A Monograph on the Sub-class Cirripedia · Charles Darwin
3. PLATYLEPAS ----?
_Compartments six: basis membranous, but sometimes in appearance calcareous, owing to the inflected parietes._ Distribution mundane; attached generally to littoral rocks and shells. This, the typical genus, is the largest and widest distributed group of the sub-family Chthamalinæ. The founder of the genus apparently did not perceive its essential character; Savigny, however, as is evident from the excellent figure in the great work on Ægypt, perfectly understood the difference between Chthamalus and Balanus. I was first indebted to Dr. J. E. Gray for explaining to me this difference; but the only published account which I have met with is in a paper by the Rev. R. T. Lowe,[130] in which he states, on the authority of Mr. Clark of Bath, that in Chthamalus the anterior compartment or rostrum has alæ like the posterior compartment or carina, the anterior or rostro-lateral compartments being destitute of alæ. These characters being exactly reversed in Balanus, as I have already explained (p. 176) under the sub-family of Balaninæ. [130] 'Zoological Journal,' vol. 3, p. 76, 1828.
The shell, owing apparently to its containing much animal matter, is particularly subject to disintegration; and when thus much affected it is quite impossible to distinguish the species by external characters. It is, in fact, best to cast on one side external appearance, though when the shells happen to be well preserved, each species has its own peculiar aspect. We have in this genus smooth and plicated, cylindrical and depressed varieties of most of the species. The development of the radii is very apt to vary, and even the compartments often become so completely united and calcified together that the sutures are almost or quite obliterated. A more serious difficulty in discriminating the species, arises from the fact of the opercular valves, not only varying extremely in external appearance in consequence of the greater or less disintegration of their apices, and consequent exposure of their articular ridges and furrows (compare fig. 1 _a_, 1 _b_, 1 _c_, in Pl.
18), but from their truly varying in outline with the varying shape of the shell: this latter circumstance is probably due to the opercular membrane which unites the valves to the shell being very narrow, and in consequence, differences in the shape of the shell affect the opercular valves, in a manner and to a degree to which the Balaninæ are not subject (compare fig. 1 _e_, 1 _f_, 1 _h_, in Pl. 18). The scuta, on the other hand, differ to an unusually slight degree in the different species. In the common _Chthamalus stellatus_, which abounds on the southern British shores, the whole external aspect of the shell is often so completely masked, owing to its varying shape, its obliterated sutures, its deeply disintegrated and punctured surface, and by the corroded condition of its opercular valves, that I have found this species, in the collections of naturalists who have attended to cirripedes, arranged actually on the same tablet, mingled with specimens of _Balanus balanoides_. I have myself several times found it less troublesome to discriminate these two genera by the included animal's body than by the shell,[131] though the latter, when well preserved and developed, possesses such obviously well-marked characters: the same thing has occurred to me with some of the other species. [131] It will be found ultimately to save time and trouble, to soak for half an hour in hot caustic potash, a specimen out of each group of shells to be examined, and then well wash and brush the separated valves and compartments; this process has been followed by me with all the species here to be described.
_General Appearance._--The shell is generally depressed, but sometimes, when growing in groups, cylindrical. The surface is either smooth or longitudinally folded; and, as already stated, very apt to be deeply disintegrated. The radii, when developed, are narrow, with their summits oblique and rounded; but they are often quite absent, and sometimes even the sutures are almost obliterated. The rostrum and carina are of the same shape and size, and the two lateral compartments on each side are of nearly equal breadths. The orifice is generally sub-rhomboidal, being widest towards the carinal, instead of towards the rostral end, as is usual in Balanus: but in _Chthamalus fissus_ the orifice is narrow and elongated. The opercular valves have their apices generally disintegrated and worn away, and are then seen to be deeply locked together. The colour of the shell is dirty white or dull purplish-red or brown; but in _C. intertextus_ rich violet-purple. The species are small, not often exceeding half an inch in basal diameter, with the exception of _C. Hembeli_, of which I have seen a specimen two and three quarters of an inch in diameter.
_Scuta._--These present no particular character, excepting in most cases the large development of the articular ridge, and sometimes the presence of a furrow above the articular ridge, of which only traces can rarely be detected in Balanus; hence in these cases, as in _C. Hembeli_ (Pl. 18, fig. 5 _a_, 5 _c_) and _scabrosus_ (Pl. 19, fig. 2 _a_, 2 _c_), the line of articulation between the scutum and tergum is more complicated than is usual. There is generally a slight pit, sometimes even furnished with small crests, for the lateral scutal depressor muscle: in _C. scabrosus_ (fig. 2 _d_, _q_) a part of this muscle is attached to a small pit at the basi-scutal corner of the tergum,--a fact of which I have observed no instance in any other genus. In _C. intertextus_ the terga and scuta are calcified together, without even a suture being visible on their internal faces. In _C. Hembeli_ the valve is externally marked by a few longitudinal furrows. The _Terga_, like the scuta, have a prominent articular ridge and deep furrow. In many specimens of _C. stellatus_ and in _C. scabrosus_ the valve is narrow: in _C. fissus_ it is triangular and nearly equilateral. The crests for the depressores muscles are well developed: in _C. scabrosus_ (Pl. 19, fig. 2 _d_, _p_) these crests are united into a plate, which, together with the outer lamina of the valve, forms a deep narrow pit: in _C. Hembeli_ the crests are furnished with small sub-crests. The opercular membrane is narrow; it is sometimes furnished with a few minute spines.
_Structure of Parietes._--The parietes are solid, and composed of successive layers of shell; the inner surface varies in condition in the same species, being either smooth or marked with branching impressed lines, or mamillated, or often irregularly punctured for the entrance of tubuli. Owing to these tubuli, the walls, when externally disintegrated, often become punctured. In certain depressed varieties of both _C. stellatus_ and _scabrosus_ the walls are supported by irregular depending columns, placed along either one side or both sides of the sutures. In _C. intertextus_ we have the remarkable character of the wall of the shell growing (I presume after a certain age) almost rectangularly inwards, thus forming a rather wide, flat, calcareous rim round the central basal membrane. In _C. Hembeli_ the internal basal edges of the parietes, in one moderately young specimen, were rugged with irregular points, but presented no other remarkable appearance; but in five old and very large specimens, the whole basis was calcareous, being absolutely continuous with the inner lamina of the parietes, showing that the latter had grown flatly inwards all round, and had then become confluent in the middle, so that there was no longer any basal membrane; excepting, no doubt, that which had existed in the younger stage, and which would be preserved in a functionless condition between the surface of attachment and the inflected parietes.
When an opercular valve or compartment is dissolved in acid, layers of tissue are left, and these are seen to be penetrated by tubuli, which enter at the punctures before mentioned on the inside of the shell: these tubuli often stand in groups of three or four together; they are about 1/10000th of an inch in diameter. Besides these irregularly scattered tubuli, there are in the opercular valves of _C. antennatus_, innumerable smaller parallel tubuli, running to the external investing membrane. _Structure of the Radii and Alæ._--The radii, when developed, are always rather narrow. Their recipient furrows are generally nearly as broad as the radii themselves. Their edges are either quite smooth, as in _C. antennatus_; or very finely crenated; or, as in _C. dentatus_ and _Hembeli_ (Pl. 18, fig. 3 _a_, 5 _a_), so strongly crenated as to make the suture, both externally and internally, toothed: in these two species, the radii are ribbed in transverse lines parallel to the basis, each rib corresponding with one of the projecting and interlocking teeth on the sutures. In _C. intertextus_, and much less plainly in most specimens of _C. scabrosus_ (Pl. 19, fig. 1 _a_, 2 _a_), we have a structure in appearance very different, for the radii here consist of several very oblique plates, (_i. e._ nearly parallel to the parietes) on both sides of the sutures, which are interfolded or locked together: I believe that this structure is a mere modification of that in _C. dentatus_ and _Hembeli_, the transverse ridges on the radii of those species being here developed into oblique plates. We shall hereafter meet with a similar structure in the genus Verruca; to which genus, until meeting with these two species of Chthamalus, I had thought that the interfolding sutures had been confined.
The alæ have their edges generally finely crenated: during diametric growth (when such takes place), they are rarely added to above the level of the opercular membrane, and hence their summits are oblique: in _C. intertextus_, however, the alæ are laterally added to above the opercular membrane, and their recipient furrows are likewise added to, of which fact I have seen no other distinct instance in any genus; hence on both sides of the sutures, in the sheath of this species, the lines of growth are upturned. In some much disintegrated specimens, both of _C. stellatus_ (var. _depressus_) and of _C. antennatus_, the radii have been corroded away, and the diametric growth is effected exclusively by the growth of the alæ, which are moreover much exposed, and rendered conspicuous. The sheath descends a moderate distance down the shell. When a shell is boiled in potash, the sutures (excepting when abnormally calcified together, as very often happens with some species) always fall apart, showing that the union is simply by animalised matter.
_Basis._--The basis is always membranous; but we have seen, in _C. intertextus_, that the walls form a flat ledge all round the base, and that in _old_ specimens of _C. Hembeli_, they grow so far inwards and become so completely confluent, that they might most easily be mistaken for a true calcareous basis. I may add, that in one elongated specimen of _C. stellatus_ from La Plata, the walls had likewise grown rectangularly inwards, forming a flat base, and had then turned upwards in the middle, forming a medial crest, with the edges not quite calcified together. The true basal membrane is very obscurely divided into concentric slips. I observed in several species, attached to the lower surface, an excessively fine network, quadrangular or hexagonal, of yellow vessels, which seemed insensibly to pass into the sheets, discs, and globules of cement, by which the membranous basis adheres to the supporting surface. I saw, in _C. antennatus_, numerous irregular, bifurcating, and inosculating cement-ducts, of unequal diameter, often crossing each other, and sending off branches ending in points: the older ducts, instead of being solidly filled up with cement, were only divided by septa. I did not succeed, in any species, in discovering the cement-glands. _Mouth._--The _labrum_ is slightly bullate, with the middle portion depressed, but not forming a notch; in some species it is hairy, and in some pectinated with short spines. The _palpi_ are of moderate size. The mandibles have from three to five main teeth, the number sometimes varying even in the same species: the lower teeth are either plainly double laterally, or very obscurely double, or to all appearance quite single: a rather large lower portion of the mandible is finely pectinated. The maxillæ are always notched under the two or three large upper spines: the notch bears some fine spines: beneath the notch there are some large spines, and at the inferior angle some smaller ones.
_Cirri._--The first and second pairs are always very short compared with the four posterior pairs. The rami of the first pair are slightly unequal. The third pair, in length and arrangement of the spines, very closely resembles the three posterior pairs; in _C. intertextus_, however, the few basal segments, chiefly on the anterior ramus, are thickly clothed with bristles, like the segments of the second cirrus. In _C. antennatus_ (Pl. 29, fig. 2), the anterior ramus of this same third pair is usually (one single specimen being excepted) much elongated, having at least twice as many segments as the posterior ramus, but the number is variable; and these segments, either all, or only the upper ones, instead of having their spines regularly arranged in pairs, in a double row, are surmounted each by a circle of spines: I suspect that these elongated rami of the third cirrus act as antennæ. It can hardly be an accidental coincidence, that certain genera, as Lysmata and Pandalus, amongst the Macrourous Crustaceans, have the same leg (homologically the second thoracic limb) elongated and antenniformed. Certain varieties of _C. stellatus_ and _cirratus_, also, have the anterior ramus of this same third cirrus considerably elongated. We are thus reminded of the remarkable variability in the numbers of the segments, and in the arrangement of their spines, in the cirri of _Tetraclita porosa_; in that species, however, it was chiefly the terminal segments of the _posterior_ ramus of the third cirrus which were so highly variable. The three posterior pairs of cirri in Chthamalus support from three to five pairs of main spines on each segment, the number often varying in the same species, with some minute intermediate bristles. The dorsal surfaces of these segments, in some of the species, are serrated in an upward direction.
_Body, &c._--The body does not present any particular character: in _C. scabrosus_ there is a slight ridge running from the base of the first cirrus towards the adductor scutorum muscle: this ridge is clothed with a few hairs; there are also some hairs at the carinal end of the sack. In _C. dentatus_, also, there are hairs on the outer tunic of the prosoma. In two species which I opened, there were no cæca to the stomach. The ova vary in length from 13 to 14/2000th of an inch in length; they are packed in two lamellæ lying on each side of the animal's body. The larva just escaped out of the egg, in _C. stellatus_, _scabrosus_ and _dentatus_, had a large probosciformed mouth. _Branchiæ._--These present a very singular amount of difference within the limits of the same genus. In _C. stellatus_ and _antennatus_ we have a simple fillet, tapering a little, barely plicated, and about half, or more than half, as long as the sack; in _C. scabrosus_ the branchiæ are entirely aborted, or are perhaps represented by the slight hairy ridge at the carinal end of the sack: in _C. dentatus_, on the other hand, each branchia consists of two large folds, barely plicated, almost covering the whole side of the sack, so that here the branchiæ are developed to an unusual degree, more than in Balanus, and as in Coronula and its allies: in the same manner as in these latter genera, the outer fold is considerably larger than the inner fold.
_Affinities._--_C. intertextus_ is the most distinct species of the genus, as shown by the peculiar radii and alæ, by the scuta and terga being calcified together, by the character of the third pair of cirri, and by the inflected parietes forming a ledge round the membranous basis; but in this latter respect _C. intertextus_ resembles _C. Hembeli_. _C. Hembeli_, in its serrated radii, is closely related to _C. dentatus_; and this latter species differs in the structure of its radii only in degree from certain varieties of _C. stellatus_. Lastly, _C. intertextus_, in its peculiar radii, closely resembles _C. scabrosus_, and this latter species does not differ much from the other species. Hence the genus Chthamalus has no claims to be subdivided into smaller genera. _Range._--The species are found all round the world, from (as far as I have seen) 54°-55° north, to Cape Horn, in 55°-56° south. All the species, of the habits of which I know anything, are littoral; and in many parts of the world are excessively numerous, quite covering large spaces of the coast-rocks, and often coating the coast-shells. _Chthamalus dentatus_ is littoral, like the other species; but it often lives attached on _Balanus tintinnabulum_ and _amphitrite_, on the bottoms of ships arriving in British ports from the west coast of Africa. I do not know of any instance of more than two species occurring in the same region. Some of the species have large ranges: _C. scabrosus_ extends from the Falkland Islands and Tierra del Fuego to Peru; and _C. stellatus_ has an enormous extension over almost the whole world, excepting the west coast of South America and Australia. I do not believe any species of the genus, owing probably to their littoral habits, have hitherto been found fossil.
1. CHTHAMALUS STELLATUS. Pl. 18, fig. 1 _a_-1 _h_. LEPAS STELLATA. _Poli._ Testacea Utriusque Siciliæ (1795), Tab. 5, fig. 18-20. ---- DEPRESSA (_var._) Ib., Tab. 5, fig. 12-16. CHTHAMALUS STELLATUS. _Ranzani._ Memoire di Storia Naturale (1820), Tab. 3, fig. 21-24. ---- GLABER (_var._) _Ranzani._ Ib. ---- STELLATUS. _Philippi_ (!). Enumeratio Mollusc. Siciliæ. LEPAS PUNCTATUS. _Montagu_ (!). Testacea Britannica (1803). _Shell white or gray, generally much corroded and punctured: radii (when present) narrow, with their sutural edges most finely crenated; tergum with the crests for the depressor muscle depending barely beneath the basal margin._ _Var._ (_a_, communis) fig. 1 _a_, 1 _f_: _Shell conical depressed, upper part corroded, walls folded, sutures moderately plain or obliterated; radii not developed; orifice broadly oval_. _Var. (b): Shell elongated, sub-cylindrical; sutures obliterated; surface much corroded; orifice almost circular._ _Var._ (_c_, communis) fig. 1 _c_, 1 _e_, 1 _h_: _Shell conical, folded, sometimes covered by membrane; radii developed, narrow; orifice sub-hexagonal, toothed._ _Var._ (_d_, fragilis) fig. 1 _d_: _Shell conical, smooth, thin; compartments easily separable, pale-coloured; radii developed, narrow; orifice large, toothed, sub-hexagonal._ _Var._ (_e_, depressus) fig. 1 _b_, 1 _g_, 1 _h_: _Shell much depressed, surface much corroded, smooth: alæ largely exposed, marked by lines of growth: radii not present; parietes on the under side often supported by pillars; orifice sub-hexagonal._ _Hab._--Southern shores of England, Ireland, Isle of Man, Mediterranean, Madeira, Cape de Verde Islands, Southern United States (Charlestown), West Indies, Brazil (Bahia), Rio Plata (Guritti Island), Red Sea, Philippine Archipelago, Coast of China, Gulf of Corea, Oregon or Northern California.
This species is very widely distributed and extremely common. On the coast-rocks of the southern shores of England it is, in parts, even more numerous than the _Balanus balanoides_, with which it often grows mingled. As already stated, it is often confounded in British collections with this species of Balanus, under the name of _Balanus punctatus_; that Montagu had this Chthamalus in view, when describing his _Lepas punctatus_, is certain, from his original specimen in the British Museum, but whether this was the case with his predecessor, Pulteney, in the Dorset Catalogue, I do not feel so sure. _Chthamalus stellatus_ varies, as we shall immediately see, extremely in appearance. Some of the varieties, as _var. depressus_, which in external aspect are especially distinct, I have no doubt are really varieties, but whether this is the case with some of the forms from the more distant localities, is a little more doubtful; but I beg that it may be observed, that I have, in the case of every one of the varieties, and of all the specimens from distant localities, cleaned with potash and most carefully examined the disarticulated valves, and likewise dissected the included animal's body.
_General Appearance._--British specimens are usually conical, and have their walls folded, corroded in their upper parts, with the radii not developed, and the sutures more or less obliterated; in this state the orifice of the shell is entire, and very broadly oval: I have seen specimens in this condition from Madeira, Brazil, and the Gulf of Corea. The shells when crowded, are rendered cylindrical, and more or less elongated, with the sutures as viewed externally quite obliterated; the surface rugged and much disintegrated; and the orifice nearly circular: in this extreme condition I have seen specimens from England, from near Genoa, and from the mouth of the Plata: the specimens from these last two localities were of remarkable size, being half an inch in diameter, and rather more than half an inch in height. Again, other British specimens (fig. 1 _c_), though not nearly so common, are rather steeply conical, and have not only their sutures distinct, but narrow radii are plainly developed on apparently both sides of the sutures; in this case the orifice is slightly toothed, and is rather elongated: I have seen specimens in this condition, but with their walls rather more deeply folded, from the Cape de Verde and Philippine Archipelago. Other specimens, from some unknown tropical sea, differed only in the walls being but slightly folded, and being so perfectly preserved as to be externally covered with membrane: this latter circumstance gave the specimens a peculiar appearance. In this condition were some specimens (fig. 1 _d_) attached to oysters sent to me by Professor Agassiz, from Charlestown; and which differed from all the others that I have seen, in the thinness and smoothness of their compartments, and in the facility with which the compartments separated from each other and from the surface of attachment: hence I have called this form, which, until finding more important differences, I must rank as a variety, _fragilis_: it has stronger claims than the other varieties to be specifically separated; but I suspect that it has been exposed to some peculiar modifying conditions.
Lastly, we have the variety (fig. 1 _b_) excellently figured and described by Poli, under the name of _L. depressa_: this is much depressed, with the walls thin, not folded, with the surface much corroded, with the sutures very distinct, with the radii not at all or barely developed, but with the alæ largely developed, and marked with lines of growth, resulting from the diametric growth of the shell: the orifice is hexagonal, but broadest towards the carinal end: most of the specimens, _but not all_, have on the under sides of their compartments rugged pillars depending from beneath the alæ, for the purpose, apparently, of supporting the much depressed shell. This much depressed variety attains a larger basal diameter (but not a greater bulk or internal capacity) than any other variety, namely, sometimes three quarters of an inch. The great peculiarities of this variety result, apparently, from its much depressed form, deeply corroded not folded walls, and considerable diametric growth; from the latter cause the alæ are largely developed; as I can find neither internally nor externally any fixed diagnostic character I have not hesitated to rank this form as a variety. Poli found his specimens mingled with the ordinary _C. stellatus_, on the shores of Sicily; and I collected at St. Jago, in the Cape de Verde Archipelago, some specimens nearly as well characterised, also associated with the common variety.
The shell itself is dirty-white or gray, or brown: in some varieties, however, the white is nearly pure: internally the parietes are generally tinted dull purple. The corium of the sack is dark greenish-black, with a white edge to the lips lining the aperture between the opercular valves. _Structure of Shell and Radii, &c._--The under side of the parietes is either quite smooth, or marked with slight, branching, depressed lines; or mamillated; or irregularly studded with large pores. I have already alluded to the pillars, depending from the under sides of the alæ in most specimens of _var. depressus_: these pillars tend to form ridges, parallel to the sides of the compartment, like those we shall presently see in certain specimens of _C. scabrosus_. The radii are very narrow, when best developed; their edges, when disarticulated, can be seen, when examined by a strong lens, to be finely crenated. The edges of the alæ are likewise very finely crenated.
Text from Project Gutenberg, public domain in the United States.