A Monograph on the Sub-class Cirripedia · Charles Darwin
LEPAS ET BALANUS AUCTORUM.
_Moveable scutum, with the lower articular ridge not half as broad as the short upper articular ridge: shell generally ribbed longitudinally._ _Var., with the shell not longitudinally ribbed._ _Hab._--Shores of Great Britain and Ireland, Shetland Islands; and, according to various authors, Denmark, Iceland, and shores of northern Europe. Red Sea, Brit. Mus. Attached to shells, laminariæ, rocks, crabs, and floating bark, from low tidal mark to fifty or ninety fathoms. _Fossil_ in Glacial deposits of Scotland, Mus. Lyell; Red Crag (Walton, Essex), Coralline Crag (Sutton), Mus. S. V. Wood. I have given so full a description of the genus that little remains to be said under the species. Generally the whole shell is covered (independently of the interfolding, oblique, articulating plates) by narrow, longitudinal ridges or folds; and by this character alone the ordinary variety of _V. Strömia_ can be distinguished (as far as I have seen) from all the other species. The shell is white or dirty yellowish-brown. The scutum has the lower articular ridge on its tergal margin very narrow (but somewhat variable in width), appearing like a mere slight shoulder, against which the longitudinal axial ridge of the tergum abuts: it is not half as wide as the short, upper articular ridge. On the under side there is a very slight depression for the adductor scutorum muscle. There is considerable variation in the degree to which the transverse ledge on the under side of the fixed tergum projects, and therefore in the depth of the hollow thus formed. The specimens with the right-side, and those with the left-side opercular valves moveable, are apparently about equally numerous.
The specimen in the British Museum, from the Red Sea, was attached to a Gorgonia, and was in the same box with a Pyrgoma--circumstances favouring the correctness of the locality--but I am much surprised from the general distribution of the species, that _V. Strömia_ should occur in so distant and isolated an area. After careful examination, I can discover no constant difference between the Red Sea and British specimens. The specimens from the Crag have not their moveable opercular valves, which offer much more important diagnostic characters than the shell; but as far as the latter is concerned, no difference whatever can be perceived from _V. Strömia_. 2. VERRUCA LÆVIGATA. Pl. 21, fig. 3 _a_, 3 _b_. VERRUCA LÆVIGATA. _G. B. Sowerby._ Genera of Recent and Fossil Shells, Plate. _Moveable scutum, with the lower articular ridge broader than the short upper articular ridge; moveable tergum broader than high, with the upper articular ridge produced into a point._ _Hab._--Tierra del Fuego; Eastern Patagonia, nineteen fathoms; Chile; Peru; Mus. Brit., Cuming, Stutchbury, Darwin: attached to shells, and often to _Balanus lævis_ and _psittacus_.
I can point out no difference in the shell between this species and _V. Strömia_, excepting that its walls seem invariably to be smooth, which is rarely the case with _V. Strömia_; perhaps also the oblique interfolding articular plates between the several compartments are here more prominent. It appears that specimens with the left side uppermost, and therefore with the left opercular valves moveable, are considerably more common than those with the right valves moveable. The moveable scutum and tergum are articulated together by much more prominent articular ridges than in _V. Strömia_, and the two valves together are broader in proportion to their height,--the height being measured from the apex to the basal margin. In the scutum the lower articular ridge is considerably broader than the short upper ridge. In the tergum, the basi-carinal corner is more rectangular, and the whole valve is nearly square: owing to the deep furrow receiving the lower articular ridge of the scutum, the axial ridge of the tergum is proportionally narrower but more prominent than in _V. Strömia_; the uppermost ridge (formed by the occludent margin of the valve) projects, especially when viewed on the under side (fig. 3 _b_), as a moderately sharp point. In the _mouth_, the lower teeth of the mandibles are more distinct than in _V. Strömia_; the lower part of the edge of the maxilla is very prominent. In the second and third pairs of cirri the terminal spines on the shorter rami are coarsely pectinated; on the sixth pair there are only three pairs of main spines on each segment; but these several points, according to the analogy of other species, I should expect to be variable.
This species is alluded to by Bruguière, in the 'Encyclopédie Méthodique,' but was confounded by him with the _V. Strömia_ of Europe. 3. VERRUCA SPENGLERI. Pl. 21, fig. 2. _Moveable scutum, with a sharp, straight, medial adductor ridge: fixed scutum not larger than the fixed tergum._ _Hab._--Madeira, Mus. Lowe; attached to shells. It would appear that the present species does not attain quite so large a size as the more northern _V. Strömia_; the walls are not longitudinally ribbed as is usual with this latter species. The proportional sizes of the compartments seem to be somewhat different; the fixed scutum is either equal to or even smaller than the fixed tergum, instead of being larger, as in _V. Strömia_; but in young individuals the proportions are reversed. In several specimens the fixed scutum and tergum together were larger than the carina. The rounded adductor plate of the fixed scutum is extremely large. The lines of growth, especially on the moveable opercular valves, are rather more plainly crenated than in _V. Strömia_. In the moveable scutum the lower articular ridge on the tergal margin varies a little in size, and is sometimes larger than in _V. Strömia_ (but never so large as in _V. lævigata_), and is placed more in the middle of the tergal margin: but by far the most important character by which this species can be distinguished from all the others, is the presence, on the under side of the moveable scutum, of a straight, prominent adductor ridge, which runs up to and even under the apex of the valve, for it is there slightly hollowed out. In the moveable tergum, owing to the medial position of the lower articular ridge of the scutum, the middle of the scutal margin is more hollowed out, and the axial ridge narrower, than in _V. Strömia_.
In the animal's body the only difference which I could perceive was that the shorter rami of the second and third pairs of cirri were not so short, compared either to the other cirri or to the longer rami of these same cirri. In the second cirrus, in a moderately-sized specimen, the segments were six and thirteen in number in the two rami, and in the third cirrus, seven and fifteen. Had it not been for the specimen in the British Museum of _V. Strömia_, from the Red Sea, I should have concluded, from geographical considerations, that _V. Spengleri_ probably was the species found in the Mediterranean, and noticed by Spengler ('Schriften der Berl. Gesell.,' 1 B., 1780), as a small variety of the northern _V. Strömia_; and likewise that it was the _Creusia echinoides_ of Risso ('Hist. Nat. Product. de l'Europe,' tom. 4, p. 382, 1826), which is certainly a Verruca, but not described with sufficient minuteness to be recognised. 4. VERRUCA NEXA. Pl. 21, fig. 5. _Shell reddish: moveable scutum, with three strongly prominent longitudinal ridges, besides the articular ridges: fixed scutum larger than the carina, with no distinct adductor plate._ _Hab._--West Indies, Mus. Brit.; attached to a Gorgonia.
This species differs considerably from all the others in the genus. The shell is brownish-red, tinted yellow: it is not at all depressed like the former species, but the walls are almost perpendicular or even overhang their bases, and the summit of the shell consequently is broad. This form may be in part, but only in part, due to the attachment on the thin branches of the Gorgonia. The umbones of the compartments are remarkably prominent and sharp. Although the parietes are nearly smooth, yet from being so steep, they are little seen, and owing to the very prominent but rounded ribs by which the compartments and opercular valves are articulated together, the whole shell has a strongly ribbed appearance. The diameter of the largest specimen was .2 of an inch.
The _rostrum_ (A, fig. 5) is patelliformed, with the umbo of growth sub-central, but rather above the middle point; hence this valve, differently from the carina, and differently from the rostrum of the other species, grows not only at its basal margin, and on both sides where opposed to the carina and fixed scutum, but also along its upper margin where opposed to the basal edges of the moveable scutum and tergum: owing to the perpendicularity of this valve, the upper part forms a ledge almost parallel to the orifice of the shell. The carina (B) is of unusually small size, being about only half the size of the rostrum, and scarcely exceeding in size the fixed tergum. The _fixed scutum_ (S′) is large, larger even than the carina; it is oblong, and its shape is more simple than in the other species; this is chiefly owing to the rostrum articulating with the whole of that margin (_b_) which answers to the basal margin of the moveable valve; whereas in the other species (fig. 1 _b_) it curls beyond this margin, and articulates with the very protuberant, so-called, parietal portion of the valve.
Three or four rounded prominent longitudinal ribs, exactly like the homologous ribs on the moveable scutum, run from the apex of the fixed scutum to the basal margin, and their extremities form the teeth by which it articulates, as just stated, with the rostrum. Its upper articular ridge (′) is more prominent, and placed much lower down in the suture between it and the fixed tergum, than in the foregoing species. The ledge (_o_) by which the orifice is kept neatly closed, is here more distinct than in _V. Strömia_: this ledge is necessary, as well as in the case of the fixed tergum, owing to the altered shape of the summits of the moveable scutum and tergum, due to their corrosion and to their coming to project freely. But the most remarkable character of the fixed scutum is, that on the under side there is no great adductor plate, but a rounded hollow with its lower edge only slightly prominent; the absence of the adductor plate, which is present in all the other species of the genus, is no doubt due to the under side of this valve being inclined even outwards, and so standing in some degree opposed to the moveable valve; thus affording on its under surface a place for the attachment of the lower end of the adductor scutorum muscle; whereas in the other species this muscle could not possibly have been attached, without the aid of an adductor plate, to the under side of the much depressed and sloping fixed valve. The _fixed tergum_ (T′) is a little more simple in form than the corresponding valve in the other species; the two arms, answering to the occludent and carinal margins of the moveable tergum, are more nearly equal in length: the internal transverse ledge, separating these rims or margins from the parietal portion of the valve, is but little developed.
All four valves forming the shell are remarkable from having, when full-grown, but not whilst young, their basal edges abruptly inflected inwards, thus forming a ledge all round the basal membrane, as in _Chthamalus intertextus_ and _Hembeli_. _Moveable Scutum._--This is slightly larger in proportion to the tergum than in the foregoing species: it is chiefly remarkable from the presence of three prominent longitudinal ridges on the main part of the valve, like the two articular ridges on the tergal margin; of these latter, the lower one extends down to about the middle of the tergal margin. The _moveable tergum_ is rhomboidal, with the whole carinal portion marked only by lines of growth: it is only remarkable by the upper of the three articular ridges on the scutal margin being unusually distinct from the occludent margin.
With respect to the animal's body, its several peculiarities have already been pointed out under the genus. The _labrum_ is decidedly bullate, triangular in section, with a row of minute bead-like teeth on the crest; the palpi are very narrow and short, and do not nearly touch each other: this variation in the structure of the labrum and in the size of the palpi, is very remarkable, considering how important, in a classificatory point of view, these parts are in all other Cirripedes. In the _mandibles_ there are either two or three main teeth, with the whole lower part of the organ pectinated with sharp spines. _Cirri_: the first pair is not short; in the individual examined, the two rami had eleven and twelve segments. In the second pair, the shorter ramus was two thirds of the length of the longer ramus, the segments being in number ten and fifteen; in the arrangement of the spines this second pair resembles its homologue in the three other species. In the third pair, the two rami are very nearly equal in length, having sixteen and eighteen segments; and the segments of the anterior ramus are only a little thicker and more thickly clothed with spines than those of the posterior ramus. The remaining cirri and the caudal appendages are as in the other species.
5. VERRUCA PRISCA. Pl. 21, fig. 4. VERRUCA PRISCA. _Bosquet._ Monographie des Crustacés fossiles du Terrain Crét. de Limbourg, Tab. 1, fig. 1-6; 1853. _Shell smooth: moveable scutum, with the lower articular ridge somewhat broader than the upper articular ridge._ Fossil--'Système Senonien et Maestrichtien,' Belgium, Mus. Bosquet; in Chalk, Norwich, Mus. J. de C. Sowerby. M. Bosquet has admirably figured and described the several separated valves belonging to this species, and I owe to his great kindness an examination of some of them. In Mr. J. de C. Sowerby's collection, also, there is a single specimen, attached to a Mollusc, with the four valves of the shell united together, but without the two moveable opercular valves; it cannot be positively asserted that this is the same species with that of M. Bosquet, but such probably is the case. This is the species to which I alluded in the Introduction to my 'Monograph on Fossil Lepadidæ.' It is an interesting species, from being the only known Secondary one, but in itself it is a very poorly characterised form, and I can point out no important character in the shell by which it can be recognised. The rostrum and carina, which are of nearly equal sizes, are locked together by the usual interfolding plates, and likewise to the fixed scutum and tergum; but these latter plates seem to have been less developed in M. Bosquet's specimen than in the English. The fixed scutum has a large adductor plate, which seems to have been chipped in M. Bosquet's specimen; this valve and the fixed tergum in all essential respects resemble the same valves in _V. Strömia_. The surface of the shell is very smooth.
The _moveable scutum_ has its occludent margin considerably arched: the lower articular ridge is broader than the upper ridge, in which respects it resembles the same valve in _V. lævigata_, but the whole valve is not so broad as in that species. There is no adductor ridge on the under surface. The _moveable tergum_ has its upper articular ridge narrow, and slightly produced into a point on the scutal margin: in this latter respect this species also resembles _V. lævigata_, but the whole valve is not so broad in proportion to its height. 3. Family LEPADIDÆ. _Cirripedia having a flexible peduncle, provided with muscles: scuta and terga, when present, not furnished with depressor muscles: other valves, when present, not united into an immoveable ring._ This Family has been fully treated of in my former volume, published by the Ray Society,[141] and I should here only have alluded to its existence, had it not been for the genus Alcippe, which differs in so many important characters from the other members of the Lepadidæ, that formerly I did not even suspect that it could belong to this Family, and therefore deferred its examination. The genus Alcippe was discovered, well described and illustrated, in 1849, by Mr. Hancock; to whose very great kindness I am indebted for permission to dissect and examine his entire stock of this truly remarkable Cirripede. In the classification of the whole class I have not felt so much doubt, as whether I ought to institute a family for the reception of this genus.
Alcippe differs from all other Cirripedes (putting on one side for the instant, the males and complemental males of Ibla and Scalpellum) in the very singular fact of being destitute of a rectum and anus;--in the three segments of the thorax, which usually support the second, third, and fourth pairs of cirri, being without any appendages;--in the fifth and sixth pairs of cirri having their inner or posterior rami metamorphosed into very singular roughened cushions or buttons, which apparently serve to triturate the food;--in the caudal appendages being muscular, and being used conjointly with the cirri;--and lastly, in the pupa having a lesser number of segments in its abdomen and caudal appendages than in (as far as I have seen) any other Cirripede. It will be thought that these characters are amply sufficient to justify the placing Alcippe in a separate family, more especially when the close general resemblance in the animal's body in most of the other members of the Balanidæ, Verrucidæ, and Lepadidæ, is borne in mind. On the other hand, the males and complemental males of Scalpellum and Ibla must indisputably be considered as members of the Lepadidæ; yet the male of _Scalpellum vulgare_ and _ornatum_ has no stomach, anus, or mouth, which is a far more abnormal structure than the absence only of the anus in Alcippe: the cirri, also, in these same males, differ from the ordinary cirripedial type decidedly more than in Alcippe. Again, in the male of Ibla, all the cirri, excepting the fifth and sixth pairs, are aborted, and these two pairs are usually only uniramous; here, then, we have a decided resemblance to Alcippe. Hence, if we might assume that the female Alcippe had partially assumed characters confined to the males of the other genera, it would assuredly stand amongst the Lepadidæ.
Independently of this comparison with the foregoing males, the affinities of Alcippe are so special to several genera amongst the Lepadidæ, that it seems unnatural to force it out of the position which it well occupies between Ibla and Anelasma, and place it in another family by itself: thus, in being bisexual, and in the general character of its very curious males, Alcippe shows an affinity to Ibla and Scalpellum; and to the former of these genera it is related in several particulars, such as in the body being lodged within the peduncle, and in the structure of the larval antennæ, &c.: to Anelasma and Alepas it is allied in the general character, and to a certain extent in the muscles, of the capitulum; Anelasma, also, has all its cirri to a certain degree rudimentary, and _Alepas cornuta_ has the inner rami of the fifth and sixth pairs of cirri,--namely, the very same rami which are so curiously modified in Alcippe,--small, destitute of muscles, and functionless for their proper purpose: to Anelasma and Lithotrya it is allied in the peculiarity of the lower end of the peduncle becoming elongated by growth, and in being imbedded; and to Lithotrya by its powers of excavation and manner of attachment.
Now, I believe it generally holds good that when a form is really distinct from another group, its affinities are general, or only in a slight degree special to the members of that group. Nor, indeed, can it be asserted that Alcippe differs much more, somewhat more it certainly does, from the other genera, than does Anelasma, with its more singular mouth, spineless rudimentary cirri, and fimbriated peduncle; and I have never regretted having included this genus amongst the Lepadidæ. Hence, after much consideration, I have resolved to consider Alcippe as one of the Lepadidæ, though so curiously modified,[142] and having characters confined to the males of the other genera. Perhaps I have been in some degree influenced by the difficulty of finding external characters by which to separate Alcippe as a family from the other Lepadidæ. [141] The fossil species have been described in a separate Monograph published by the Palæontographical Society. Since its publication, M. Bosquet has produced an excellent memoir, containing descriptions, with the most beautiful illustrations, of several new Cretacean species of Pollicipes and Scalpellum. The memoir is entitled a 'Monographie des Crustacés Fossiles du Terrain Crétacé, du D. de Limbourg.'
_Genus_--ALCIPPE. Pl. 22, 23. ALCIPPE. _Hancock._ Annals and Mag. of Nat. Hist., vol. 4, 1849, Pl. 8, 9. _Fem.--Capitulum without valves, with the orifice spinose: peduncle with the basal end added to during growth; its rostral surface depressed and covered by a horny disc: capitulum and peduncle imbedded in a cavity excavated in the shells of molluscs._ _Labrum very large, with a row of long hairs on each side: palpi rudimentary: mandible one-toothed: second, third, and fourth cirri absent: fifth and sixth cirri with the posterior ramus represented by a button-like body: caudal appendages four jointed, muscular: anus none._ _Males,--several, adhering to the upper end of the horny disc of the female: capitulum naked, transparent, elongated, with a small orifice at the end: peduncle lobed, with the lower end extending far beyond the pupal antennæ: eye, testis, and vesicula seminalis single; probosciformed penis very long: mouth, stomach, thorax, abdomen, and cirri none._ ALCIPPE LAMPAS, _Hancock_ ut suprà. _Hab._--North-eastern shores of England, fifteen to twenty fathoms, imbedded in dead shells of _Fusus antiquus_ and _Buccinum undatum_ (A. Hancock); south-eastern shores, off the Eddystone, Lighthouse (C. S. Bate). FEMALE. Pl. 22. I may premise that after the sketch of the leading peculiarities of Alcippe, and after the discussion on its affinities, just given under the Family, I think it would be superfluous to institute a full generic description, separately from the following detailed account of this most anomalous cirripede.
_General Appearance._--The whole animal is from .2 to .3 of an inch in length, of a soft texture, colourless or yellowish, and lives concealed in a cavity of its own formation in the shells of certain Gasteropods. This cavity communicates with the water by a narrow fissure-like orifice (Pl. 22, fig. 4), broadest at the posterior end, where the cirri are exserted; narrow, closed, and generally curved at the other (_a_) end: the two sides of the fissure (_b_) are commonly bordered by a calcareous inorganic deposit: the walls of the cavity are worn so thin over the peduncle, at the narrow end of the fissure, that the orange-coloured ovaria can generally be seen through the shell of the mollusc, and hence there is here a distinct fan-shaped stain (fig. 3) on the surface. The animal consists of a compressed capitulum, without valves, and of a sort of peduncle depressed on its rostral face, and covered with a broad, oval, thin, horny disc. We must remember that in the Lepadidæ the peduncle does not essentially differ from the capitulum, being only the flexible lower or anterior end of the animal, and is separated from the capitulum only by shape, and generally by the direction of the lines of growth. The disc, when most regular (fig. 1, H), lies in a plane at right angles to the sides of the capitulum, and almost in a line with the orifice leading into the sack; but the peduncle is often very irregular (fig. 2), and the disc comes even to occupy a position nearly parallel to one or the other side of the capitulum. On the carinal side, the capitulum is generally separated from the peduncle by a rather deep fold (_f_, in the section fig. 5), but this depends in some degree upon the state of distension of the mass of ovarian cæca. I have given a drawing, fig. 1 (partly taken from Mr. Hancock), of a very regular individual, and of an extremely distorted specimen (fig. 2). The distortion, I believe, is generally caused by the animal, during its excavation, breaking into some old cavity.
Ideas discussed in this chapter
Text from Project Gutenberg, public domain in the United States.