A Monograph on the Sub-class Cirripedia · Charles Darwin
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_Diametric growth._--When first examining groups of _T. porosa_, in none of which the radii had been developed, and in which, consequently, the shell could not have grown in diameter, but only at its basal margin, I was at first unable to comprehend, how the upper part of the shell and the orifice could have acquired their proper proportional width. The young shell, at its first formation, starts with an orifice so small that a pin could hardly be inserted in it; and this in many individuals is never increased in diameter by the diametric growth of the shell; but in place of this, as the conical shell is added to at its base, the whole upper part disintegrates and wears away, the orifice becoming thus enlarged. We thus see that the corrosion and wearing away of the upper part of the shell is a necessary element in its growth. The development of the radii, which in some of the species, as in _T. purpurascens_, at first seems to be quite capricious, really depends upon the fact, whether the specimens have been exposed to disintegration; for I have almost always found that when the outer lamina of shell has been well preserved, the radii have been developed, and the orifice has been enlarged by their growth, instead of by the wearing down of the upper part of the conical shell.
_Basis._--This consists of a very thin, flat, though irregular, translucent, calcareous plate, which towards the edges is sometimes membranous. In _T. purpurascens_, the basis is entirely membranous. When a portion of the calcareous base is dissolved in acid, a tissue is left, composed of several laminæ, to which numerous bifurcating cement-ducts are attached: even before dissolution, these delicate bifurcating ducts can just be perceived by the aid of a simple lens. _Mouth._--The several organs present no particular characters. There are generally three teeth on each side of the notch in the labrum. The palpi usually have parallel sides, but are club-shaped in _T. purpurascens_ and _costata_. The mandibles have generally four teeth, but there are five in _T. vitiata_, and only three in _T. costata_. The maxillæ are notched. The outer maxillæ are bilobed in front. _Cirri._--The segments of the three posterior pairs usually support only three pairs of main spines, but there are four pairs in _T. vitiata_ and _costata_: between each pair, there is either a tuft of fine spines, or a single fine spine. The rami of the first cirrus are unequal in length. In the third cirrus, the posterior ramus is sometimes much elongated, but sometimes both rami are short and blunt. Some of the segments in the third cirrus often support very coarsely and doubly pectinated spines. Under the head of _T. porosa_, it will be seen to what a remarkable degree the relative numbers of the segments in the several cirri vary, even in specimens taken out of the same cluster.
The _Branchiæ_ are well developed (at least in _T. porosa_ and _costata_), as a large, plicated, tapering fold, with a small second fold on the inner side at the base. In _T. porosa_ the _stomach_ is destitute of cæca. The _vesiculæ seminales_ in this same species are large, with their broad, blunt ends reflexed. The _ovarian_ tubes surround the sack, and cover the basal plate. _Distribution and Habitats._--This genus is confined to the tropics, and to the warmer parts of the temperate seas: in the southern hemisphere, it ranges south, to the Cape of Good Hope and to Van Diemen's Land: in the northern hemisphere it does not appear to range so far; I do not know of any authentic case of a species having been found in the Mediterranean, or on the shores of the United States, north of the West Indies. _Tetraclita porosa_ is found round the whole world; _T. radiata_, also, has a very wide range, inhabiting the West Indies, the East Indian Archipelago, and New South Wales. This latter species, as well as _T. cœrulescens_, is often attached to the bottoms of ships, adhering to _Balanus tintinnabulum_. The several species live attached to tidal rocks, to littoral shells, or to massive corals. I have met with two instances, in the West Indies and the Philippine Archipelago, of _T. porosa_ adhering to wood. _Tetraclita porosa_ seems to feed chiefly on crustaceans: the number and the size of the amphipods, isopods, and entomostracans, together with an annelid, in the stomachs of some specimens from South America, was quite surprising. As many as five species occur in the same region, in the eastern half of the world; thus on the shores of New South Wales, we have _T. porosa_, _vitiata_, _radiata_, _purpurascens_, and _rosea_; in the Philippine Archipelago, we have _T. porosa_, _vitiata_, _costata_, and _cœrulescens_.
I have not seen any species of this genus fossil. _Variation._--The species vary in shape nearly, but not quite as much as in Balanus, for we very rarely here see cylindrical varieties; but in the same species, we have extremely depressed, steeply conical, and convex forms; the orifice varies in relative size; the state of the surface, whether ribbed or smooth, whether well preserved or corroded, the upfilled parietal tubes being thus exposed, varies more than in Balanus. The colour also varies; specimens of _T. porosa_ (Pl. 10, fig. 1 _a_ to 1 _f_) being dark purple, or even inky black, white, pale pinkish-purple, and green: as far as I have seen, the pinkish varieties of _T. porosa_ are confined to the eastern half of the world. Even the ridges and crests on the under side of the scutum vary in some degree; and the tergum in _T. porosa_ varies considerably, and in some of the other species, slightly, in general shape: I believe that the tergum becomes narrow and elongated, when the shell is steeply conical, with the orifice of small diameter. After having spent several days in disarticulating and closely examining the many specimens of _T. porosa_ in my possession, I concluded that its varieties formed at least four species, these being in external appearance extremely distinct; but ultimately the many intermediate forms compelled me to unite all into a single species. Again, I may instance the conical, ribbed variety of _T. purpurascens_ (Pl. 11, fig. 1 _b_), with the outer lamina of the shell preserved and with the radii widely developed, as having not the smallest resemblance to the other common depressed variety (fig. 1 _a_), having a granulated surface, produced by the exposed tips of the upfilled parietal tubes, and without a trace of the radii or even of the sutures; I may add that, according to the characters used by some authors, these two varieties would be classed in two separate genera.
To distinguish the species of this genus, the chief reliance must be placed (as in the case of most other sessile cirripedes) on the general outline of the opercular valves, and on the ridges and crests on their under sides: the internal structure, however, of the radii, and in a lesser degree of the parietes, must not be overlooked. I have not found the parts of the mouth, or the differences in the cirri, of much service; and it will be shown under _T. porosa_, that the relative numbers of the segments in the several cirri, and even their shape, is extraordinarily variable. _Affinities._--All the species are pretty closely allied, and there is no ground for making any sectional division of the eight species, more especially not on the ground whether or no the radii are developed. _Tetraclita purpurascens_, taking into account all the characters, including the animal's body, is perhaps the most distinct species in the genus, but it is closely allied to _T. costata_. In the well-marked character of the parietes being formed of a single row of large tubes, _T. rosea_ differs from all the other species. The genus is closely allied to Balanus; I can point out no difference in the animal's body, nor any constant difference in the opercular valves. The ridges and crests on the under sides of the scuta are more prominent in Tetraclita; and all the species, excepting two, have crests (though sometimes very slight) for the attachment of the rostral depressor muscle, and these occur only in two species of Balanus: crests, also, for the attachment of the lateral depressor muscle are common in Tetraclita, but rare in Balanus. The main character, however, of the genus, as compared with Balanus, is derived from the existence of only four compartments, and in a lesser degree from the several rows of parietal pores; but, as just stated, _T. rosea_ has only a single row, and, on the other hand, in _Balanus crenatus_, there is a slight tendency exhibited, in the dividing of the longitudinal septa near the outer lamina, to form a second row of parietal pores; and in _B. cariosus_, moreover, we actually have two or three rows of irregular and very short pores. The thin, yet solid calcareous basis which occurs in all the species of Tetraclita, excepting _T. purpurascens_, resembles the basis in _Balanus flosculus_ and _imperator_, but I suspect that the structure of the cement-ducts is different in the two genera.
1. TETRACLITA POROSA. Pl. 10, fig. 1 _a_-1 _m_. LEPAS POROSA. _Gmelin._ Syst. Naturæ, 1789.[107] BALANUS SQUAMOSUS. _Bruguière._ Encyclop. Method., 1789, Pl. 165, fig. 9, 10. LEPAS FUNGITES. _Spengler._ Skrivter af Naturhist. Selskabet, 1 B., 1790. ---- POROSA. _Wood's_ General Conchology, Pl. 9, fig. 4, 1815. TETRACLITA SQUAMULOSA. _Schumacher._ Essai d'un Nouveau Syst., &c., 1817. BALANUS STALACTIFERUS. _Lamarck._ Animaux sans Vertèbres, 1818. ---- ---- _Chenu._ Illust. Conch., Pl. 4, fig. 6, 7. ASEMUS POROSUS. _Ranzani._ Memoire di Storia Naturale, Tab. 3, fig. 32-35. CONIA POROSA. _Sowerby._ Genera of Recent and Fossil Shells, Plate, 1823. ---- ---- _Leach._ (sine descript.) Encyclop. Brit. Supplement, vol. 3, 1824, Tab. 57. [107] As Gmelin and Bruguière published the same year, I do not know which comes first, but I have adopted the best known name. Most authors give, amongst their references, Chemnitz, vol. 8, Tab. 98, fig. 836, 837, but the accompanying description is more applicable to _T. serrata_ of this work than to _T. porosa_; without a figure or description, however, of the under side of the scutum it is impossible to decide. Several authors, also, give _Lepas cariosa_ of Pallas, as a synonym; this, however, is the _Balanus cariosus_ of this work. _Radii rarely present, when present narrow; even the sutures often absent: shell steeply conical, with the surface generally corroded, and having a stalactiferous appearance._
_Var._ (1) communis (Pl. 10, fig. 1 _a_): _outer lamina of shell almost wholly removed; the portion preserved, as well as the exposed parietal tubes, gray, or pale dirty brown, or dull purple_. _Var._ (2) nigrescens (Pl. 10, fig. 1 _c_): _outer lamina of shell almost wholly removed; the portion preserved, and the exposed parietal tubes, very dark purple or inky black_. _Var._ (3) viridis: _outer lamina of shell almost wholly removed; the portion preserved, and the exposed parietal tubes, green; under surface of opercular valves clouded green_. _Var._ (4) rubescens (Pl. 10, fig. 1 _b_): _outer lamina of shell almost wholly removed; the portion preserved, and the exposed parietal tubes, pale reddish-purple, or peach-blossom purple; under surfaces of opercular valves often reddish purple; terga often rather narrow, with the spur somewhat pointed, and with the basal margin on the carinal side sloping, or not making an angle with the spur_. _Var._ (5) elegans (Pl. 10, fig. 1 _d_): _outer lamina preserved, excepting sometimes near the summit of the shell; white, tinged with yellowish brown from the epidermis; surface strongly ribbed longitudinally; orifice rather small; sheath reddish-purple; terga narrow, with the basal margin sloping as in var._ rubescens. _Var._ (6) communis (young) (Pl. 10, fig. 1 _e_): _radii developed, very narrow; outer lamina of shell preserved, gray or dull purple; surface slightly ribbed longitudinally_.
_Var._ (7) patellaris (Pl. 10, fig. 1 _f_): _radii developed, very narrow, white; outer lamina of shell preserved, generally reddish-purple; steeply conical, with the orifice extremely small; surface smooth, with longitudinal white ribs. Terga very narrow, with the spur sharply pointed, and with the basal margin on the carinal side sloping towards it, or not making an angle with it. Scuta, with the adductor ridge very prominent. Attached to a ship's bottom_. _Hab._--West Indies, Brazil, West Colombia, Panama, Galapagos Archipelago, California, Philippine Archipelago, China, East coast of Australia, Red Sea; generally attached to tidal rocks, sometimes to shells, sometimes to wooden posts. Very common. _General Appearance._--This, the widest-distributed and much the commonest species of the genus, varies greatly in external appearance. The usual shape is steeply conical, but some individuals are much depressed. In the common varieties the outer lamina of shell has been removed even close to the basal edge; the upfilled parietal tubes being thus exposed (fig. 1 _b_), as flattened adpressed points. These points are largest in large specimens, but they vary somewhat in size in specimens of equal growth. When the outer surface is preserved, it is generally ribbed longitudinally, but is sometimes quite smooth. The most general colour is dirty gray or dark purple; but many specimens are pale pinkish-purple, owing to the exposure of the parietal tubes upfilled with shelly matter of this tint: there are also, as given under the characters of the _vars._, black, white and green varieties. The sheath is always tinted by the prevailing colour. The radii are rarely developed, but generally the four sutures are distinct; sometimes these are externally quite obliterated, the shell, as seen from the outside, consisting of a single piece like a patella or fissurella. When the radii are developed they are very narrow, with their upper edges oblique: their development seems always coincident with the more or less perfect preservation of the shell, and their function is to enlarge the orifice; the enlargement being usually effected by the disintegration and removal of the whole upper part of the conical shell. The size of the orifice varies considerably; in the seventh variety it was extraordinarily small: in outline it varies from oval to rounded trigonal or rhomboidal; in some specimens, with the radii well developed, it was rounded pentagonal.
_Size._--_Tetraclita porosa_ is the largest species of the genus; I have seen specimens attached to a large pebble of granite in the British Museum, which measured two inches in basal diameter, and nearly one inch and a half in height. _Scuta_: these are sub-triangular and generally a little elongated, but they vary slightly in relative breadth (fig. 1 _i_, 1 _l_), and likewise in the degree to which the basi-tergal angle is rounded off. The under surface is clouded with dull or pinkish-purple, or with green, or is nearly white. The articular ridge is not prominent, and the articular furrow is narrow. The adductor ridge is prominent, and runs upwards for some distance close and parallel to the articular ridge; and sometimes it extends nearly or quite up to the apex of the valve; in one single specimen the adductor ridge had an abraded appearance, and was very little developed. The crests for the rostral and lateral depressores are sharp and distinct. Along the occludent margin, the ends generally of the alternate lines of growth are enlarged into knobs, serving to lock the two valves together; but in many specimens only two or three knobs, at intervals of several lines of growth (fig. 1 _b_), are developed.
_Terga_: when the upper end of the valve is not corroded, there is a distinct beak, hollow within for a thread of corium. The scutal margin is not much inflected, and the articular ridge not very prominent. The spur is placed quite close to the basi-scutal angle of the valve, so that there is no basal margin on that side of the valve. The width of the valve and of the spur, and the acumination of the extremity of the latter, varies in a remarkable manner. In the broad and commonest variety (fig. 1 _k_), the basal margin of the valve form an angle of about 130° with the carinal side of the spur, and the basal end of the spur is broad and truncated. In the less common and narrow variety (1 _m_), the basal margin in some extreme cases forms very nearly a straight line with the carinal side of the spur; and the spur itself is bluntly pointed: in _var._ 7 it is sharply pointed.
_Structure of the parietes and radii._--In all cases the four sutures are quite distinct, from top to bottom, on the internal lamina of the shell, and generally they run through the whole thickness of the walls, and are visible externally. Often they do not quite reach the outer lamina, and then the shell externally consists of a single piece, like a patella. Sometimes the sutures can be traced running through the parietal tubes only for a short distance from the internal surface; where they cease, the two walls of the suture become fused together. When a perfect suture is split open, the radius is represented (fig. 1 _h_) by a few narrow, sinuous ridges, sending out on each side little branches or denticuli; these are received into corresponding furrows in the opposed compartment. These ridges run nearly parallel to each other, and somewhat obliquely, from the outer lamina of the shell to the basis. When the radii are developed, their edges are similarly formed, by sinuous denticulated ridges, with the interspaces between them filled up solidly. The alæ are but little prominent.
The _mouth_ does not deviate from the generic type. The _cirri_ are remarkable from the variability in the several pairs of the relative numbers of their segments, as shown in the following table. The segments do not correspond even on opposite sides of the same individual, as may be seen in the two lower lines of the table. I believe that variability to this degree is very uncommon in other cirripedes, though, as stated in the Introduction, the number of the segments always increases with the growth of the individual. The terminal segments in the longer rami of both the first and third cirrus are antenniformed,--being elongated, and of a different shape, with fewer bristles, compared with the basal segments of the same cirri. It is apparently these terminal segments which are particularly liable to vary in number. In both rami of the third cirrus, some of the segments, from the sixth to the eleventh inclusive, (counting from the bottom), more especially the eighth, ninth, and tenth, carry a few spines coarsely and doubly pectinated; but as some of the adjoining segments carry spines which may be called doubly serrated, it is not easy to draw an exact line of demarcation. Sometimes, though rarely, a few of the nearly terminal segments in the second cirrus are furnished with similar, doubly pectinated spines.
_Varieties._--Under the generic description, I have stated that after having spent some time in examining a very large suite of specimens of _T. porosa_, I concluded that at least four of the varieties were true species. It so happened that all the specimens which I first examined of the _var._ (4) _rubescens_, had the narrow sloping terga, and scuta with only two or three great teeth on their occludent margins; but ultimately, in a group thus characterised, I found one or two individuals with terga precisely of the shape of those in _var._ (1) _communis_. Again, in a group of dull purple specimens of _var. communis_, a few had the narrow sloping terga, and scuta with teeth on their occludent margins, intermediate in size and number between the varieties with only one or two great teeth, and those with every alternate growth-ridge enlarged into a tooth. Hence _var. rubescens_ completely broke down as a species. With respect to _var._ (5) _elegans_ (a M.S. specific name of Leach) I inferred at first, from external appearance alone, that it was distinct; the outer lamina of the shell and even the epidermis is preserved; the surface is strongly ribbed, and the whole shell, excepting the sheath, is nearly white; the terga are narrow, with a sloping basal margin as in _var._ (4) _rubescens_. Whole groups of specimens are thus characterised. But as _var. communis_ is often white, and as the surface, when the outer lamina is preserved, is generally, as we shall presently see, ribbed longitudinally, the differences in _var. elegans_ are quite unimportant.
The (6th) _var._ differs from _var. communis_ only in the narrow radii having been developed, and consequently in the orifice being pentagonal, and in the outer, longitudinally ribbed lamina of the shell having been preserved. In the same group of specimens, I have seen every intermediate stage between this and the common form. It must not, however, be supposed that the young of _var. communis_ always pass through these stages, for such is not the case. In the genus _Balanus_ it has been seen, how capricious in some species is the development of the radii. Of the other varieties, enumerated at the beginning, no further mention is required, excepting with respect to _var._ (7), the most singular of all. I have seen only three specimens, sent to me by Dr. Aug. Gould, of Boston, United States, and these from the appearance of their bases I have no doubt had been attached to a ship,--the only instance which I have met with, in the present species. The shell is steeply conical, with the orifice so small as to be reduced to a mere pore; the radii are extremely narrow and white; the shell is thin, with the surface smooth, but ribbed longitudinally and regularly; the outer lamina of the shell and the epidermis are perfectly preserved; the upper part of the shell is reddish purple, which dies away towards the base: careful examination of the apex shows that at the first growth the young shell was blueish-green. The terga are narrow, with a sloping basal margin, as in _var. rubescens_, but with the point of the spur sharper. I have formerly remarked that the shape of the terga seems influenced by the size of the orifice. The lower edge of the sheath depends freely: I have seen no other instance of this latter structure, so common, but so variable, in _Balanus_, in the present species, except to a partial extent in one distorted specimen, in Mr. Stutchbury's collection, adhering to _Balanus tintinnabulum_, and probably taken from a ship's bottom. I may add that this distorted specimen was remarkable from its radii being wider than in any other instance,--from its smooth uncoloured surface without longitudinal ribs,--and from the perfect preservation of the epidermis over its entire surface. Although Dr. Gould's specimens, in external aspect, are absolutely and entirely different from the common varieties of _T. porosa_, there are so many intermediate forms, and the differences are so little important that I feel no hesitation in attributing them to variation, consequent on the individuals having been exposed to unusual conditions, attached to the bottom of a ship.
2. TETRACLITA SERRATA. Pl. 10, fig. 2 _a_-2 _d_. _Shell dark greenish-gray, with narrow, longitudinal, serrated ribs: radii absent: scutum with the adductor and articular ridges forming a cavity, which runs up to the apex of the valve._ _Hab._--Cape of Good Hope; Algoa Bay; attached to sandstone and to Patellæ; Mus. Brit., Cuming, and Stutchbury.[108] [108] I have seen three separate lots of this species all from the Cape of Good Hope; one lot was collected by Dr. Krauss, at Algoa Bay, and I strongly suspect is the species described by him in his 'Südafrikanischen Mollusken' as _Conia porosa_. If the species, figured by Chemnitz, and mentioned in a note (p. 329), under _T. porosa_, be the present species, the specimens probably did not come from Tranquebar, on which point Chemnitz speaks only from memory. I have seen one specimen ticketed New South Wales, it is possible, considering the case of _T. rosea_, that this may be correct, but I should like to have further confirmation before giving it as a habitat. _General Appearance._--Colour dark greenish gray; form steeply conical; surface covered, especially in the lower half of the shell, by numerous, narrow, sharp, longitudinal ridges, but slightly prominent, and serrated or transversely divided into small teeth: when the shell has been much disintegrated, the upper part of the surface consists of the exposed, smooth, rather large, upfilled parietal tubes. I have seen no instance of the development of the radii; sometimes even the sutures are with great difficulty distinguishable, though I believe they always reach the outer surface; sometimes the sutures are wide from the disintegration of the edges of the compartments. Orifice rounded or oval.
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