A Monograph on the Sub-class Cirripedia · Charles Darwin
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_Structure of the Shell and Radii._--The walls are thick: the parietal tubes are rather large and regular; they become solidly filled up in their upper parts. The sutural edges of the radii are formed by unusually narrow, sinuous ridges, sending off delicate denticuli on each side: the interspaces between these ridges are solidly filled up. The crenated edges of the alæ are rather thick. Neither the _mouth_ nor the _cirri_ offer any noticeable character; but I may observe, that, in the mandibles, the third and fourth teeth are close together; and that, in the three posterior cirri, the tufts of small spines between the main pairs are small. _Affinities._--This species is very distinct from the others, with the exception of the following _T. radiata_, to which it is in several respects allied. The under side of the scutum, with its great articular ridge, and the cylindrical tube formed by the union of the latter with the short adductor ridge, affords the most noticeable character. 8. TETRACLITA RADIATA. Pl. 11, fig. 5 _a_-5 _d_. CONIA RADIATA.[112] _De Blainville._ Dict. Sc. Nat., 1816-1830, Pl. 164, fig. 5, 5 _a_, (sine descript.) ---- LYONSII. _G. B. Sowerby._ Genera of Recent and Fossil Shells, Plate, 1823, (sine descript.)
_Shell white, with numerous approximate longitudinal ribs: radii broad, with their summits slightly oblique, internally porose: tergum with the articular ridge extraordinarily prominent, with the spur not joined to the basi-scutal angle._ _Hab._--West Indies, adhering to _Balanus eburneus_ and to _Lepas anserifera_. New South Wales, adhering to _Tetraclita porosa_. Attached to _Balanus tintinnabulum_, on a ship's bottom from Sumatra; not rarely attached to _Balanus tintinnabulum_ on ships' bottoms; Mus. Brit., Stutchbury, and Cuming. _General Appearance._--Shell white, rather steeply conical, with numerous, approximate, rather narrow, longitudinal, rounded ribs on the walls: in a specimen half an inch in diameter, there were from eight to twelve ribs on each compartment. The outer lamina of shell seems always well preserved. Orifice rounded, trigonal. Radii white, smooth, broad, with their summits only slightly oblique. I have seen one specimen 1.2 in basal diameter, but quarter of an inch is a common size, and very young specimens are unusually frequent in collections. _Scuta_ broad, externally not striated longitudinally. The articular ridge is prominent, and the furrow deep, but not in so great a degree as in _T. cœrulescens_. The adductor ridge is only slightly prominent; it extends upwards only a little way above the lower end of the articular ridge, and does not form with the latter a cavity. There are no crests for the rostral depressor muscle, but there is a little pit, formed by the folding over of the occludent margin.
_Terga._--These valves, when articulated with the scuta (fig. 5 _b_), project above them to an extraordinary degree, and are separated from them by a deep, fissure-like hollow, caused by the remarkable prominence of the articular ridge of the terga. The upper part of the tergum is not beaked, and does not project freely much above the sack. The valve is large; externally there is a rounded longitudinal furrow. The tergal margin is broadly inflected. The articular furrow is deep, and the articular ridge far more prominent than in any other sessile cirripede, for it projects, as measured from the outside surface, more than half the width of the valve; and consequently the valve, when viewed vertically from above, almost appears as if formed by the union of three plates, viz., the articular ridge, and the outside surface on each side of the spur. The spur is of moderate width, with the corners rounded: it is placed near, but not close to the basi-scutal angle, so that there is on this side a small portion of basal margin, forming nearly a straight line with the margin on the carinal side. In some young specimens, about the tenth of an inch in diameter, from the West Indies and from New South Wales, the spur (the position of which I found varied a little in some other specimens) was placed nearly in the middle of the valve, and very nearly at right angles to the basal margin; it is possible that these may be a distinct species, but without larger specimens to judge from, I think it more probable that this difference in the tergum is due to variation and youth.
_Structure of Shell and Radii._--The parietal tubes are commonly elongated in the ray of the circle: the septa are rather thick, and strongly crenated at their basal edges. The inner lamina of the walls is strongly ribbed longitudinally. The broad radii have their sutural edges formed by ridges, with numerous and closely approximate denticuli: the interspaces between the main ridges are not soon filled up, and at the bottom, each interspace usually terminates in a pore or tube; so that the radii are not solid, as in most of the foregoing species, but porose. The alæ have their edges crenated. _Basis_, calcareous, of unusual thickness; the inner, or upper surface, is striated from the centre in rays, corresponding with the ribs on the inner lamina of the walls. This striated or furrowed structure in the basis, shows a tendency to its becoming tubular or porose, as may be inferred from analogous cases in Balanus. _Animal's body_ unknown. _Affinities._--This species is rather more closely allied to the last than to any other. There is a close analogy in the peculiar manner in which the scuta and terga are articulated together in the two species: in this species it is effected by the great development of the articular ridge of the tergum, and in _T. cœrulescens_ by that of the scutum. The internally striated calcareous basis, and the internal tubular interspaces between the denticulated ridges of the radii, are peculiar characters. The white colour, the narrowly and closely ribbed parietes, and the broad radii, give this species an aspect, by which it can be easily recognised.
4. _Genus_--ELMINIUS. ELMINIUS. _Leach._ Zoological Journal, vol. 2, July, 1825. _Compartments four: parietes not porose. Basis membranous._ _Distribution_, Southern temperate seas. _General Appearance._--Shell conical, with a strong tendency in most of the species to become cylindrical: orifice generally large. Walls either thin and smooth, or thick and plicated longitudinally. Colours various, pale purple, greenish, white, and, in _E. plicatus_, owing to the exposure of an intermediate lamina of shell, bright orange-yellow. Radii, either of considerable width, with their summits oblique and rounded, as in the first two species of the genus, or very narrow, as in the last two species. _Elminius plicatus_ is the largest species, and is sometimes one inch in basal diameter. The outer surface of this latter species is occasionally much corroded. _Scuta_: these are of the usual shape; in _E. Kingii_ and _modestus_ there is no adductor ridge and no crests for the depressor muscles; in _E. plicatus_ and _simplex_, on the other hand, there is a well developed adductor ridge and crests for the lateral depressor muscles; in some individuals, also, of _E. plicatus_ there are small crests for the rostral depressores. The _Terga_ are remarkable for their variability in all the species; in many specimens of _E. Kingii_ and _modestus_ the basal margin on the carinal side of the spur is deeply hollowed out. The width and acumination of the spur varies in all the species. In _E. plicatus_ and _simplex_ this valve is remarkably like that of _Tetraclita porosa_. In some specimens of _E. Kingii_ the terga and scuta are firmly calcified together.
_Structure of the Parietes and Radii._--As in Tetraclita, the two lateral compartments are necessarily broad. The parietes are never porose, but consist, in appearance, of a single layer of shell. In _E. modestus_ the basal internal edges of the parietes are smooth, but in the other species they are striated longitudinally with short ridges, or sometimes with sub-cylindrical projections. In those specimens of _E. plicatus_, which have externally suffered much corrosion, the walls have been rendered extremely thick, by the inward production of these ridges or plates; and in this case the ridges are not confined to the basal edges, but extend upwards close to the sheath. The basal surfaces of the walls in these latter specimens resemble those of Chelonobia, but the walls in that genus have an internal lamina, which here is not the case. The radii are wide in _E. Kingii_, and of moderate width in _E. modestus_, with their summits oblique and smoothly rounded, and their sutural edges not in the least crenated. In _E. simplex_ they are extremely narrow, smooth-edged, and rounded: in _E. plicatus_ they are narrow, and in this species alone the sutural edge is sinuous, and sends inwards short ridges or teeth. The alæ, in all the species except this last, are likewise smooth-edged. The lower edge of the sheath depends, more or less freely, in all the species, except in _E. Kingii_.
_Basis_, membranous in all four species. In _E. modestus_, the true basal membrane is extremely thin, and is divided into concentric slips: on its inner surface there are attached numerous cement-ducts, varying from 1/3000 to 1/2000 of an inch in diameter, repeatedly trifurcating, rarely forming hexagonal or quadrangular loops, and with the branches placed approximately parallel to each other. Beneath the true basal membrane there is a complicated layer of cement, in the form of a network, or of separate tubes, or in beads and patches. In _E. Kingii_, the basal membrane presented a wonderfully complicated appearance, in part due to the cement forming a mass of inosculating fibres; many of these fibres seemed to end in circular discs of cement. Neither the _Mouth_ or _Cirri_ offer any noticeable generic characters, as distinct from Balanus and several other genera. The _Branchiæ_, in _E. plicatus_, are well developed and moderately plicated. In _E. modestus_ they are small, not plicated, but with a rounded sinuous margin: in a specimen having a basal diameter of 25/100, the branchiæ in total length were only 4/100 of an inch. At the bottom of the sack I observed some inwardly pointed, tapering filaments, such as occur in Balanus. In this same species I measured the ova, which were unusually elongated, being 19/2000 in length; I may add, that the probosciformed penis was actually thrice the length of the animal's body in some small but mature specimens (with ova), having a shell with a basal diameter of 16/100 of an inch.
_Distribution and Habitats._--This genus is remarkable, inasmuch as it is not distributed over the whole globe: three of the species occur very commonly on the shores of New South Wales, Van Diemen's Land, and New Zealand; not extending, as far as I can judge, much north of Sydney: the fourth species is confined to South America, ranging from the Falkland Islands and Tierra del Fuego, as far north as Chiloe. Elminius, therefore, appears to be strictly a southern genus. _Elminius Kingii_ and _modestus_ represent each other on the American and Australian continents; so I believe _E. plicatus_, in New Zealand, represents _E. simplex_ in New South Wales and Van Diemen's Land. The species are all attached to tidal rocks and shells. _E. Kingii_ is sometimes attached to floating wood. At the Falkland Islands, the last-mentioned species adhered to some rocks, in a running brook of fresh water, at most eighteen inches under high-water mark, so that for the greater part of each tide it was exposed to absolutely fresh water. At Sydney I found _E. modestus_ adhering to oysters in a muddy lagoon, almost separated from the sea, and apparently very unfavorable for cirripedes.
_Affinities._--This genus can be distinguished from Tetraclita only by the four compartments not being porose, and by the basis being always membranous; whereas, in _Tetraclita purpurascens_ alone it is membranous. _Elminius Kingii_ and _modestus_, on the one hand, are closely allied together, as are _E. simplex_ and _plicatus_ on the other hand. The last-named species, in the characters of its opercular valves and in its shell, comes nearest to Tetraclita. In _T. rosea_ we have seen that there is only a single row of parietal tubes, and the outer lamina of the shell is strengthened (as, indeed, it is in most of the other species of the genus, and in Balanus) by small longitudinal plates or ridges, which are similar and homologous to those on the internal basal edges of the parietes in three of the species of Elminius; so that the difference in the structure of the parietes, in Tetraclita and Elminius, is small. 1. ELMINIUS KINGII. Pl. 11, fig. 6 _a_-6 _e_. ELMINIUS KINGII. _J. E. Gray._ Zoological Miscellany, p. 13, 1831. ---- LEACHII. _King_ and _Broderip_. Zoological Journal, vol. 5, 1832-1834, p. 334, and appendix to King and Fitzroy's Voyages. ---- ---- _G. B. Sowerby._ Genera of Recent and Fossil Shells, Plate. _Shell smooth, gray or dirty white: radii broad, smooth-edged: scutum without an adductor ridge; tergum with the spur distinct from the basi-scutal angle: scutum and tergum sometimes calcified together._
_Hab._--Tierra del Fuego, Falkland Islands, Chiloe. Attached to tidal rocks and sometimes to floating timber; Mus. Brit., Darwin, &c. _General Appearance._--Shell fragile, either steeply conical with a large orifice, or sub-cylindrical; surface smooth, grayish or white, with large portions covered by pale brown epidermis. Radii broad with their summits oblique, smooth, slightly arched, exhibiting a large surface of the alæ. The alæ usually have their summits much less oblique than those of the radii; the portion added during diametric growth is of a dead white colour. The growth ridges on the scuta are very little prominent, and are crossed by a very obscure band of blueish-gray. The largest specimen which I have seen was .8 in basal diameter, and the longest cylindrical variety .55 of an inch in height. The _Scuta_ are remarkable for not having any adductor ridge or crests for the depressor muscles; the articular ridge is prominent, but it is short, not extending down half the valve. I have mentioned under the genus, that in many specimens at the Falkland Islands the scuta and terga were calcified together.
The _Terga_ are rather small: the basal margin on the carinal side of the spur is always hollowed out, but to a very variable degree, as may be seen in the three figures (6 _c_-6 _e_); this margin is generally dentated with one or two little points; and an inner lamina of shell sometimes depends beneath the outer lamina, to which the opercular membrane is attached, as may be seen in the figure (6 _d_) of the external surface of the valve. The crests for the depressor muscles are well developed. The tergal margin is broadly inflected, and the articular ridge prominent, making the articular furrow deep. The spur is rather narrow, and is either (6 _d_, 6 _e_) bluntly or sharply pointed. The basal margin on the scutal side of the spur, is hollowed out, but to a variable depth. _Structure of the Parietes and Radii._--The parietes are thin; at their internal basal edges they are finely striated in longitudinal lines. The radii are solid, with quite smooth edges; they are generally covered by the epidermis. The sutural edges of the alæ are likewise smooth, these are added to largely during the diametric growth; and their summits, as already stated, are much less oblique than the summits of the radii. The internal surface of the shell is smooth, and is tinted pale dull purple. The lower edge of the sheath can hardly be said to be free. The carinal margins of the compartments project a little inwards.
_Mouth_: the labrum is deeply notched, and supports five little teeth on each side; the palpi are thickly clothed with spines on their inner sides; the mandibles have five or only four teeth: the maxillæ are notched, and the outer maxillæ bilobed. _Cirri_: the first pair has one ramus nearly twice as long as the other. The three posterior cirri are elongated, and each segment supports five or six pairs of long spines, with a few minute intermediate bristles. 2. ELMINIUS MODESTUS. Pl. 12, fig. 1 _a_-1 _e_. _Shell folded longitudinally, greenish or white: radii of moderate breadth, smooth edged: scutum without an adductor ridge: tergum narrow, with the spur confluent with the basi-scutal angle._ _Hab._--New South Wales; Van Diemen's Land; New Zealand; very commonly attached to littoral shells and rocks; associated with _Balanus trigonus_ and _vestitus_; Mus. Brit., Cuming, Stutchbury, Darwin. _General Appearance._--Shell conical, generally rather steep, occasionally depressed: walls longitudinally folded, sometimes very deeply, sometimes only to a slight degree: colour dull greenish or white. Radii of moderate width, with their summits very oblique, smooth and slightly arched: alæ much exposed, with their summits less oblique than those of the radii: the portion added to the alæ during the diametric growth differs much in appearance from the other portion. The scuta have the growth ridges but little prominent; they are crossed by a faint longitudinal band of gray. The largest specimen out of the many which I have seen, was under .4 of an inch in basal diameter.
_Scuta_, destitute of an adductor ridge and of crests for the depressor muscles: the articular ridge is moderate; but the articular furrow is rather wide: the internal occludent margin is much thickened. The _Terga_ are narrow and small; they are somewhat variable in shape, caused by the degree to which the basal margin is hollowed out (fig. 1 _c_-1 _e_), and likewise by the extent to which the upper end of the valve has been worn away. No spur is apparent, for it is confluent with the basi-scutal angle of the valve. The articular ridge is very prominent, and runs down to the basi-scutal angle; and as the valve in this part is extremely narrow, with the spur not developed, it here assumes a channelled structure. The basi-carinal corner of the valve is furnished with rather feeble crests for the depressor muscles, and in those varieties in which the basal margin is much hollowed out, this part is remarkably narrow. _Structure of the Parietes and Radii._--The internal basal edges of the parietes and the sutural edges of the radii and alæ, are all smooth. The lower edge of the sheath depends freely. In the green varieties the colour is most distinct on the internal surface of the shell. The four compartments separate very easily when the shell has been ill preserved in spirits, or after a very short immersion in caustic potash.
_Mouth_, as in _E. Kingii_, excepting that there are only three teeth on each side of the notch (which is deeper) on the labrum. The cirri resemble those of _E. Kingii_; the segments in the sixth pair are equally elongated, and bear five or six pairs of spines. _Affinities._--This species is closely allied to its South American representative _E. Kingii_; the differences consist in its smaller size, often greenish colour, more folded walls, and narrower radii: the internal basal edges, also, of the parietes are here smooth, instead of being striated, as in _E. Kingii_. The terga present even more obvious differences, in their narrowness, channelled under surface, and in the absence of the spur, or more properly in its confluence with the basi-scutal angle of the valve. 3. ELMINIUS PLICATUS. Pl. 12, fig. 2 _a_-2 _f_. ELMINIUS PLICATUS. _J. E. Gray._ Appendix to Dieffenbach's Travels in New Zealand, p. 269, 1843. _Shell deeply folded longitudinally, corroded, coloured in parts orange: radii very narrow, with their edges sinuous, and slightly dentated: scutum having an adductor ridge._ _Hab._--New Zealand; New South Wales(?). Attached to rocks, often coated by _Chamæsipho columna_; Mus. Brit. and Cuming. _General Appearance._--Shell tubulo-conical, or conical, rarely depressed; strong, rugged, coloured in parts bright orange; deeply plicated longitudinally, but with the upper parts corroded and smooth. Orifice large. The sutures are indistinct and almost obliterated; the radii, when most developed, are narrow. Some specimens have their whole surface deeply corroded; in which case they are finely striated longitudinally, or pitted, and are of a gray or brown colour. The largest specimens are one inch in basal diameter, but one depressed specimen was 1.3 in diameter; another was rather under one inch in diameter, and one inch in height.
_Scuta_; beginning with the common tubulo-conical and not much corroded specimens, the valve (fig. 2 _c_) is moderately elongated, but in a rather variable degree. A prominent adductor ridge runs, from a little above a middle point of the basal margin, along the slightly prominent articular ridge: the articular furrow is moderately wide. There are distinct crests for the lateral depressores. In the conical, corroded specimens, the scuta (fig. 2 _e_) are considerably broader, with the articular ridge much more prominent, and the furrow wider: in one such specimen, there were crests for the rostral depressor muscle. The _Terga_, in the commoner variety, resemble those of _Tetraclita porosa_; the spur adjoins the basi-scutal angle of the valve: the articular ridge is moderately prominent, and the furrow moderately deep. The valve is beaked, with an unusually large internal tube for the thread of corium: the beak, however, is often worn away. In the depressed much corroded specimens, the terga (fig. 2 _f_), like the scuta, are broader and shorter than in the commoner variety; and the spur more especially is broader. The scutal margin is much more widely inflected, and the articular ridge much more prominent; consequently the articular furrow is much deeper.
_Structure of the parietes and radii._--The orange or yolk-of-egg colour, which is so conspicuous a character in the present species, is due to a layer of shell between the inner and outer lamina, and is exposed only by the corrosion of the latter. Hence the very base of the shell is not of this colour; nor are the uppermost and still more deeply corroded portions, for here the orange-coloured layer has been removed. The sheath is orange-coloured, and the operculum, to a certain extent, is similarly tinted. The epidermis on the parietes, where preserved quite close to the basis, supports remarkably strong spines, about 1/100th of an inch in length. The basal internal edges of the walls are rather coarsely striated with irregular short ridges and sub-cylindrical points; and the walls in most of the specimens are regularly and deeply folded, which, with the little ridges, gives the appearance represented in fig. 2 _b_, Pl. 12. I have stated, under the Genus, that in the corroded and depressed specimens, the walls are rendered extremely thick by the inward production and upward extension of these same ridges and points; the under surface of the shell acquiring almost the appearance of _Chelonobia caretta_. The Radii are often not developed, even the sutures being obscure; when most developed, they are narrow, with the outer lamina along the growing edge sinuous, giving to the sutures a crenated appearance. The sinuosities on the growing edge generally send inwards short ridges or septa, like those on the sutural edges of the radii in most Balanidæ, but of which there is no trace in the other species of Elminius. In very minute, colourless specimens, about the 1/20th of an inch in diameter, the radii are quite smooth-edged. The alæ have their edges strongly crenated. The lower edge of the sheath depends freely.
_Mouth_: the labrum shows some tendency to be bullate; the notch is broad and shallow: the palpi have a thick brush of bristles on their inner sides. The mandibles have four or five teeth. In the maxillæ, the upper spines above the broad notch, are very strong. In the outer maxillæ, the two lobes are widely separated. _Cirri_: in the first pair, one ramus is about one fifth longer than the other. In the third pair, the posterior ramus is one fourth longer than the anterior ramus, and its terminal segments are tapering, each having a single circle of bristles: the other segments, and those of the shorter ramus, support many coarsely pectinated spines. In the sixth cirrus, the segments are protuberant in front, and carry four pairs of stout spines, with a tuft of fine bristles between them. _Affinities._--This species differs considerably from the first two of the genus. In several characters it approaches nearer than the other species to Tetraclita, especially to _T. porosa_;--namely, in the scutum having an adductor ridge and crests for the lateral depressores, in the whole form of the tergum, in the thick walls liable to much corrosion, in the narrow radii, and in their edges, as well as those of the alæ, being crenated; and, lastly, in the character of the cirri, more especially of the third pair, with its coarsely pectinated spines. It also approaches, in all its characters, _Balanus imperator_ and _flosculus_.
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