A Monograph on the Sub-class Cirripedia · Charles Darwin
3. PLATYLEPAS ----?
With respect to the affinities of this genus, we have seen that both in the included animal's body, and, at an early age, in the shell, it is extremely close to Octomeris: in the presence of caudal appendages, and in the basis being calcareous, we have a clear affinity with Catophragmus; in the two lateral compartments of _P. aurantiacum_, tending to become blended together, we have some relationship exhibited to Chamæsipho, as is likewise shown in the structure of the second and third pairs of cirri. If the genera of the Chthamalinæ were ranged in a circle, Pachylasma would be the point of contact with the Balaninæ. I must repeat, that it is extremely unfortunate that when the shell alone of _P. giganteum_ is examined, it is hardly possible to separate this genus from Balanus. 1. PACHYLASMA GIGANTEUM. Pl. 19, fig. 5 _a_-5 _d_. CHTHAMALUS GIGANTEUS. _Philippi_ (!). Enum. Mollusc. Siciliæ, 1836. _Shell and operculum dirty white: carino-lateral and lateral compartments furnished with similar alæ._ _Hab._--Mediterranean; Sicily; deep water; often attached to the _Millepora aspera_, and sometimes associated with _B. tulipiformis_. _Fossil_ in the tertiary beds, near Messina. Mus. Brit., Cuming, Stutchbury, Lyell, Philippi. I owe to the kindness of Dr. Philippi, authentic specimens of his _Chthamalus giganteus_, from the tertiary beds of Messina: had this distinguished naturalist seen recent specimens, or a fossil one with all the valves united, he would no doubt have perceived that this species cannot be classed with Chthamalus. I am also indebted to Sir Charles Lyell for some magnificent specimens, which he himself collected near Messina.
_General Appearance._--Shell conical, rugged, irregular, with the lines of growth plain; colour dead dirty white. Orifice large, diamond-shaped, narrow towards the carinal end; notched. Radii narrow, barely distinct from the parietes; in the same individual sometimes absent, and sometimes forming a mere ribbon, confined to the lower edge of a compartment. The diametric growth is effected by the alæ, which seen externally are broad, and strongly marked by lines of growth. Basal diameter of largest recent specimen 1.15; height of highest compartment (a carina) 1.4; and width at base of widest rostrum .85 of an inch. Amongst the fossil specimens, height of highest compartment (a carina) 1.8, width of the same one inch; width of widest rostrum 1.1 of a inch. One of these carinæ, a little below the middle point, was actually .3 of an inch in thickness, from which circumstance I have given the generic name of Pachylasma. _Scuta_, triangular, but the width varies a little: growth-ridges prominent, sinuous, with a few slight furrows radiating from the apex of the valve. Internally, the articular ridge is not very prominent; nor is there a deep depression for the adductor muscle, and none for the lateral depressor muscles. The upper part of the valve projects freely.
_Terga_, broad, broader than the scuta: the growth-ridges are prominent, and angularly upturned close along the scutal margin; the carinal half of the valve is smooth, with the faintest traces of longitudinal striæ. A portion of the valve, nearly half of its entire width on the scutal side, is slightly depressed below the general level, and depends slightly beneath the basal margin on the carinal side: this evidently forms the spur. The carinal margin is nearly straight, with the lines of growth upturned along it. Internally, the articular ridge in the upper part is extremely prominent: the crests for the depressor muscles are very prominent, and depend beneath the basal margin like a comb: they extend over nearly half the basal margin, and the muscle, in a corresponding manner, is unusually spread out.
_Structure of Shell and Radii_: the compartments are attached to each other less strongly than in any other cirripede which I have examined, so that when dried specimens are soaked in spirits of wine they generally fall to pieces with a touch. In full and half-grown specimens the carino-lateral compartments are nearly as broad as the lateral compartments; in very young specimens, about 1/20th of an inch in basal diameter, they are proportionally much narrower. The _walls_ are strong even in young specimens; in old ones they attain a thickness I have scarcely seen equalled except in Chelonobia. Their internal surfaces are smooth, as is the basal internal margin in young specimens, but in old specimens it is roughened with short, blunt ridges and little points. The _Radii_ are often absent; when present they are very narrow, and consist merely of a ribbon-like portion, formed by obliquely upturned layers of growth, more prominent than on the parietes. The _Alæ_ are very largely developed; they are added to, during the diametric growth of the shell, in a regular sweep all the way down to the basal margin, and consequently they do not form a rectangular shoulder as is usual; externally they are plainly marked by lines of growth: they are added to a little above the line of attachment of the opercular membrane; their summits are very oblique. The sheath has its basal edge slightly hollowed out.
_Basis_, solid, calcareous, very irregular, and of variable thickness. I have, under the Genus, alluded to the structure of the rostrum: in one shell, the basal diameter of which barely exceeded 1/20th of an inch; the compound rostrum (being 5/200ths of an inch in height), had its basal margin (being 9/200ths of an inch in width) rendered deeply sinuous (see Pl. 19, fig. 5 _b_) by two indentations, corresponding with and caused by two notches at the top of the valve. These two notches extended down barely 1/100th of an inch (strictly 4/1000ths) from the summit and then disappeared; so that when the shell was under 1/100th of an inch in height, (only one distinct zone of growth having been formed), the now compound rostrum consisted of three separate compartments; and there were eight compartments altogether. Of the above three little compartments, the middle one, or true rostrum, had large alæ, which could be most distinctly seen, extending on both sides, under the little rudimentary rostro-lateral compartments. These latter overlapped the compartments on both sides of them, as in all the Chthamalinæ. _Mouth_: labrum bullate, with no central notch; nor is the inner fold of the labrum, forming the supra-œsophageal cavity, thickened, as in Balanus: minute muscles run from this inner fold straight back to the cavity formed by the outer bullate fold: the crest of the labrum is hairy, with a row of the minutest bead-like points or teeth. _Palpi_, small, broad, placed almost parallel to the sides of the mouth, with their apices not nearly touching each other. _Mandibles_, with three large nearly equal-sized _single_ teeth; the whole inferior angle strongly pectinated. _Maxillæ_, small, with a broad, square notch beneath the two or three great upper spines.
_Cirri_: first pair short, with the rami equal in length. Second pair, with the anterior ramus having broader segments than those of the posterior ramus, and with all the segments, except the few uppermost, thickly covered with spines; the posterior ramus has rather less than half the segments thickly covered. Third pair, very slightly shorter than the sixth pair; anterior ramus with the lower segments, less than half of the whole in number, thickly covered with spines; posterior ramus with only the lowest segments, about one fifth of the entire number, thickly covered; the other segments of these two rami, and the upper segments of the posterior ramus of the second pair, closely resemble in the regular arrangement of their spines in pairs, the three posterior pairs of cirri. The pedicel of the third pair supports numerous, irregularly scattered bristles. The segments of the sixth pair bear four or five pairs of main spines, with a few intermediate spines. _Caudal Appendages._--Multiarticulate, narrow, tapering, situated on each side of the anus: each segment has two little tufts of spines on each side of its upper edge. These appendages are about one third longer than the pedicel of the sixth pair of cirri: in a specimen, in which the rami of the sixth pair had twenty-three segments, the caudal appendages had nineteen segments.
_Penis_ short, hairy, finely-ringed, with no projecting point at its dorsal basis. _Branchiæ_ moderately large, nearly circular, not much plicated. _Fossil Specimens._--With respect to the Sicilian specimens sent me by Dr. Philippi and Sir C. Lyell, I can see no difference whatever from the recent specimens, excepting in their greater size and thickness; it must, however, be borne in mind that I have seen only half-a-dozen recent shells. The one fossil scutum which I have seen is rather broader than is usual with the recent, but I have seen one nearly as broad. This species seems to have been extremely common when the beds at Messina were deposited, and probably it attained a larger size than it does at present. The compartments are always found separated, which is easily understood, by the facility with which, as above stated, recent specimens fall to pieces. 2. PACHYLASMA AURANTIACUM. Pl. 20, fig. 1 _a_-1 _d_. _Shell tinged with orange; viewed externally seems formed of only four compartments, owing to the carino-lateral and lateral compartments on each side being separated only by an obscure fissure._ _Hab._--New South Wales; apparently from deep water, attached to sandstone.[135] [135] I am indebted to Mr. Bowerbank for this unique and interesting species, which I have deposited in the British Museum.
_General Appearance._--Shell conical, smooth, with a tinge of orange colour; orifice large, deeply notched, sub-triangular. Viewed externally, the compartments in appearance are only four; but on close examination, the lateral compartments are seen to be divided by a very fine fissure into two nearly equal compartments. The rostrum is broad and flat, and when carefully examined, it also is seen to be divided by two fine fissures into three compartments; of these the middle one, or true rostrum, is a very little broader than the rostro-lateral compartment on each side; hence, on careful examination, the shell is found to consist of eight compartments. The carina is much compressed. The radii (in my one specimen) are not developed. The carina, and the two lateral compartments alone, have alæ; for the carino-lateral compartments are too closely joined to the lateral compartments, and the true rostrum is too closely joined to the rostro-lateral compartments, to have their alæ developed. The four alæ which are developed, are very broad, widely exposed, and marked externally by lines of growth. The basal diameter of my one, apparently old specimen, is one inch.
_Structure of Shell, &c._--The inner surface (fig. 1 _b_) of the rather thick parietes is smooth, except close to the base, where it is roughened by a few irregular points. The sheath descends low down, and has its lower edge slightly free. The sutures are much plainer on the internal than on the external surface, and can be here plainly seen to be eight in number. That portion of the carino-lateral compartment, which helps to form the sheath, is narrow, though the whole compartment is of very nearly equal width with the lateral compartment. It is a singular fact, that no portion of the rostro-lateral compartment helps to form the sheath; for the alæ of the lateral compartments, overlap the whole upper part of the rostro-lateral compartments, and abut against the true rostrum. Hence, when the sheath alone is examined, the number of the compartments appears only six. In a section the true rostrum can be seen to underlie the rostro-lateral compartments, and thus exhibits vestiges of alæ. The fact of the rudimentary rostro-lateral compartments not forming a part of the sheath offers a marked difference from Chelonobia, which otherwise has this part of the shell very similarly constructed. The diametric growth of the shell, which seems to be considerable, is effected by the four large alæ of the carina and of the lateral compartment on each side. The sutural edges of the alæ are added to in a regularly inclined line down to the basis. The _basis_ is calcareous, and not very thin.
Both opercular valves (fig. 1 _c_, 1 _d_) closely resemble those of the last species. The _Scuta_ are remarkably narrow and elongated: the external surface is slightly furrowed longitudinally, the prominent lines of growth are much wrinkled. The articular ridge is blunt, and not very prominent. The upper part of the valve is reflexed, and a considerable portion must have projected freely. The _Terga_ are nearly twice as broad as the scuta: the carinal half of the valve is very smooth, and is coloured beautiful reddish-orange. The spur is tolerably distinct. The articular ridge is not very prominent. The _Mouth_ resembles that of the last species. The mandibles have three sharp teeth, with the inferior part narrow and pectinated with long spines: the edge, also, between the upper main teeth, is pectinated with short spines, which latter often have their summits crenated. The _maxillæ_ are notched; the inferior corner is produced into a small step-formed projection. The outer maxillæ are more pointed than is usual. The _Cirri_ most closely resemble in every detail those of _P. giganteum_; I can point out only one slight difference, namely, that the lower segments, in the posterior ramus of the third cirrus, which are thickly clothed with spines, are more numerous in proportion to the upper segments with the spines arranged in regular pairs (being as 4 to 14), than in _P. giganteum_, in which only one fifth (or 4 to 20) are so clothed.
_Caudal Appendages._--These are very small, not being more than one fifth of the length of the pedicel of the sixth cirrus: their segments are indistinct, and they support a very few coarse spines. The rami of the sixth cirrus, in the one specimen, had twenty-five segments, whilst each caudal appendage had only five. Hence the caudal appendages are far less developed than in _P. giganteum_. 15. _Genus_--OCTOMERIS. Pl. 20. OCTOMERIS. _G. B. Sowerby._ Zoological Journal, vol. 2, p. 244, July, 1825. _Compartments eight: radii with their edges crenated: basis membranous._ _Distribution_, Cape of Good Hope; Philippine Archipelago. The two species of this genus differ considerably in external appearance, though not in essential character. Both, as the name expresses, have eight compartments: the carino-lateral pair are rather narrower than the lateral. The basis is membranous. The radii are narrow, and are distinctly crenated on both sides of the sutures with the teeth neatly interlocking; but these teeth can hardly be distinguished in the large, corroded specimens of _O. angulosa_. The crenated structure of the radii is identical with that described under _Chthamalus dentatus_ and _Hembeli_; if, indeed, we were to add carino-lateral compartments to the shells of these two species, they would belong to Octomeris. I have seen only a few specimens of either species of Octomeris, and none preserved in spirits; and therefore I know nothing of the anatomy of the softer parts: I was not able to make out distinctly any branchiæ. The cirri differ considerably in the two species, in nearly the same way as in the two species of Chamæsipho. In the structure of the second pair of cirri, and in the tendency of the basal margin of the parietes to form bay-like indentations, _Octomeris angulosa_ shows some special affinity to _Chamæsipho scutelliformis_. Under Pachylasma, I stated that that genus was closely related to Octomeris; and I have just alluded to the close affinity of the latter to the division of the genus Chthamalus, which has crenated radii.
1. OCTOMERIS ANGULOSA. Pl. 20, fig. 2 _a_-2 _b_. OCTOMERIS ANGULOSA. _G. B. Sowerby._ Zoological Journal, vol. 2, July, 1825. And Genera of Recent and Fossil Shells, Plate. ---- STUTCHBURII. _J. E. Gray._ Annals of Philosophy, new series, vol. 10, August, 1825. ---- AUGUBRA (?) _Chenu._ Illust. Conch., Tab. 4, fig. 2. _Shell dirty white, rugged and massive: alæ thick, with their sutural edges coarsely crenated._ _Hab._--Algoa Bay, Cape of Good Hope. Attached to littoral rocks; often associated with _Balanus Capensis_ and _Chthamalus dentatus_; Mus. Brit., Cuming, Stutchbury, Bowerbank. _General Appearance and Structure of Shell._--Shell extremely rugged, irregular, massive, generally much corroded, steeply conical or even sub-cylindrical: orifice large, broad, rhomboidal, of nearly equal breadth at both ends. Colour dirty white, often slightly tinted yellow from the investing membrane, and from thin layers of punctured membrane alternating with the laminæ of shell. The parietes, in old specimens, have very irregular longitudinal ridges, or rather plates projecting out, sometimes much branched, and generally curved inwards so as to meet each other, thus forming round the basal margin a circle of cylindrical apertures. In old large specimens the radii are not developed, and till the compartments are disarticulated there is no trace of the toothed structure of their sutural edges: in this condition the sutures exist as deep, rugged, narrow fissures. In younger shells, the radii, though narrow, are distinct, and have their surfaces transversely ribbed, and their edges toothed and interlocked with the teeth of the recipient furrow. Some of the specimens present a curiously deceptive resemblance to _Elminius plicatus_. Basal diameter of largest specimen, one inch and a quarter; height one inch.
The parietes are remarkably thick, but the compartments separate easily: I have, however, seen one instance in which they were partially calcified together. Their internal surfaces are very smooth. The sheath does not descend low. The alæ project rectangularly; they have thick edges, and these are coarsely crenated in transverse lines. Of the radii sufficient has been said. _Scuta_ (fig. 2 _b_): these, in all the specimens seen by me but one, have been deeply corroded, and their outline, as in Chthamalus, considerably modified: in the one specimen well preserved, the exterior growth-ridges were extremely prominent. The articular ridge does not project much, nor is the articular furrow very deep. There are more or less distinct crests for the lateral depressor muscles. The _Terga_ are rather narrow, with a small rounded spur, moderately distinct. The tips of the crests for the depressor muscle barely depend beneath the basal margin. _Mouth_: labrum considerably bullate, with the crest hairy and furnished with a few most minute teeth. Palpi small, with their tips not nearly touching each other. Mandibles with four teeth, of which the lower ones are laterally double: inferior pectinated portion small. The maxillæ are notched, but their outline differed on opposite sides of the individual examined. Outer maxillæ bilobed on their inner face.
_Cirri._--The first pair has the rami unequal in length by about three segments, having six segments on one and nine on the other ramus. In the second pair the anterior ramus is remarkably short and extremely broad; the five segments of which it is composed being thickly covered with bristles, some of which are very coarsely and doubly pectinated: the posterior ramus is nearly twice as long as the anterior ramus, and has eleven segments; of these, excepting the two basal segments which are rather thickly covered with spines, the others resemble in the arrangement of their spines the four posterior pairs of cirri, with the exception that there is a single transverse row of rather long spines along the upper and inner lateral edge of each segment. The segments in all the four posterior pairs of cirri resemble each other in bearing each four pairs of main spines (of which the two lower pairs are short), with a tuft of small intermediate spines. The penis is remarkable from its very small size and shortness, not being more than once and a half as long as the pedicel of the sixth cirrus. 2. OCTOMERIS BRUNNEA. Pl. 20, fig. 3 _a_, 3 _b_. _Shell reddish-brown, depressed, thin, finely furrowed longitudinally: tergum with the basal margin having a slight angular bend._ _Hab._--Philippine Archipelago; rare; Mus. Cuming.
_General Appearance and Structure._--Shell circular, much depressed; coloured brown, with an orange-red tint; surface regularly and narrowly furrowed in longitudinal lines, the intermediate rounded ridges projecting at the basal margin in finger-like points. The eight compartments, when disarticulated, are of nearly equal sizes; the carino-lateral and rostro-lateral compartments being rather smaller than the others. The radii are neatly toothed; but with the exception of these teeth, which are equally developed on both sides of the sutures, and which closely interlock in the lower part of the shell, but stand a little apart in the upper part, the radii can hardly be said to be developed. The alæ do not form a rectangular shoulder, the lower margin being regularly curved into the parietes; the upper margin is only slightly oblique; the external surface is furrowed by lines of growth. The shell increases a little in diameter, chiefly by the growth of the alæ. The parietes are not very thick, with the inner surface smooth, but with punctures placed in rows for the entrance of the tubuli: the laminæ of shell alternate with layers of yellow finely punctured membrane. Basis formed of thin membrane, in concentric slips. Basal diameter of largest specimen, .6 of an inch. _Scuta_ broad, with the lines of growth few in number, but extremely prominent, so as to form folds: basal margin with a very slight angular bend: articular ridge not very prominent. _Terga_, with a narrow slip along the scutal margin, having the lines or folds of growth so prominent as almost to form a series of small transverse pits: the basal margin of this portion forms an angle with the rest of the basal margin: no spur can be said to exist: the crests for the depressor muscle depend a little beneath the basal margin: articular ridge very prominent.
_Mouth_: labrum and palpi as in the last species. Mandibles with three main teeth, apparently single, of which the lower tooth has its upper edge pectinated: the inferior part of the mandible is pectinated as usual. The maxillæ have two notches, one beneath the two upper large spines, and the other in the middle, separating some thicker and thinner spines. _Cirri_: first and second pairs short, with the anterior rami in each longer by about two segments than the posterior rami; all the segments thickly covered with bristles. The third cirrus is much longer than the second cirrus, with the posterior ramus longer than the anterior ramus; on the latter the three basal segments, and on the posterior ramus the two basal segments are thickly covered with bristles; the other segments have bristles arranged as on the three posterior pairs of cirri, namely, each segment has four pairs of main spines, of which the two lower pairs are short. The unarticulated support, whence the articulated portion of the penis arises, is unusually long, equalling the pedicel of the sixth cirrus. 16. _Genus_--CATOPHRAGMUS. Pl. 20. CATOPHRAGMUS. _G. B. Sowerby._ Genera of Recent and Fossil Shells. Plate. _Interior compartments eight, with several exterior whorls of small supplemental compartments: basis either membranous or calcareous._ _Distribution_, West Indies and Australia. Attached to littoral shells and rocks.
This genus is very remarkable amongst sessile cirripedes, from the eight normal compartments of the shell being surrounded by several whorls of supplemental compartments or scales: these are arranged symmetrically, and decrease in size but increase in number towards the circumference and basal margin. A well preserved specimen has a very elegant appearance, like certain compound flowers, which when half open are surrounded by imbricated and graduated scales. The Chthamalinæ, in the structure of the mouth and cirri, and to a certain extent in that of the shell, fill up the interval between the Balaninæ and Lepadidæ; and Catophragmus forms, in a very remarkable manner, the transitional link, for it is impossible not to be struck with the resemblance of its shell with the capitulum of Pollicipes. In Pollicipes, at least in certain species, the scuta and terga are articulated together--the carina, rostrum, and three pairs of latera, making altogether eight inner valves, are considerably larger than those in the outer whorls--the arrangement of the latter, their manner of growth and union,--all are as in Catophragmus. If we, in imagination, unite some of the characters found in the different species of Pollicipes, and then make the peduncle so short (and it sometimes is very short in _P. mitella_) that the valves of the capitulum should touch the surface of attachment, it would be impossible to point out a single _external_ character by which the two genera in these two distinct families could be distinguished: but the more important differences in the arrangement and nature of the muscles which are attached either to the opercular valves or surround the inside of the peduncle, would yet remain.
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