A Monograph on the Sub-class Cirripedia · Charles Darwin
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_Scuta._--These are slightly narrower, thicker, and more convex than in _A. spongites_: they are strongly striated in longitudinal lines. The articular ridge is very feebly developed. _Terga_: in full-grown specimens, the spur is half the width of the whole valve, and is truncated; its basal edge being parallel to the basal margin of the valve. The articular ridge and crests for the depressor muscles are very feebly developed. _Internal structure of the parietes._--The inner surface of the parietes is quite smooth, without even a trace of ribs or teeth. But the most important character is that the internal lateral margins on both sides of each compartment, from the sheath to the basis, project inwards and form a rim; so that when the shell is viewed from within (Pl. 9, fig. 5 _b_, representing the lateral and carino-lateral compartments, and part of the carina), the six sutures are seen to be strengthened by six double columns. _Basis._--This is moderately cup-formed. The edge, in order to meet the basal points of the inwardly projecting lateral margins of the six compartments, has six knob-like teeth. These are placed at unequal distances, for two on each side stand near each other, owing to the narrowness of the carino-lateral compartments. The degree of their development varies extremely; when most developed, as in the specimen figured (Pl. 9, fig. 5 _a_), each tooth is bifid and a little hollowed out, so as to receive the points of the two inwardly projecting margins which form each suture. Ridges, more or less prominent, running from each of the six marginal teeth, extend towards the centre of the cup. These six teeth cannot be seen from the outside. The edge of the cup is rarely crenated; but I have seen two instances in which this was the case.
_Cirri._--In the first pair, the rami are not quite so unequal as in _A. spongites_; the longer ramus being about twice as long as the shorter. In the third pair, there are some very minute, thick, upwardly-pointing spines, which I did not notice in _A. spongites_. In the fourth pair, the spines are a little more crowded, with longer dorsal tufts, than in the sixth pair; and they are mingled with some very minute, thick, upwardly pointing spines. In young individuals, there are only three pairs of main spines on the segments of the sixth pair, instead of four pairs. 6. ACASTA LÆVIGATA. Pl. 9 fig. 6 _a_, 6 _b_. ACASTA LÆVIGATA. _J. E. Gray_ (!). Annals of Philosophy, (new series), vol. 10, Aug. 1825. _Parietes internally quite smooth, with the lateral margins of each compartment inwardly prominent: basis with the edge strongly crenated, and furnished with six inwardly prominent teeth: scutum feebly striated longitudinally, or smooth._ _Var._ (_a_), _epidermis coloured dull orange_.--Red Sea. _Hab._--Red Sea, Philippine Archipelago; Mus. Brit., Cuming, &c.
This species, of which I have examined many specimens from the above two and other unknown localities, agrees in all essential points of structure with _A. glans_, and consequently I for some time classed them together; but the characters, though usually of small value, by which this form differs from _A. glans_ being apparently constant, I have with some doubt allowed it to remain specifically distinct. These characters are, firstly, the much smaller size of the whole shell in _A. lævigata_; secondly, the edge of its basal cup being always crenated, which seems to be a rare accident in _A. glans_; thirdly, though of secondary importance, the scutum being here less plainly striated; and, lastly, the spur of the tergum being of less breadth, and of a more rounded outline; on the other hand, it must be confessed, that when small specimens of _A. glans_ are taken, there is hardly any difference in the spurs of the terga. _General Appearance and Structure of Shell._--The surface of the shell is often very smooth, but is sometimes studded with some small sharp calcareous points. The colour is white, or pale reddish-brown; but in the specimens from the Red Sea, the tint is more orange, with the upper part of the shell white. The orifice of the shell is unusually small. The largest specimen which I have seen was only .25 of an inch in basal diameter, and therefore less than half the size of _A. glans_. The internal surfaces of the _parietes_ are smooth, with the two lateral margins inflected, as in _A. glans_. The edge of the basal cup has six knob-like teeth, like those in _A. glans_, but smaller; and, in addition, it is finely crenated.
_Scuta_: these differ only in being less plainly striated in longitudinal lines; indeed, some specimens show hardly a trace of this structure. _Terga_; these valves, in some varieties (Pl. 9, fig. 6 _b_) can hardly be distinguished from those of equal size from young individuals of _A. glans_; other varieties have the spur (Pl. 9, fig. 6 _a_) not truncated, but broadly pointed, and therefore of considerably different shape. Neither in the mouth, nor cirri could I detect any difference with _A. glans_. 7. ACASTA FENESTRATA. Pl. 9, fig. 7 _a_-7 _c_. _Shell reddish, with six large, membrane-covered apertures between the sutures, above the basal cup: carino-lateral parietes half as broad as lateral parietes; internally, parietes and edge of basis smooth; tergum with the articular ridge short and prominent; spur pointed._ _Hab._--Philippine Archipelago, Mus. Cuming. _General Appearance._--Shell rather elongated or tubular; with the upper part reddish, and the surface roughened with very minute points. The basal cup is generally as deep as the shell is high, ending downwards in a blunt point, often curved to one side. The summits of the radii, as usual, are oblique. The parietes of the carino-lateral compartments are about half as wide as the parietes of the lateral compartments, and are therefore of greater proportional width than in the foregoing or any other species of the genus. The large membrane-covered openings, or, as they may be called, windows, presently to be described, between the lower halves of the compartments, is much the most remarkable character of this species. The largest specimen which I have seen was only .23 in diameter, and .6 of an inch in height, measured from the basal point of the cup to the tips of the compartments.
_Scuta._--These barely exhibit a trace of longitudinal striæ. The valve is rather thick and convex. The basi-tergal angle is much rounded off. Internally, the articular ridge is thick and rather prominent. _Terga_: the valve is furrowed in the line of the spur: the spur is pointed and rather long; it is distinctly separated from the basi-scutal angle of the valve, and the basal margin on the two sides of the spur forms a straight line. The articular ridge is prominent, and short. _Structure of the Parietes, Radii, and Basis._--The parietes are internally quite smooth down to its basis. The edges of the radii are also smooth, as is the edge of the basal cup. The alæ project less than usual. The radii are of moderate breadth, they extend downwards only a little below the sheath, namely, about half way down the shell, where they terminate, as usual, in a point. The increase in width, during growth, of the radii, and their not extending down to the basis, would necessarily cause a gap between the opposed edges of the walls, in the portion beneath the radii; but besides this, the edges of the walls themselves, beneath the radii, and on the opposed side beneath the alæ, are hollowed out, but on the latter side or beneath the alæ sometimes in a lesser degree. The result of this is, that the compartments, in their lower halves, are separated from each other by membrane-covered windows or apertures, arched at their upper ends, and of considerable size, namely, about as wide as the parietes of the carino-lateral compartments. I have only further to remark, that during the downward growth of the parietes, the apertures increase in size, but at the same time become closed up at their upper ends; and the arched layers of shell added at these upper ends, assume a very different aspect from the rest of the parietal surface,--appearing like two wedges, with their points upwards, let in, on one side of the suture, between the ordinary parietal surface and the radius, and, on the other side of the suture, between the ordinary parietal surface and the recipient furrow of the radius.
The _animal's body_ was in a bad state of preservation; but, as far as I could make out, the cirri resembled those of _A. glans_. _Affinities._--This species differs from _A. glans_ in not having the internal margins of the compartments projecting inwards. It differs from all the ordinary varieties of _A. spongites_, in the smoothness of the basal edges of the parietes and of the edge of the cup; in the greater width of the carino-lateral compartment, though this is a variable point in _A. spongites_; slightly in the shape of the scuta and terga; and, lastly, in the large, membrane-covered openings between the compartments. 8. ACASTA PURPURATA. Pl. 9, fig. 8 _a_-8 _c_. _Shell dull blueish-purple, with six small, membrane-covered apertures between the sutures, close above the basis: tergum with the articular ridge very short and prominent; spur very broad and rounded._ _Hab._--Sumatra; Philippine Archipelago; imbedded in the bark of an Isis; Mus. Cuming, Stutchbury, Brit. This species is perfectly distinct from the others, as shown by its general appearance, its habits, and the structure of its opercular valves: it is allied to _A. fenestrata_, in having membrane-covered apertures between the compartments, and in some respects in its opercular valves: it is also allied to _A. sulcata_ and _cyathus_ in the parietes being often internally ribbed, in the basal cup having a crenated edge, and in the anterior ramus of the fourth cirrus being furnished with the minute hook-like spines.
_General Appearance._--Sub-globular, slightly compressed, with a rather small orifice; smooth, but sometimes furnished with sharp shelly points; dull purple, more or less dark, with the upper parts of the walls often white. The radii are rather narrow, and generally white, with their summits only slightly oblique, but variable in this latter respect. The parietes of the carino-lateral compartments are narrow, being only one sixth of the width of the parietes of the lateral compartments. In some specimens there are membrane-covered apertures of considerable size, in others mere narrow clefts, between the basal halves of the compartments. The basal cup is moderately deep. The largest specimen was only .16 of an inch in basal diameter. _Scuta_, rather broad, externally convex, not longitudinally striated: articular ridge prominent, short, not extending down above one third of the length of the valve. Depression for the adductor muscle deep. On the internal surface, near to the rostral angle, a rather large purple spot of corium adhered to the valve. _Terga_, broad, externally rather convex: scutal margin protuberant: carinal margin slightly inflected, or furnished internally with a rim: articular ridge prominent, very short, not extending down above one fourth of the valve. Spur very broad, rounded, confluent with the basi-scutal angle of the valve.
_Internal Structure of the Parietes, Radii, and Basis._--The parietes, internally, are either quite smooth, or more commonly ribbed, with the basal edge in consequence crenated; the ribs are either placed at an unusual distance from each other, and consequently are few in number, or are pretty close together. The edge of the basal cup is either quite smooth, or closely crenated, or distantly toothed, in conformity with the state of the internal surface of the parietes. The radii have nearly smooth edges, with their summits more or less oblique. They sometimes extend down only three fourths, or only two thirds, of the length of the shell, and the margins of the parietes under the radii being a little hollowed out, the sutures are converted into clefts or apertures (of course covered by membrane) like, but not so large as, those in _A. fenestrata_. The margins of the parietes are hollowed out only on the side of the radius, and not on both sides of the sutures, as is most usual in _A. fenestrata_. In some specimens the radii extended down close to the basal cup, and only very minute clefts were left between the opposed edges of the parietes. In the _animal's body_ the only noticeable character was, that on the anterior ramus of the fourth pair of cirri, some of the segments were furnished with very broad and thick, small, downwardly curved, teeth or hooks, like those described in certain varieties of _A. sulcata_; but they are here stronger and thicker. The segments in the three posterior pairs of cirri are not so much elongated, as in the other species.
9. ACASTA SPORILLUS. Pl. 9, fig. 9 _a_-9 _d_. _Shell purplish-brown, with the parietes internally strongly ribbed and reticulated: carino-lateral compartments extremely narrow, not extending down to the basis._ _Hab._--Sooloo Islands, East Indian Archipelago; Mus. Dana. I am indebted to Mr. Dana, the distinguished naturalist of the United States Antarctic Expedition, for two specimens of this interesting species, which, in the singular reticulated structure of the inner surface of the walls, and in the almost rudimentary condition of the carino-lateral compartments, not extending down to the basal cup, is very distinct from the foregoing species. I have used Mr. Dana's very appropriate MS. name of _sporillus_. The specimens were dredged up, lying quite loose and unattached at the bottom of the Sooloo sea; the one which I opened, must have long lain dead; but Mr. Dana assures me that some were living, and he has sent me drawings of parts of the mouth and cirri: I am much surprised at this circumstance; for analogy would have made me believe that this species must have been imbedded in some sponge-like body, such as the bark of a zoophyte, and that it could not have lived unattached. I may add that a small fragment of a brown leathery substance adhered to the upper end of one of the two specimens, and this seems to indicate attachment.
_General Appearance._--Shell shaped like a pointed acorn; slightly flattened; orifice extremely small; surface very finely punctured, covered by a purplish-brown epidermis, with transverse stripes of different shades, and with the apex dark; according to Mr. Dana, when fresh, the colour was purplish-carmine. Radii narrow, white. The carino-lateral compartments are extremely narrow; the wall-portion (fig. 9 _b_) forming a mere linear rib, terminating downwards in a sharp point, which does not reach the basal cup: hence this compartment evidently tends to become rudimentary. The basal cup is moderately deep and pointed. Basal diameter .16; height, from the bottom of the cup to the top of the shell, .24 of an inch. _Scuta_: narrow, with the upper part produced; not striated longitudinally; coloured by a pale purple, longitudinal band. Internally, there is scarcely a trace of an articular ridge, which, in the other species, is always more or less developed. _Terga_ with the spur bluntly pointed; nearly the whole basal margin, on the carinal side, slopes towards the spur.
_Internal Structure of the Parietes, Radii, and Basal Cup._--The parietes are strongly ribbed internally; and these ribs are connected by very narrow, less prominent, transverse, slightly branched ridges, giving a reticulated structure to the inner surface. Between several of the main longitudinal ribs, in the lower part of the shell, new ribs may be seen in process of formation, and these tend to convert the reticulated structure into a double row of minute cells. I have not met with an exactly similar structure in any other cirripede; but I have no doubt that the little transverse ridges are homologous with the transverse calcareous septa in the parietal pores of many Balani, in the same manner as the internal longitudinal ribs, in this and other species of Acasta, are homologous with the longitudinal septa forming the above pores. The edge of the basal cup is pectinated with teeth, which lock into the teeth formed by the ends of the internal parietal ribs. The radii are narrow, and have smooth edges. The alæ project beyond the parietes to a remarkably small extent. The sheath is free, or hollow beneath. I have already described the almost rudimentary condition of the carino-lateral compartments; this is best exhibited in an internal view of the two compartments, as given in Pl. 9, fig. 9 _b_. _Animal's body_ unknown to me: from Mr. Dana's drawing the three posterior pairs of cirri seem to have been much elongated: and the rami of the first pair, as usual, unequal in length.
M. Deshayes has given an indifferent figure and imperfect description of ACASTA TUBULOSA (Guerin, Magasin de Zoologie, 1831, Tab. 39; and Guerin, Iconographie du Règne Animal, Mollusques, Tab. 38, fig. 4, but here by a misprint called _A. spinulosa_); it is utterly impossible to recognise the species of this genus from such materials. 3. _Genus_--TETRACLITA. Pl. 10, 11. TETRACLITA. _Schumacher._ Essai d'un Nouveau Syst., &c., 1817. CONIA. _Leach._ Journal de Physique, tom. 85, 1817.[105] ASEMUS. _Ranzani._ Memoire di Storia Naturale, 1820.[106] POLYTREMA. _De Ferussac._ Dict. Classique d'Histoire Naturelle, 1822. LEPAS. _Gmelin._ Systema Naturæ, 1789. BALANUS. _Bruguière._ Encyclop. Method., 1789. ---- _Lamarck._ Animaux sans Vertèbres, 1818. [105] From a note by the Editor, it appears that Schumacher's essay appeared before the number of the Journal containing Leach's paper, so that Schumacher's name must be adopted. [106] I have not seen a _complete_ copy of this work, and give the title from a catalogue; the running heading of the part containing the Cirripedia, is "Opuscoli Scientifici." _Compartments four, sometimes externally calcified together: parietes permeated by pores, generally forming several rows. Basis flat, irregular, calcareous, or membranous._ _Hab._--Throughout the tropical and warmer temperate seas.
_General Appearance._--The shell is conical, more or less depressed, and very rarely, even when growing in crowded groups, becomes cylindrical or elongated. The orifice is seldom large, generally diamond-shaped or oval. The colour is either nearly white or purple, occasionally even inky black, or very dark green, and sometimes of a pale pink peach-blossom. The surface is sometimes smooth, but more commonly longitudinally ribbed; the outer lamina of shell is very often wholly corroded away, excepting close to the basis, leaving the solidly upfilled parietal tubes exposed: these give the shell a striated appearance, or they appear like flattened tapering points adpressed to its surface (Pl. 10, fig. 1 _b_): Lamarck attempted to express this appearance, by using the specific name of _stalactiferus_. The colour of the shell depends, to a considerable extent, on the colour of the shelly matter in these exposed parietal tubes. We shall presently see that the corrosion and disintegration, to which some of the species are so liable, plays an important part during their growth. The radii are either well developed, as in most of the species; or they are entirely absent, as in the great majority of specimens of _T. porosa_ and _serrata_. In many individuals of _T. porosa_ and _purpurascens_ not only are the radii absent, but the four compartments are calcified together without any trace, on the external surface, of the sutures. The largest specimen which I have seen of _T. porosa_, which is the largest species, was two inches in basal diameter, and nearly one inch and a half in height.
_Scuta._--These valves are sub-triangular, and either longitudinally or transversely elongated. Externally, the growth-ridges are moderately prominent, and in _T. costata_ and _cœrulescens_ they are crossed by longitudinal striæ. Along the occludent margin, the inflected extremity of each alternate growth-ridge is generally much thickened,--a set of teeth being thus formed, by which the two valves are locked together. In _T. porosa_, this character is variable, for sometimes every alternate ridge, and sometimes only two or three ridges, separated from each other by several growth-ridges, are thus developed into teeth. The _articular_ ridge is either moderately prominent, or is extremely prominent, as in _T. cœrulescens_; but the lower edge in no case depends as a free, hinge-like style, as sometimes in Balanus. The _adductor_ ridge is generally well developed and distinct from the articular ridge: in _T. purpurascens_ it is very blunt: in _T. serrata_ it is united to the articular ridge half way up it, thus forming a deep tubular cavity running up to the apex of the valve: in _T. cœrulescens_, the adductor ridge is very short, and is united to, or almost continuous with, the lower end of the articular ridge, a small sub-cylindrical tubular cavity being thus formed. Small crests exist for the attachment of the rostral and lateral depressor muscles, in most of the species, excepting _T. purpurascens_ and _costata_, in which, however, more especially in the former, there are, instead of crests, minute pits for the attachment. These crests vary much in prominence in the same species.
_Terga._--These valves present no essential differences from those of Balanus; they are sometimes beaked, and the beak is hollow and occupied by a thread of corium, as in that genus. The external surface of the valve is often depressed in the line of the spur, but there is never a longitudinal furrow with the edges folded in, as in Balanus. The spur is very short in _T. purpurascens_. In _T. radiata_, the articular ridge is remarkably prominent. The crests for the depressor muscles are well developed in all the species. The shape of the terga is variable in nearly all the species, and greatly so in _T. porosa_. _Compartments._--Owing to there being only four compartments, the lateral pair are large; the size of the carina relatively to the rostrum varies, according as its alæ have been added to during diametric growth. The walls are very thick, and are composed of numerous tubes, in some species as many as fourteen or fifteen rows being exposed on the basal margin (Pl. 10, fig. 1 _g_). The tubes are generally angular, and slightly elongated in the ray of the circular shell; sometimes they are nearly circular and small. New tubes are formed only at the basal edge of the outer lamina, by the bifurcation of the septa which form the tubes. In very young specimens there is only a single row of tubes; and in _T. rosea_ this holds good throughout life: in this species (fig. 3 _d_) the tubes, in the single row, are large and quadrangular, and the outer lamina of shell is strengthened by numerous, small, internal, longitudinal plates. I believe the branching septa, which separate and form the parietal tubes, correspond with the longitudinal septa in the more simple walls of Balanus. The tubes become solidly filled up, in their upper parts, with hard, and generally coloured shelly matter. The degree to which they are filled up differs in the different species; the external side of each tube is always first thus coated. The thin outer lamina of shell, in several of the species, commonly disintegrates and disappears; the upfilled parietal tubes being thus exposed. The inner lamina of the walls is generally smooth, but in _T. radiata_ it is longitudinally ribbed, as in most species of Balanus. The sheath is generally dark-coloured; its lower edge does not project or overhang the inner lamina, as is usual in Balanus, excepting in _T. serrata_, and in some few varieties of _T. porosa_.
The _Radii_, when developed, are either narrow or broad, with their summits either oblique or extending in a straight line from the top of one compartment to that of another. In _T. serrata_, I have not seen a single specimen with the radii developed; in _T. porosa_, they are very seldom developed, and then, apparently, only in quite young specimens, in which they are narrow; in _T. purpurascens_, they seem to be about as often developed as not, and when present they are broad; in _T. costata_, _cœrulescens_, and _radiata_, they are always largely developed. In some specimens of the species, in which the radii are not developed, even the sutures do not reach the external surface; the outer lamina of the parietes being continuous all round, so that the shell seems formed of a single piece. Even in such specimens, the four compartments, viewed from within, can be seen to be distinct; and the sutures can generally be traced across the whole thickness of the parietal tubes; in this latter case, when the sutures are broken open, the radii are seen to be represented (Pl. 10, 1 _h_) by a few small sinuous ridges. Owing to the disintegration of the upper and outer part of the shell, and the consequent exposure of parts of the sheath and alæ, the radii sometimes appear as if developed, when such is not the case. With respect to the internal structure of the radii, they are formed, in _T. purpurascens_ and _costata_, of tubes, like those of the parietes, and therefore according to the normal plan; whilst in the other species they are formed by longitudinal sinuous ridges, sending out on each side irregular denticuli; and the interspaces between the ridges are filled up solidly during the growth of the radii, in all the species, except in _T. radiata_, in which they are left to a considerable extent open. These sinuous ridges, with their denticuli, homologically represent the branching septa which form the parietal tubes. The edges of the alæ are crenated in all the species, except in _T. costata_.
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