A Monograph on the Sub-class Cirripedia · Charles Darwin
3. PLATYLEPAS ----?
_Scuta._--The outline of these valves varies considerably in specimens taken out of the same group: we have either a nearly equilateral triangle (fig. 1 _f_) or the tergal margin (1 _e_) is much shorter than the other margins. There is always a deep depression for the adductor muscle, and a small pit of very variable depth for the lateral depressor. But the tergal margin offers the greatest variability; here we see a very prominent articular ridge or fold, having either a straight edge or a single or double prominence (fig. 1 _e_-1 _h_). In specimens in the same group we find considerable variation in these points; but the amount of difference is sometimes so great, that I long hesitated whether to rank some of the varieties as species. The _Terga_, likewise, vary greatly in shape and width: in some of the commonest varieties (and in _var. depressus_) the valve is very narrow, with the under surface channelled or concave: in other varieties the valve is much broader and flatter. The spur is but slightly developed. The crests for the depressor muscles barely descend beneath the basal margin of the valve. The articular ridge, in some varieties (as in specimens from Madeira and the Cape de Verde Islands), is extremely prominent and straight (1 _f_); in others, it is little prominent and deeply sinuous (1 _h_). This great variability in the articular margins of the scutum and tergum seems to be mainly due to the corrosion to which these valves have been subjected, and their consequently modified growth: in some specimens the articular ridge of the scutum, and in others that of the tergum, has been largely developed, in either case their mutual outlines being greatly affected.
_Branchiæ._--These are narrow, hardly at all plicated, elongated, being about half as long as the sack. In a specimen from La Plata, this organ ended rather more abruptly in a point than it did in other specimens. _Mouth._--The crest of the labrum is usually hairy, but in a specimen from Bahia (Brazil) there were some very fine teeth. The palpi vary somewhat in shape, and sometimes have a row of bristles along their basal exterior margins. The mandibles usually have four main teeth, the lowest one being confluent with the inferior pectinated angle: in _var. depressus_, in the same individual, there were only three teeth on one side of the mouth and five on the other; the lower main teeth are laterally double, but generally one tooth of each pair is so small and obscure as to be perceived with difficulty. In the maxillæ, there are some large spines above the notch, and in the notch some fine ones: in _var. depressus_, and in a cylindrical _var._ from La Plata, there was quite a tuft of small spines above the notch.
_Cirri._--The outer surface of the pedicel of the second cirrus bears a tuft of long, fine, plumose hairs: the terminal segments of the rami of this cirrus sometimes (as in the La Plata specimens) support a clump of coarsely pectinated spines. In specimens having six segments in the shorter ramus of the second cirrus, the shorter ramus of the third cirrus had fifteen segments. The two rami of the third cirrus are usually equal in length and in the number of their segments; but in the Brazilian specimen there were fifteen segments in the posterior, and twenty-six in the anterior ramus; in another specimen, fixed on a tropical Perna, there were in the two rami of this third cirrus eighteen and twenty-four segments. In the three posterior pairs of cirri each segment carries either four or five pairs of main spines: the segments vary a little in the degree to which they are elongated, being most elongated in the var. from La Plata, with an elongated shell.
_Varieties._--It will have been observed, that the shell, in the specimens from several distant quarters of the world over which this species, as I believe, ranges, differs considerably in external aspect: so do the opercular valves; and so do the parts of the mouth and cirri: but I cannot make out that these differences are coordinated. Thus, _var. depressus_, which is so entirely different from the others in appearance, differs only internally in the presence of a tuft of fine spines above the notch of the maxillæ; and this character is found in the La Plata variety, which, as far as the shell and opercular valves are concerned, is at the other end of the scale of variation. Again, _var. fragilis_, from Charlestown, presents, in the animal's body, hardly any difference. The Brazilian specimens, which in the shell and operculum offer only quite common characters, have the remarkable peculiarity of a considerable difference in the length and number of the segments in the rami of the third cirrus; they, also, have the segments of the sixth cirrus considerably elongated, and the labrum finely toothed. Of these peculiarities one alone, namely, the inequality in the rami of the third cirrus, but in a lesser degree, is common to the specimens adhering to the tropical Perna, which had a shell very unlike the Brazilian variety, but which, on the other hand, differed scarcely in a single character from some other specimens from an unknown tropical sea, in which the rami of the third cirrus were quite equal. The La Plata specimens differ most in internal characters, viz., in the tuft of fine spines above the notch of the maxillæ, in the coarsely pectinated spines on the tips of the second pair of cirri, in having the segments of the sixth cirrus much elongated, and in the apex of the branchiæ being abruptly pointed; yet in the shell and operculum they were identical with certain Mediterranean varieties. From these several facts, I must believe that all the widely distributed forms here grouped together, do really belong to the same species.
2. CHTHAMALUS ANTENNATUS. Pl. 18, fig. 2. _Shell conical, generally smooth: when not deeply corroded of a pale dirty flesh-colour: sutures always distinct: radii, when present, with their sutural edges quite smooth._ _Hab._--New South Wales, (Moreton Bay, 27° S.; Sydney; Twofold Bay), Van Diemen's Land (Hobart Town). Attached to littoral rocks and shells; Mus. Brit., Cuming, Darwin. _General Appearance and Structure of Shell._--Shell conical, rather smooth; when not much corroded, of a pale dirty flesh-colour; often covered by membrane; sometimes deeply corroded, extremely rugged, and then of a brown colour; in this condition not much punctured, as generally is the case with _C. stellatus_. Sutures almost always quite distinct; rarely the shell becomes cylindrical with the sutures obliterated. Orifice moderately elongated, sub-hexagonal. Radii rather narrow, but not so narrow as in _C. stellatus_, smooth, with their upper margins very oblique: when disarticulated their edges are quite smooth. The edges of the alæ are sometimes crenated, and sometimes not so, being only marked by lines of growth; they are often rather thick. The parietes are usually rather thick, with their internal surfaces smooth, and not mamillated, as is so often the case with _C. stellatus_. The largest specimens which I have seen, were .6 of an inch in basal diameter. _Opercular Valves._--These are hardly distinguishable from those of _C. stellatus_. The only very slight difference which I can point out is, that the crests for the tergal depressores are less spread out, and depend rather more beneath the basal margin of the valve; and lastly, that the surface of the tergum, just above these crests, is rather prominent.
_Branchiæ_: these are oblong; taper but little, and have a broad rounded end: they are scarcely plicated. _Mouth._--The crest of labrum is hairy: the palpi are square, and have no bristles along their basal exterior margins, but long ones at their truncated ends. The mandibles have three or four main teeth apparently single: the inferior coarsely pectinated portion is short. The maxillæ are deeply notched. _Cirri._--The first and second pairs, and portions of the third, are darker coloured than the three posterior pairs. The rami in both of the first two pairs are slightly unequal in length. The third cirrus is much longer than the second: in a specimen in which there were six segments in the shorter ramus of the first and second pairs, there were twenty segments in the posterior and shorter ramus of the third cirrus; and in this same individual there were no less than forty-nine segments in the anterior ramus. In another specimen (Pl. 29, fig. 2) the number of segments in the two rami of the third cirrus, was 20 and 41; in another, the numbers were 18 and 53; in several other specimens the numbers varied in about these proportions; but in one single specimen the numbers were equal. Not only did the number of segments thus vary in the two rami, but likewise the arrangement of the spines on the segments in the anterior and longer ramus; in some specimens the spines on all the segments were arranged in a single circle, and then the organ had a specially antenniformed appearance: in other specimens, some of the lower segments (in one case thirteen in number) had the spines placed in regular pairs precisely as on the posterior ramus, and as on the three posterior pairs of cirri. Under the genus I have pointed out the resemblance between this structure and that occurring in certain Macrourous Crustaceans. The pedicel of the third cirrus had its spines more crowded and irregular than on the three posterior pairs of cirri. The segments in the latter vary in bearing either three or four pairs of main spines. The whole dorsal surfaces of the lower segments of the several posterior cirri are serrated in an upward direction by short spines, but to a variable degree.
3. CHTHAMALUS CIRRATUS. Pl. 18, fig. 4 _a_, 4 _b_. _Shell white or gray: sheath and opercular valves generally clothed by fimbriated membrane: tergum, with its basi-carinal angle depending and pointed._ _Hab._--Peru, Chile, Chiloe, Northern Chonos Islands. Attached to littoral rocks, and sometimes to littoral shells, often mingled with _Chthamalus scabrosus_; Mus. Brit., Cuming, Darwin. _General Appearance and Structure of Shell._--Shell dirty white or gray: sometimes tinted pale purple within; irregularly conical, or much depressed, or cylindrical and much elongated. Generally much corroded, sometimes well preserved and covered by membrane. Orifice rather large, of variable shape. Sutures often quite obliterated. Radii when developed narrow, with their sutural edges, as well as those of the alæ, generally very finely crenated, but to a variable degree. The membrane lining the sheath and covering the opercular valves, is remarkable from each zone being fimbriated; for this expression is more correct than to say that the membrane bears a row of spines, though the fimbriæ do approach in character to spines; sometimes, though rarely, the fimbriæ are branched. The largest specimens which I have seen (from Coquimbo and Valparaiso) were half an inch in basal diameter, and some of these were so much elongated as to be one inch in height. _Scuta._--The scuta are rather narrow: they have a somewhat peculiar appearance, from the articular furrow being wide, and from the articular ridge projecting with a uniform curvature: the pit for the lateral depressor muscle has some minute crests, of which I have seen traces in the foregoing species. The _Terga_ vary somewhat in shape: they have the basi-carinal angle of the valve, where the narrow crests for the depressores are placed, pointed and dependent, and the surface of the valve above these crests is prominent. Altogether the opercular valves have a sufficiently distinct character to be recognised without much difficulty.
_Mouth._--The crest of the labrum is not toothed; the palpi have long hairs along the exterior basal margin. The lower main teeth of the mandibles are plainly laterally double. _Cirri._--The pedicel of the second cirrus is extremely broad, and on the exterior margin supports a tuft of very long, finely plumose spines: in some specimens each of the lower segments of the anterior ramus of this cirrus bore one or two very large spines, doubly and extremely coarsely pectinated. In two specimens the rami of the third cirrus were of equal length: but in one specimen (from Iquique, Peru), having seven segments in the shorter ramus of the first and second pairs of cirri, the posterior ramus of the third pair had fourteen segments, and the anterior ramus twenty-two segments. The posterior cirri have segments carrying five pairs of main spines: the dorsal surfaces of the lower segments are serrated. 4. CHTHAMALUS FISSUS. Pl. 18, fig. 6 _a_, 6 _b_. _Shell brownish, plicated; orifice twice as long as broad: tergum triangular, equilateral, with the basal and carinal margins slightly protuberant._ _Hab._--California, attached to _Lottia grandis_. Peru(?); Mus. Brit. _General Appearance and Structure of Shell._--Shell globulo-conical, irregular, with the walls much folded. Colour brownish. Sutures generally distinct in young specimens, and nearly obliterated in full-grown individuals. Radii, when present, very narrow. The orifice of the shell, in full-grown specimens which have their summits a little worn, is narrow, much elongated, about twice as long as wide, oval or ovate, with the rostral end the narrowest: the elongation of the orifice offers almost the only character by which this species can be externally recognised. The lateral compartments are rather wider than usual in proportion to the rostro-lateral compartments. Basal diameter of largest specimen .2 of an inch.
_Scuta_, elongated transversely, with the pit for the adductor muscle bordered on the under side by an adductor ridge rather more prominent than usual in this genus. _Terga_, triangular, equilateral, with the margins slightly curved and protuberant: the basal margin is regularly and equably curved from one end to the other. Neither the _Mouth_ nor _Cirri_ offer any peculiar characters distinct from the genus. I may mention, however, that the crest of the labrum is toothed, and that the segments of the posterior pairs of cirri support five pairs of main spines. The characters by which this species differs from _C. stellatus_ and from the other species, consist almost exclusively in the triangular and equilateral terga, and in the much elongated orifice of the shell; and these differences I believe to be of specific value. I must, however, confess that I have examined one young specimen attached to a _Pollicipes elegans_, in which the orifice was not nearly so much elongated, and in which the terga were not so equilateral, with the basal margin not quite equably curved, but more protuberant on the scutal than on the carinal side: from an examination, however, of only one specimen, and that a young one, I cannot decide on its specific nature. 5. CHTHAMALUS DENTATUS. Pl. 18, fig. 3 _a_-3 _c_. CHTHAMALUS DENTATUS. _Krauss_ (!). Die Südafrikanischen Mollusken, 1848, tab. 6, fig. 27.
_Shell dirty white or brownish: sutures formed by interlocking teeth: tergum with the carinal margin protuberant._ _Hab._--South Africa, Natal; West Africa, Loanda and the Gold Coast; West Indies (?). Attached to ships' bottoms and to littoral shells, and to _Tetraclita serrata_, _Balanus perforatus_, and _amphitrite_; often attached to _Balanus tintinnabulum_ and _amphitrite_ on ships' bottoms. _General Appearance and Structure of Shell._--Shell dirty white, pale brown, or gray: conical, moderately depressed: walls either broadly and irregularly folded (fig. 3 _a_), with the surface corroded, or (when attached to ships' bottoms and sometimes to other Cirripedes) narrowly and regularly folded (3 _b_), with the surface well preserved, smooth, and generally covered by thin brown membrane. These latter specimens generally have the shell more steeply conical, with the orifice rather smaller, and the radii broader, than in the first-mentioned specimens, which are attached to coast-rocks and shells, and have had their summits worn down. The sutures in all cases are tolerably distinct, and have their edges toothed and interlocked: the crenations are visible before the compartments are disarticulated, when viewed either internally or externally, but occasionally they are obscure. The radii, when best developed, are rather narrow, and of equal width on both sides of the sutures, with their summits rounded: their surfaces are finely ribbed transversely, each rib corresponding with, or rather forming a point of, the toothed edge. On the under side these teeth usually are a little hollow or are pitted: the radii, in fact, may be said to be folded, like the parietes, only more symmetrically and narrowly, so that the points in the opposed edges interlock. The edges of alæ are serrated, but more finely than the radii. The parietes are rather thin, with their under surfaces generally smooth. The sheath does not descend far down the shell. The colour of the corium lining the sack and the animal's body varies considerably, being either almost black or pale purple; and the specimens adhering to ships' bottoms are internally almost white. Of these latter specimens, I have seen some .6 of an inch in basal diameter: of corroded specimens attached to littoral shells, I have not seen one quite .4 of an inch in basal diameter.
_Scuta_, with the articular ridge very prominent: the pit for the adductor is deep, and there are generally some distinct, though minute, pits for the lateral depressores. The _Terga_ (fig. 3 _c_) have the articular ridge very prominent; and the carinal margin is rather more arched and protuberant than in the other species. _Mouth._--The crest of the _labrum_ is toothed: the palpi are short, truncated, with long spines arising from their ends, and along the basal exterior margin. In the mandibles the first tooth is seated rather far from the succeeding teeth: the inferior part is coarsely pectinated: the maxillæ are deeply notched. The tips of second pair of _Cirri_ have many coarsely pectinated spines: the shorter ramus had six segments, whilst the shorter ramus of the third pair had eighteen segments: the segments on the posterior cirri carry five pairs of main spines. _Branchiæ, &c._--These consist of a double fold, the outer one being the largest, and the inner semi-circular, as has been described under the genus. In a young specimen, only one tenth of an inch in basal diameter, the branchiæ consisted of a single fold; in a specimen a little larger, there were two folds, but these were of equal size: ultimately the outer fold increases in size so as to become nearly double the inner fold. The _Ovarian tubes_ are remarkable from their large diameter. I was surprised to observe in the specimen only one tenth of an inch in basal diameter, that the larvæ were ready to escape. On the prosoma, there are some longish _hairs_ arranged in short rows, and some few on the membrane lining the sack, and some on the opercular membrane and valves.
It may be seen in Pl. 18, fig. 3 _a_, and 3 _b_, that I have here united two varieties considerably different in external aspect: I have done this without hesitation, inasmuch as there are intermediate forms, and as the differences are analogous with those so commonly occurring in sessile cirripedes; dependent on whether or not the specimens have been exposed to corrosion. I have seen both varieties from Natal, and both from the west coast of Africa; although the smooth, well-preserved, narrowly plicated varieties seem more common in western than in southern Africa. With respect to the range of the species, I have seen a specimen from the West Indies, but it was the variety which so commonly adheres to ships' bottoms. This variety often arrives alive in British ports; and I have seen a specimen picked up dead on the beach near Dublin. 6. CHTHAMALUS HEMBELI. Pl. 18, fig. 5 _a_-5 _d_, _e_. EURAPHIA HEMBELI. _Conrad._ Journal Acad. Nat. Sc. Philadelphia, vol. 7, 1831, Pl. 20, fig. 6. _Shell dull reddish purple: sutures, when not obliterated, formed by interlocking teeth: basis sometimes rendered calcareous by the inflexion of the parietes: scutum with two or three furrows extending down the middle of the valve._ _Hab._--California, near S. Diego, according to Conrad. Mus. Brit., Cuming. I have seen five, old, large specimens, from an unknown locality, with their whole surfaces deeply corroded, and with most of the sutures obliterated; and three separated valves of a young specimen. From these materials, imperfect as they are, I feel no hesitation in identifying this species with the _Euraphia Hembeli_ of Conrad, which is remarkable in several respects, and especially from being gigantic in size, compared to other members of its sub-family.
_General Appearance and Structure of Shell._--The young specimen (fig. 5 _b_, about .7 of an inch in basal diameter) consists only of the carina, and the two lateral compartments; but these, as far as I can judge, resemble the specimen figured and described by Conrad, which was two inches in diameter. Shell depressed, spreading, surface moderately smooth, covered by brownish membrane: shell itself pale dull reddish-purple. The radii are not very narrow, with their summits rounded and very oblique: their edges are toothed, and their external surfaces are transversely ribbed, in correspondence with the interlocking points of the sutures. On the internal surface, the toothed suture is not visible, except near the base of the shell, owing to the overlapping of the alæ. The alæ have oblique summits, which are slightly notched owing to the upturned prominent lines of growth. The parietes are thick; their basal internal surfaces are rugged, with slightly branching ridges.
The old specimens (fig. 5 _a_) are so much corroded that not a particle of the external surface is left: one of them which was 2-3/4 of an inch in basal diameter. Shell much depressed, spreading; colour pale purple; orifice large, rhomboidal, with a slight hollow on each side for the corners of the scuta: sutures generally in part or wholly obliterated; where still preserved, the interlocking toothed structure is distinct; the sheath is strongly marked by lines of growth, and is of a dark brown colour. The carina and rostrum are of unusually large size compared with the lateral compartments; and this, as far as I can judge, must have been the case with the younger specimens. That portion of the rostro-lateral compartment which forms part of the sheath, is reduced to a mere ridge. The most remarkable character is, that all these old specimens (of which the smallest measured nearly one inch and a half in diameter) had a flat, wide, calcareous basis, which is absolutely continuous with the inner lamina of the parietes, whereas in the younger specimen there was no appearance of any tendency in the parietes thus to grow inflected. There can be hardly any doubt that in a series of specimens some would be found with the parietes first forming a flat narrow ledge round the true basal membrane (as in the following species); and that in others, this ledge would be wider and wider, till its edges met in the middle, and coalesced into a continuous plate.
_Opercular Valves._--I have seen these only in the old corroded specimens (fig. 5 _c_, 5 _d_): they are locked together by remarkably strong articular ridges and furrows. The _Scuta_ have externally two or three impressed lines or narrow furrows, proceeding from the apex to the middle of the basal margin; these can be seen only in one of my specimens, owing to the degree to which the valves have suffered disintegration; but they are mentioned by Conrad. The basal margin is rather short compared with the other two margins. The tergal margin is remarkable from the depth of the upper furrow above the articular ridge, and from the size of the prominence (appearing like the true apex of the valve) above this upper furrow. There is a hollow for the adductor muscle, and traces of crests for both the rostral and lateral depressores. The _Terga_ are generally but little corroded, and hence the dark brown membrane with which they are covered is well preserved; the shelly matter itself is also brown: there is only a trace of this colour in the more corroded scuta: the external surface of the terga is very smooth. The spur is pretty well developed, and is half as wide as the whole valve. The lines of growth are upturned along the carinal margin. The articular ridges and furrows are much developed. The crests for the depressor muscles are extremely strong; they depend beneath the basal margin, and are remarkable (fig. 5 _c_) from being furnished each with fine sub-crests.
Text from Project Gutenberg, public domain in the United States.