orifice hexagonal: radii thin, not exceeding one fifth of the thickness of a compartment: terga absent._ _Hab._--Attached to whales, Pacific Ocean; Mus. Cuming, Stutchbury, and Darwin.
I have seen eight specimens of this form--two in Mr. Cuming's collection, one of which was procured at Iquique in Peru, the other from an unknown locality; one was given me in Chile, and was stated to have come from the Pacific; two others are in Mr. Stutchbury's collection, from unknown but distinct localities, and three in a group in the British Museum. These eight specimens, of which six were collected separately, all strikingly agree in general aspect and in details of structure, so that I can easily recognise the shell, and can at once pick out a single compartment, when mingled with those of the two foregoing species; nevertheless, the differences are so small, that I at first hesitated whether to name the species; but, upon reflection, I am convinced that it is distinct. _Coronula reginæ_ is much more closely related to _C. diadema_ than to _C. balænaris_, and I suspect that it replaces in the Pacific the former of these two species.
_General Appearance_; shell conical, straight-sided; some specimens being nearly as much depressed as _C. balænaris_, and considerably more depressed than any variety of _C. diadema_; other specimens being globulo-conical, but rather less globular than the ordinary form of _C. diadema_. I may here remark, that shape is of more value in this genus, in which the shell is attached to the yielding skin of whales, than in those genera in which it adheres to rocks. The orifice is neatly hexagonal, and the whole internal cavity, when the opercular membrane is removed, can be seen from one point of view, owing to the contraction of the lower part of the cavity and small size of the basal membrane. The surface of the shell is smooth, but has, even up to the summit, a peculiar frosted appearance, different from that of the foregoing species, caused by the longitudinal striæ being rather more distinct, and being crossed by beaded, very delicate transverse lines of growth. The ends of the transverse loops, forming the exterior surfaces of the compartments, are much flattened, even more so than in _C. balænaris_. The lines of junction between the loops, are finely serrated, as in _C. diadema_; and internally they are solidly filled up, instead of being formed into a set of tubes by longitudinal septa, as in _C. balænaris_. The under side of the shell, with its folded walls, presents an appearance intermediate between the variable appearance of this part in the above two species.
The sutural edges of the radii (Pl. 16, fig. 4) offer by far the most remarkable character, in their thinness from top to bottom; for they hardly exceed one fifth of the thickness of the compartment, measured from the external surface to the base of the sheath; hence a very large cavity is left between the radii and alæ: in the thickness of the radii the three species already described form a series, _C. reginæ_ at one end and _C. balænaris_ at the other. The sinuous plates, forming the lower part of the radius, are coarser and stand rather further apart than in _C. diadema_. The alæ are thick, and have the same outline, being narrow at their basal margins and broad at top, as in _C. diadema_, with their sutural edges similarly constructed: the basal edge of the sheath likewise projects freely. _Operculum._--This resembles most closely that of _C. diadema_. There are no rudiments of terga. The scuta cannot be distinguished from those of _C. diadema_. _Mouth._--The labrum has a row of inwardly curved little teeth along the whole crest, and these I did not notice in _C. diadema_: there is only a trace of the prominence on the outside at the bottom of the central notch. The hairs on the basal exterior margin of the palpi are moderately long. The mandibles have five teeth. In the _Cirri_, the first pair resembles in its peculiar structure those of _C. diadema_. In the sixth pair, however, the segments support only three pairs of main spines: but the specimen was not very large, and probably in old specimens there would have been four pairs.
_Summary._--The present species differs from both the foregoing only in its rather more conical and straight-sided outline, smooth, frosted surface, and in the narrowness of the sutural edges of the radii, and consequent large size of the chambers between the radii and alæ. It resembles _C. diadema_, as far as the shell is concerned, in the external ribs or transverse loops having their lines of junction serrated and in being solidly filled up--in the shape of the orifice and of the internal cavity of the shell--in the shape and structure of the alæ--and in the basal edge of the sheath being free. It comes nearer to _C. diadema_ than to _C. balænaris_ in the structure of the radii. It differs from _C. diadema_, and comes nearer to _C. balænaris_, in the external ribs being flattened, instead of being convex, and in the lines of growth being very delicate. But as it resembles _C. diadema_ in the several foregoing characters of its shell, in the opercular valves, in all parts of the mouth (excepting the labrum), and in the cirri, it is very much more nearly related to that species than to _C. balænaris_. 4. CORONULA BARBARA. Pl. 15, fig. 6. CORONULITES DIADEMA (?) _Parkinson_, Organic Remains (1811), vol. 3, p. 240, Pl. 16, fig. 19. _Shell (probably) crown-shaped, with longitudinal convex ribs, having their edges crenated, and their surfaces rugged, both externally and internally, with transverse ridges: radii moderately thick; the spaces between the radii and the alæ solidly filled up._
_Fossil_ in Red Crag, (Bawdsey and Sutton); Mus. S. Wood and Geological Society. The species here described, though near to _C. diadema_ and easily confounded with it, I have no doubt is distinct. I owe to the kindness of the Rev. Mr. Image an examination of the original specimen figured by Parkinson; and in Mr. Stutchbury's collection there is a similar and more perfect specimen; both of these resemble _C. diadema_ in general form, but have been too much worn to be positively identified. The following description is drawn up from some compartments collected by Mr. Searles Wood, belonging certainly to three, and probably to four individuals, one of which was young; as these specimens agree in all essential respects, I feel pretty confident that the characters, by which the present species differs from _C. diadema_, are of specific value.
_Structure of Shell._--The longitudinal ribs on each compartment (_i. e._ the terminal transverse loops), are convex and prominent, as in _C. diadema_, but they are crossed by more prominent ridges of growth than even in the roughest varieties of that species, so that the surface of the shell is more rugged. In the three previous species, the surface of the wall entirely round the cavities occupied by the whale's skin, is striated only by longitudinal very fine lines; but here, the outer portion, or that (fig. 6) formed by each transverse loop, is crossed by transverse ridges of growth, like, but less prominent than, those on the external surface of the shell. The minute teeth, along the lines of junction between the transverse loops, are here less regular, and can hardly be said to exist; for the two edges are locked together by what may be more strictly called minute zig-zag ridges than teeth. The exact number of the circumferential plications in the walls is variable in the same manner as in the three foregoing species. The sutural edges of the radii are about as thick as, or rather thicker than, in _C. diadema_.
As in this latter species, and in _C. reginæ_, each ala rests, not on the internal surface (as in _C. balænaris_, and in all other Balanidæ) of the radius, but on a special plate; but in _C. barbara_, instead of a deep chamber, running up to the apex of the compartment, being left between the radius and ala, this part is filled up almost entirely by solid shell. Although the extent to which this chamber is filled up varies a little, and although its depth varies a little in _C. diadema_, yet there is a marked difference between the specimens of this latter species, in which the chamber is most filled up, and those of _C. barbara_, in which it is least filled up. The alæ are thick, as in _C. diadema_, and their sutural edges have a central ridge, sending off on both sides sinuous ridges. The basal margins of the alæ are not short compared with their upper margins, and therefore the whole ala is not wedge-formed; and in this rather important respect _C. barbara_ resembles _C. balænaris_ (Pl. 16, fig. 3), and differs from _C. diadema_ (fig. 2). The lower edge of the sheath does not seem to have projected freely,--in this respect, also, resembling _C. balænaris_. From the basal margin of the alæ not being narrow, and from the inner ends of the folded walls being, as it would appear, also broad, I have little doubt that the cavity in which the animal's body was lodged, resembled in shape that in _C. balænaris_, the membranous basis being larger than the orifice of the shell.
Opercular valves unknown. _Summary._--This species is more nearly related to _C. diadema_ than to the others; but in some points, just specified, it resembles _C. balænaris_. The characters by which it differs from all the species are, firstly, the more prominent transverse ridges on the external surface of the shell, and more especially on the surfaces bounding the outer sides of the cavities occupied by the epidermis of the whale. Secondly, the character of the teeth, or rather ridges, along the lines of junction between the transverse loops. And, thirdly, the spaces between the radii and the plates on which the alæ rest, being solidly filled up. _Species Dubiæ._ The _Coronula bifida_ is an Italian tertiary species, so named by Bronn, in his "Italiens Tertiär-Gebilde" (1831), p. 126. It is very possible that this may be identical with _C. barbara_, but Bronn does not seem to have been aware of the absolute necessity of giving minute details in his descriptions of fossil cirripedes. The chief character of _C. bifida_ is thus given:--"Eine tiefe Furche oder Spalte theilt die Längenrippe von oben herab bis zur Hälfte, welche bei der sonst ähnlichen _C. diadema_ entweder ganz fehlt, oder nur zuweilen kurz angedeutet ist." Had it been stated that the longitudinal ribs were divided from the middle down to the base, instead of from the top to the middle, the description would have been intelligible to me, though the character thus afforded would not have been of specific value, as this dividing of the ribs occasionally occurs in all four species, and is produced by the formation of new folds to the walls.
9. _Genus_--PLATYLEPAS. Pl. 17, fig. 1 _a_-2 _b_. PLATYLEPAS. _J. E. Gray._ Annals of Philosophy, (new series), vol. x (1825). CORONULA. _De Blainville._ Dict. des Sciences Nat. (1824). COLUMELLINA. _Bivona_ (fide _Philippi_). Nuovi generi, &c. di Molluschi, Palermo (1832). _Compartments six, each bilobed and inwardly produced, so as to form six midribs, which support the outwardly convex, membranous basis._ _Distribution_, throughout the tropical and warmer temperate seas. Imbedded in turtles, sea-snakes and manatee. This small genus, consisting of three species (though I have named only two), is a very natural one, yet closely allied to Coronula. Most authors have united these genera, but in doing so they destroy two very natural little groups. Platylepas, moreover, when all its characters are considered, has as good a claim to be generically separated from Coronula, as has Tubicinella,--a genus universally admitted. I shall presently recur to this subject.
_General Appearance._--The most remarkable character, and which gives a peculiar aspect to the whole shell, is that each of the six compartments has a medial fissure or fold; the shell thus consisting of twelve lobes. Each fold extends from the summit to the base of the shell, and is produced inwards in the shape of a midrib (Pl. 17, fig. 1 _a_). The membranous basis is supported by the basal edges of the six midribs, and is thus rendered convex. As the midrib on the rostrum would have interfered, if fully developed, with that part of the animal's thorax which I have called the prosoma, it is of less size than the other midribs; and from this same cause the whole rostrum is pushed a little on one side, generally to the left, so that the shell is not perfectly symmetrical. The midrib of the carina is less than the lateral midribs. The orifice is oval, and is more or less plainly indented by six angular points,--the worn-down summits of the six midribs. The narrow elongated opercular valves stretch from one end of the orifice to the other. The rostrum is much broader than the carina: the lateral compartments are but little broader than the carino-lateral compartments. The radii are narrow, though variable in width. The general shape of the shell varies much in the same species, being either much depressed, or rather steeply conical; either circular, or more commonly oval. The (so called) epidermis is usually persistent in the lower part of the shell. The shell itself is white. The basal diameter of the largest specimen of _P. bissexlobata_ was nearly three-quarters of an inch.
_Structure of the Parietes, Radii, &c._--The walls of the shell in _P. bissexlobata_ are permeated by minute pores, and have exactly the same structure as in Coronula; the pores being completed by the union of ledges on the outer sides of the longitudinal septa, and the latter have little knobs formed on them at each period of growth. In _P. decorata_ the walls are solid. Each compartment, as already stated, has a deep nearly medial furrow, the sides of which are closely pressed together, thus forming the midribs, and thus differing from the furrows in Coronula, which not being pressed together, form cavities on the under side of the shell. The inner longitudinal margin of each midrib is thickly coated by a layer of shell, and is thus rendered much more prominent than it would have been if formed exclusively by the inward folding of the wall. The upper part of each midrib forms a slight, longitudinal ridge (fig. 1 _d_) on the sheath, having become encased by the sheath during its downward growth. Owing to the small size of the midrib of the rostrum, there is scarcely any ridge on the sheath of this compartment. The sheath does not descend half-way down the walls in _P. bissexlobata_, but further down in _P. decorata_; but perhaps the thickened inner margins of the midribs may be considered as the downward prolongations of the sheath, and on this view the sheath descends actually to the basal membrane. The radii are narrow: their edges have simple septa, with the interspaces filled up solidly. The alæ have their sutural edges smooth.
The _Basis_ is membranous, and more or less convex. This convexity is rendered possible by the support afforded by the basal edges of the six midribs. The so-called epidermis or membrane which covers the shell, is prolonged in six flaps under the midribs; and hence the proper basal membrane has six deep indentations, receiving the above flaps. The cementing apparatus is constructed on the same simple type as in Coronula, as has been described in the Introduction. _Scuta and Terga._--These are narrow and long, stretching the whole length of the orifice of the shell. The two valves touch each other, but are not properly articulated together: their upper surfaces scale off in layers: they resemble each other very closely in shape (fig. 1 _c_), being simply oblong. The membrane covering the valves supports some very minute spines, 2/1000th of an inch in length. When a portion of valve is dissolved in acid it is seen to be penetrated by very numerous tubuli. The opercular membrane is rather thick: in _P. decorata_ I found it occasionally double; in this species it supports some very minute spines; it is attached about one third down the sheath, of which the upper part is not marked (as in Balanus) by regular concentric rings. The aperture between the valves, leading into the sack, has protuberant lips, as in Coronula and Tubicinella. The normal five bundles of muscles surrounding the sack, differently from in the two genera just mentioned, do not thin out and spread out downwards, but retaining their full thickness, reach the basal membrane, to which they are attached.
_Mouth_: the parts present no difference from those in Coronula; between the outer maxillæ we have the same small flattened mentum-like projection; and in the mandibles, the same small intermediate teeth between the second and third, and between the third and fourth main teeth, as in that genus. The main teeth here are more plainly double laterally. There are teeth on each side of the central notch of the labrum. The cirri differ little from those of Coronula, but the segments are not nearly so much flattened and broad, and the second and third pairs are not so short. The segments of the sixth pair support four pairs of main spines, with a few minute intermediate spines, but with no intermediate tuft. _Branchiæ_: these consist on each side (at least in _P. decorata_) of a double fold, much less plicated than in Coronula; the fold nearest to the animal's body is rounder, and not quite so large as the outer fold. _Range and Habits._--The _P. bissexlobata_ ranges from the Mediterranean and west coast of Africa to the West Indies, to the north-east coast of Australia, and, judging from the name given by Chenu, to California: the second species, _P. decorata_, inhabits the Pacific Ocean; and the third imperfectly known species, was deeply buried in the skin of a sea-snake, off Borneo. The _P. bissexlobata_ is attached both to turtles and to the skin of the manatee: some specimens in the British Museum, attached to the latter, from the west coast of Africa, were entirely buried in the skin, with the exception of the operculum. The specimens of _P. decorata_, which I obtained at the Galapagos Archipelago, were buried about half their depth in the softer parts of the skin of the green turtle. I do not doubt that the imbedment is effected in the same manner as in the case of Chelonobia.
_Affinities._--This genus, in general structure and habits, approaches closely to Coronula; but it differs from all the species of that genus in the following respects, which appears to me fully to justify its generic separation:--The outline is often oval, instead of circular; the compartments are not of equal sizes and shapes; each compartment has here only one inward furrow or fold, and this has its sides pressed closely together, and is encased, but not obliterated, by the sheath; the rostrum is not quite medial; and the radii are narrow and nearly solid. The opercular valves, again, are here more developed, and their microscopical structure is different. The opercular membrane is less constantly double; and, what is much more important, the muscles of the sack are not here spread out, and show no tendency to become rudimentary and lose their transverse striæ. The cirri differ only in not being so broad and flattened. The branchiæ are not quite so much developed. Finally, I may add, that in several of the above respects in which Platylepas differs from Coronula, it approaches ordinary sessile cirripedes. 1. PLATYLEPAS BISSEXLOBATA. Pl. 17, fig. 1 _a_-1 _d_. CORONULA BISSEXLOBATA. _De Blainville._ Dict. des Sciences Naturelles, tom. 32, (1824), Tab. 117, fig. 1. PLATYLEPAS PULCHRA. _J. E. Gray_ (!). Annals of Philosophy, (new series), vol. 10, (1825).
COLUMELLINA BISSEXLOBATA. _Bivona_ (fide _Philippi_). Nuovi generi di Mollusch. (1832), Tab. 3, fig. 1.[126] CORONULA CALIFORNIENSIS. _Chenu_ (!). Illust. Conch., Tab. 1, fig. 4. [126] This memoir was published in the 'Effemeridi Scientifiche e Litt. per la Sicilia,' according to the 'Bibliographia Zoologiæ et Geologiæ,' by Agassiz and Strickland. _Shell with the transverse lines of growth conspicuous: parietes permeated by pores; sheath descending barely half-way down the parietes._ _Hab._--Mediterranean, attached to turtles. River Gambia, attached to manatee. Honduras, attached to manatee. Moreton Bay, lat. 27° S., Australia, apparently attached to the dugong of that coast. California(?) Mus. Brit., Stutchbury, and Cuming. _General Appearance._--Shell generally much depressed, and broadly oval or circular; sometimes steeply conical. Orifice oval, generally not large. Surface rather plainly marked by closely approximate lines of growth, which with the lobed outline gives to the whole an elegantly sculptured appearance: occasionally the longitudinal ridges formed by the parietal septa are distinct. Basal diameter of largest specimen three quarters of an inch. _Structure of Shell._--I have nothing material to add to the generic description. The midribs are not so prominent as in the following species, and hence the basal membrane is less convex. The origin of the midrib as a fold is very plain. The sheath descends barely half-way down the walls, and is a little hollow on its under margin, on each side of the midrib. Beneath the sheath the parietes are finely ribbed (fig. 1 _d_), but to a variable degree. I may here remark, that in the specimens taken from manatee, on the coast of Africa and at Honduras, the internal ribs extended further up and were plainer, and the opercular valves seemed to be a little narrower than in the other specimens, so that I at first suspected that they were specifically distinct, but I could make out no other than these small and variable points of difference.
_Scuta_: oblong (fig. 1 _c_), about twice as long as broad, with the rostral end rounded, rather narrower than the other end, and curled a little inwards. _Terga_, of nearly the same shape and nearly as long as the scuta: the carinal end is rather more pointed than the scutal end of the valve, and when viewed internally, the growing surface of this end is seen to be bluntly pointed. In both valves the upper layers of shell usually scale off. _Mouth and Cirri._ The only differences, compared with the following species, that I perceived, were that the hairs at the end of the palpi were here rather longer, and the fourth tooth in the mandibles rather less distinct. The rami of the first cirrus are very unequal in length. 2. PLATYLEPAS DECORATA. Pl. 17, fig. 2 _a_-2 _b_. _Shell with fine longitudinal ridges, ornamented in the lower part by minute beads; parietes not porose: basal membrane equalling in convexity the shell._ _Hab._--Pacific Ocean; Galapagos Archipelago; Lord Hood's Island, Low Archipelago; Mus. Cuming, Darwin.
_General Appearance and Structure._--Shell oval, with the orifice large. The walls are thick, and of less height from top to base than in the last species. The surface (and this is the chief external character) is marked by fine longitudinal ridges, each of which, when closely examined, is found to be double (fig. 2 _b_), and near the basis, where not abraded, to be ornamented with minute beads on each side: these beads are largest on those ridges which border the sutures. The parietes are not permeated by pores; on their inner surfaces there are a few rather prominent but short ridges, instead of the many finer ridges, as in _P. bissexlobata_. The sheath is of great thickness; in young specimens layers scale off its summit, all round the shell, as in Tubicinella; it is only slightly hollowed out at its lower margin; it descends more than half-way down the walls. The midribs, formed by the folded walls, are more prominent than in the last species, and descend lower down; hence the basal membrane is more convex than in the last species, for it projects downwards as much as the shell projects upwards. In medium-sized specimens, the midribs project inwards to a distance exceeding half the breadth of a compartment. A less proportional length of the midrib, in an inward direction, is formed by the inward folding of the wall, and a greater length by the thickening of its inner longitudinal margin, than in the last species. The basal edge of the inner end of the midrib is smooth.
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