A Monograph on the Sub-class Cirripedia · Charles Darwin
3. PLATYLEPAS ----?
_Animal's body_, unknown. 7. CHTHAMALUS INTERTEXTUS. Pl. 19, fig. 1 _a_, 1 _b_. _Shell, when well preserved, violet-purple: sutures, when not obliterated, formed by oblique interfolding laminæ: basis membranous, but surrounded by a ledge formed by the inflected basal edges of the parietes: scutum and tergum completely calcified together._ _Hab._--Philippine Archipelago; Mus. Cuming. _General Appearance and Structure._--Shell depressed, with a large diamond-shaped orifice. Colour beautiful violet-purple, but externally much obscured by disintegration, causing the shell to be ashy gray. Walls smooth or slightly folded. Sutures generally quite, or almost quite obliterated; but when well preserved, they differ remarkably in appearance from those in the foregoing species; for the radii externally here consist of oblique plates or laminæ arising on both sides of the sutures, standing nearly parallel to the parietes, and interfolding with each other. These laminæ are rather plainly marked by lines of growth. Essentially the radii do not differ much from those in _C. dentatus_ and _Hembeli_; we have but to produce obliquely upwards the transverse and interlocking ribs on their radii, and so convert them into laminæ. During the diametric growth of the shell, the sutural edges of the alæ are added to, in the usual manner, by upturned lines of growth; and, in addition, the recipient furrows of the alæ are similarly added to, so that the lines of growth are upturned, and alæ appear to have been developed on both sides of the sutures in the same way as the radii appear to have been developed on both sides in many Chthamalinæ, though rarely in the Balaninæ. The inside of the shell is beautifully coloured rich violet; it is punctured with small holes as so often is the case with _C. stellatus_. In every specimen (all full-grown) which I opened, the inner basal edges of the parietes were inflected rectangularly inwards, forming a smooth-edged ledge all round the basal membrane, which, in proportion to the width of this ledge, was by so much reduced in diameter. The largest specimen which I have seen was .35 of an inch in basal diameter.
_Opercular valves._--The scuta and terga, in all the specimens which I have seen, were firmly calcified together; in some, a trace of a suture could be seen externally, but hardly a trace internally. In one specimen, there were vestiges of some impressed lines on the scutum, in exactly the same position in which such occur in _C. Hembeli_. The scutum is rather narrow. The basal margin of the tergum is either straight, or depends a little on the scutal side, thus producing a small spur: the crests for the depressor muscles are strongly marked, and depend beneath the basal margin. _Mouth._--The labrum is strongly toothed: the palpi have long hairs along the exterior basal margin: the mandibles have only three main teeth, and the inferior coarsely pectinated portion is short; the maxillæ deeply notched. _Cirri_: the first and second pairs have their rami slightly unequal in length; the third pair differs from the same pair in the other species of the genus, in having some few of the basal segments on the anterior ramus thickly clothed with spines, so as to be brush-like: there is even a trace of a similar structure in the lowest segments of the posterior ramus. In the three posterior pairs of cirri the segments are much elongated, and support four pairs of spines. _Branchiæ._--Unknown. 8. CHTHAMALUS SCABROSUS. Pl. 19, fig. 2 _a_-2 _d_.
_Shell (when well preserved) dull purplish-brown: sutures formed by oblique interfolding though rarely well developed: tergum with a deep narrow pit, at the basi-carinal angle, for the depressor muscle._ _Hab._--Peru, Chile, Tierra del Fuego, Falkland Islands. Very common; attached to littoral rocks and shells, and often associated with _Balanus flosculus_, and sometimes with _Chthamalus cirratus_; Mus. Brit., Cuming, W. Dunker, Darwin. _General Appearance and Structure._--Shell generally depressed; when growing crowded, sometimes cylindrical: colour dingy purplish-brown, but when much corroded, dirty gray; very young shells are very dark green, owing to the corium, which is of this colour, being seen through the valves. Surface generally rugged, from irregular slight longitudinal folds, and from the transverse overlapping tile-like lines of growth; but sometimes the surface is nearly smooth, with very slight longitudinal folds, these being gray coloured, the intermediate parts being pale dingy purple, the shell thus becoming striped. Orifice rhomboidal, passing into trigonal, owing to the great width of the carinal end. Sutures generally very distinct, rarely obliterated in the cylindrical varieties. Radii narrow, generally exposing much of the alæ, which are plainly marked by lines of growth: the radii themselves, when well developed, which is not often the case, consist of small laminæ or ridges, placed on both sides of the sutures, and interfolded or interlocked together: usually only a trace of this structure is exhibited, but occasionally, along some or along all the sutures (as in the specimen figured), the laminæ of the radii interfold, as plainly as in _C. intertextus_. The alæ differ slightly from the alæ of the other species, in not forming so much of a rectangular projection, the lower margin running with a gentle curve into the parietes. The internal surface of the parietes is either smooth, or near the basal margin is roughened with depending points: in some specimens from the Falkland Islands, both edges of each suture were inflected, forming a double ridge, with roughened edges, resting on the basal membrane, and supporting the shell. I must mention that in my notes made at these Islands, I remark that the basal membrane seemed sometimes to be surrounded by a calcareous rim; none of the specimens brought home are thus characterised; but bearing in mind the affinity of this species to _C. intertextus_, no doubt we have here an indication of the shelly ledge surrounding the basis, as described under that species. The largest specimens which I have seen are a quarter of an inch in basal diameter.
The _Opercular Valves_ generally have their summits much worn down. The _scuta_ are elongated in the line of the longer axis of the orifice; the articular ridge is very prominent, and is placed in the middle of the tergal margin. The _terga_ are very narrow, as in some varieties of _C. stellatus_: they are remarkable in two respects, namely, in the depressor muscle being attached to a plate, formed apparently by the union of the usual crests, parallel to the outer lamina of the valve itself, a deep narrow cavity (fig. 2 _d_, _p_) being thus formed; and secondly, in the far more extraordinary circumstance of the existence of a small pit (_q_) at the extreme basi-scutal corner of the valve, in which about half of the scutal lateral depressor muscle is attached: I have observed no other instance in any cirripede of the partial attachment of a muscle properly belonging to one valve to another valve. The figures of the valves 2 _b_, 2 _c_, 2 _d_, are from specimens most unusually perfect, with the upper portion not worn away; the ordinary appearance of the valves as seen from above, is given in fig. 2 _a_; at fig. 2 _d_, a view is given of the tergum seen from vertically beneath, showing the cavity for its own depressor muscle, and for part of the lateral depressor muscle of the scutum.
_Mouth._--The crest of the labrum is hairy, without teeth: the palpi have long spines at the end, but none along the inferior margin: the mandibles have either four or five graduated teeth, the lower ones of which are plainly double laterally: the maxillæ have a very sinuous edge. _Cirri_: the first and second pairs are very short: on the four posterior pairs the segments support either four or five pairs of main spines, with the small intermediate spines rather larger than usual. _Branchiæ._--None; but where they ought to occur, there are two very slight ridges clothed with hairs, about the 1/100th of an inch in length. On the prosoma, there is a slight ridge, extending from the base of the first cirrus towards the adductor scutorum muscle, also clothed with hairs; this unusual character of the prosoma being hairy is common to _C. dentatus_. This species is the commonest cirripede on the shores of the Falkland Islands: many of the specimens are there crowded together, and rendered elongated and cylindrical, with the walls very thin, and the sutures often obliterated; as the opercular membrane is very narrow, the opercular valves are much influenced both in their outline and in their crests and articulations, by the varying form of the shell: I have even seen specimens with the scutum and tergum on one side twice as large as on the other side.
13. CHAMÆSIPHO--_Nov. Genus_.[132] Pl. 19. [132] Χαμαι [Greek: Chamai], on the ground, and σιφων [Greek: siphôn], a tube. _Compartments four, with the sutures often much obliterated: basis membranous._ _Distribution_, Australia, China (?). Attached to littoral shells and rocks. The two species united under this genus agree in having only four compartments, and in these having a strong tendency to become confluent; but they resemble each other hardly in any other respect, more than do all the species of the present sub-family. _Chamæsipho columna_ differs from all the other Chthamalinæ in the structure of its second pair of cirri, and _C. scutelliformis_ differs from all in its shell,--namely, in the small size of the rostrum, with its alæ but little developed, and in the very peculiar apertures in the three other compartments. Hence this genus can hardly be considered a very natural one, though I could not introduce the two present species into Chthamalus, or into any other genus, without doing still more violence to the principles of classification followed throughout this work. Chamæsipho bears nearly the same relation to Chthamalus, as Tetraclita and Elminius do to Balanus. 1. CHAMÆSIPHO COLUMNA. Pl. 19, fig. 3 _a_-3 _c_. LEPAS COLUMNA. _Spengler._ Skrifter Naturhist. Selbskabet, b. 1, (1790), Tab. 6, fig. 6. _Sutures, excepting during early youth, generally obliterated both externally and internally: tergum with small pits for the attachment of the depressor muscle._
_Hab._--New South Wales, Tasmania, New Zealand; extremely common; attached to littoral shells and rocks; often associated with _Chthamalus antennatus_ and _Elminius modestus_; and in New Zealand, often thickly coating _Elminius plicatus_. I have identified, with some doubt, the present species with the _Lepas columna_ of Spengler, obtained from Otaheite, as no description is given by him of the opercular valves, and more especially as Spengler's specimens were an inch in height and seven lines in width, which is much larger than any of the many specimens seen by me: from Spengler's clear description of the structure of his shell, it evidently belongs to the present genus. _General Appearance and Structure of Shell._--The sutures separating the four compartments are generally, excepting at an early age, quite obliterated, both internally and externally, the shell in this case consisting of a single piece, with its summit and opercular valves always much worn. Occasionally the sutures are preserved, and then the four compartments are seen to be of nearly equal sizes. The orifice is always broadly oval, with the carinal end the broadest; and it often approaches closely to circular. The upper part of the shell is frequently steeply conical, with the lower part spreading and folded; sometimes deeply folded. Very young shells are apt to be remarkably smooth. The radii appear never to be developed: the inside of the shell is smooth. The colour is either blackish-green (this being the tint of the corium lining the sack), with the upper part gray from disintegration; or the lower part is brown, from the investing membrane, the shell itself being pale coloured. This species seems particularly liable to grow crowded together; often covering rocks and shells with a honey-combed layer. The basal diameter of some of the largest specimens was .3 of a inch. In Mr. Cuming's collection, however, there is an Australian specimen .55 in diameter and .3 in height, which is, moreover, remarkable from the projecting, extremely rugged, overlapping, dark-coloured layers of growth, which surround the lower part of the shell: we have seen that Spengler's specimens, said to have come from Otaheite, are even broader and considerably higher.
_Scuta_: from the disintegration which the valves have undergone, the scuta and terga are externally seen to be locked together by a deeply sinuous articulation. The _Scuta_ have a wide articular furrow and a very prominent articular ridge; but the exact outline of these parts varies greatly, very much as in the genus Chthamalus. The _Terga_ are very narrow, with the under surface channelled: the attachment of the depressor muscle offers the only peculiarity,--the muscle being attached to four or five little pits, placed transversely to the longer axis of the valve; the septa between these little pits evidently answering to the crests as usually developed. _Mouth._--The crest of the labrum is hairy, and slightly bullate. The palpi are rather small, with long spines at their ends. The mandibles have four (sometimes five) teeth, with the inferior portion pectinated. The maxillæ are notched.
_Cirri._--The first pair presents no remarkable character. The second and third pairs are subject to extreme variation, as in _Chthamalus antennatus_ and _Tetraclita porosa_. In all the specimens, the anterior ramus of the second cirrus is short, with all the segments thickly covered with bristles; it is the posterior ramus which varies so much in relative length and in the arrangement of the bristles; but in no case are all the segments clothed with bristles as on the anterior ramus, and as is normally the case with all sessile cirripedes. In some specimens from New Zealand, the anterior ramus having only five segments, the posterior ramus was twice as long, having sixteen segments, with the bristles arranged in circles, but standing rather thicker together on the basal segments. In a Tasmanian specimen, the posterior ramus was only a little longer than the anterior ramus, and the spines were arranged in regular pairs (as on the three posterior pairs of cirri) on all the segments, excepting the few basal ones, on which they were more crowded: a nearly similar arrangement occurred in some other specimens from unknown localities, excepting that the rami were of nearly equal length.
In the third cirrus, in all the specimens, the anterior ramus has three or four of its basal segments much broader than the upper segments, and thickly clothed with spines (as is the case with one species of Chthamalus, viz. _C. intertextus_) all the other segments having regular pairs of spines. The posterior ramus of the third cirrus varies in being either much longer than, or only equal in length to, the anterior ramus; in the former case (in the New Zealand specimen) the spines were arranged in circles, giving an antenniformed structure to the ramus; and in the latter case they were arranged in regular pairs. In Mr. Cuming's great Australian specimen, there was a further peculiarity, in the presence on the posterior cirri of a tuft of intermediate spines between the main pairs; and, in there being on those segments, which are thickly covered with spines, certain very large spines, doubly pectinated, with the pectinations elbowed, closely like the spines met with on the cirri in certain species of Pollicipes. Finally, the segments in the three posterior pairs of cirri support five or six pairs of main spines; the dorsal surfaces of the segment are rough and hairy. _Branchiæ_, rudimentary; consisting of a small, simple, tongue-formed fold, projecting about 1/100th of an inch. Ova, 16/2000ths of an inch in length.
2. CHAMÆSIPHO SCUTELLIFORMIS. Pl. 19, fig. 4 _a_-4 _d_. _Rostrum very small, elongated, triangular: lateral compartments, each with an aperture, and carina with two similar apertures, all four leading into shelly tubular columns._ _Hab._--Attached to _Pollicipes mitella_, probably from the seas of China; Mus. Brit. _General Appearance._--This very singular shell would not, without some examination, be thought to be a cirripede. From the symmetrical position of the four apertures, with the diamond-shaped orifice in the middle, with the sutures on each side of the rostrum, and from its depressed and circular form, this shell bears some resemblance to the perforated species of Scutella. Shell much depressed, generally nearly circular, with the basal margin highly sinuous and even sometimes almost branched. Surface slightly irregular, marked by fine lines of growth, and covered by brown membrane. Of the four compartments, the _Rostrum_ is very narrow, triangular, and comes up to the orifice almost in a point: it is rather depressed, that is, it lies rather below the level of the other compartments: the straight sutures separating it from the lateral compartments are distinct in the upper part, though always obliterated in old shells in the lower part: these sutures are generally far plainer than those separating the lateral compartment from the carina, which in most cases are obliterated and calcified together, excepting close to the orifice. The alæ of the rostrum are not externally visible, and there are no radii to any of the compartments.
The _Carina_ is twice as broad as the rostrum, and is furnished with alæ of the usual shape, which are, to a certain extent, externally visible. The _Lateral Compartments_ are broad, being broader than the carina; they are both penetrated, down to the surface of attachment, by a hole or rather tube,--the two holes standing opposite the rostral end of the operculum: the carina is penetrated by two rather smaller but similar holes. It is these four holes which give to this cirripede its very singular aspect: they are rather smaller than the orifice of the shell; they are oval, with their longer axes placed in the direction of the ray of the circular shell: their manner of formation will be immediately explained. The orifice is neatly diamond-shaped and broad: it is rather small compared with the whole shell, and is closed by the operculum, which is seated near the summit of the shell. Basal diameter of largest specimen 2/10 of an inch; few, however, attained this size, and perfect larvæ were included in much smaller specimens. _Structure of Shell._--The rostrum (fig. 4 _c_) is remarkable from its small size, and from the plainness of its sutures, in comparison with those separating the other compartments, and this is exactly the reverse of what I should have expected in a compartment tending to become rudimentary. Not only is the rostrum small, but the alæ project to an unusually small degree, and gradually slope away into the lower part of the parietes. These peculiarities are even more strongly marked in very young shells: thus in one of the size of a pin's head, the rostrum consisted of a minute parallelogram, without, as far as I could see, any alæ, and was only one fourth of the size of the carina,--this latter compartment being only half as wide as the lateral compartments. The carina, at this early period, had quite distinct alæ.
The tubular prolongations from the four external holes are of course very conspicuous on the under side of the shell (4 _b_); and their structure is there plainly seen. In extremely young shells the holes are not present; but very soon, at four points of the circumference, namely, two in the carina and one in each lateral compartment, the basal edge becomes indented, and during growth more and more deeply indented; at last the horns or points of the bays, thus formed in the circumference of the shell, grow inwards and meet, the four indentations being thus converted into four rings or holes; as the shell is added to, at its circumference, these come to stand further and further from the exterior margin; and as the shell at the same time rises above the surface of attachment, the holes are added to at their basal edges, and are thus converted into shelly tubes, generally freely open at the bottom as well as at the top. Sometimes these tubes are closed at the bottom, and this is usually caused by their sides having been added to in a spiral direction. A somewhat sinuous double ridge or fissure, leading from the tubes or holes to the exterior border, can always be perceived on the under side of the shell.
Occasionally, though rarely, in very old shells, a second series of holes is formed outside the first four holes, and often a tendency to this may be perceived in the just-mentioned fissures expanding a little at their outer ends, thus forming four new circumferential indentations. The purpose of this peculiar structure, apparently, is to give support to the much depressed and thin shell. In _Chthamalus stellatus_ and _scabrosus_, we have seen a slight indication of a similar structure, in the formation on the under side of the shell, but confined to the lines of sutures, of obscurely tubular pillars: we have also something analogous in the singular midribs, in Platylepas, causing the membranous basis to be convex. I need only further add, that the parietes, in Chamæsipho, are rather thin, and are composed of translucent shell, punctured for the entrance of tubuli, with the punctures often arranged in straight lines. _Opercular Valves_ (fig. 4 _d_).--These are attached by a narrow opercular membrane to the sheath, but little beneath the summit of the shell. The _Scuta_ are considerably arched or convex: the articular ridge is very prominent, and there is a thick strong adductor ridge. The _Terga_ have a short, rather broad, rounded spur, placed very nearly in the middle of the valve: the crests for the tergal depressores are moderately developed, and are simple.
_Mouth._--The crest of the labrum is hairy, and is much hollowed out. The palpi are small and narrow, with long bristles at their apices. The mandibles have four or five teeth, with the lower part pectinated. The maxillæ are notched. Of the _Cirri_, the second pair is short, and all the segments are thickly clothed with bristles. The third pair in the arrangement of the bristles resembles the four posterior pairs. The segments in these pairs are elongated, and support four pairs of main spines. I was not able to observe any _Branchiæ_. 14. PACHYLASMA--_Nov. Genus._[133] Pl. 19, 20. [133] Παχυς [Greek: Pachus] thick, and ελασμα [Greek: elasma], a valve. CHTHAMALUS. _Philippi._ Enumeratio Mollusc. Siciliæ. _Compartments, when the shell is very young, eight; when mature, either six, or in appearance only four owing to the close union of the lateral compartments: basis calcareous._ _Distribution_, Mediterranean, and New South Wales; deep water.
The two species here included form a very natural genus, though, as far as the shell alone is concerned, at first sight there is an unusual amount of difference between them. This genus offers an instance of a case, far from uncommon in nature, though so unfortunate for the systematist, in which the most obvious and useful characters of a group are completely masked. When I first examined _Pachylasma giganteum_,[134] I did not doubt that it was a _Balanus_; and when I first looked at _P. aurantiacum_, I thought, from there being in appearance only four compartments, that it was an Elminius; in neither case, from the absence of alæ to the rostrum, did I even suspect that the species belonged to the sub-family of the Chthamalinæ. But when I examined the included animal's body, I found, in both species, the labrum bullate, not notched, with the palpi small, and the mandibles with their lower teeth not laterally double. Again I found in the third pair of cirri only the basal segments thickly clothed with spines; and lastly, there were caudal appendages.
Now these characters are pre-eminently those of the Chthamalinæ; in fact, they are those met with in the typical genus Octomeris, with the exception of the presence of caudal appendages, and these occur in Catophragmus,--a genus standing next to Octomeris, and in no other genus of sessile cirripedes. Moreover, if we look to the shell of Pachylasma, the absence of pores in the parietes, or at least of symmetrical longitudinal ribs on their inner surfaces, and the peculiar character of the narrow radii, hardly differing in structure from the parietes, are characters which are rare in the Balaninæ, but universal in the Chthamalinæ. Hence, taking the whole organisation of the two species of Pachylasma, it is certain that they must be ranked amongst the Chthamalinæ, though the leading character of the group, namely, the rostrum being furnished, like the carina, with alæ, here fails. Owing to this conviction, I examined very young individuals of _P. giganteum_, and in specimens only the 1/100th of an inch in height, I was interested by finding eight separate compartments, with the rostrum having distinct alæ; hence, at this early age, as far as the shell is concerned, this species may be said to be an Octomeris; and we have seen that this likewise holds good with the included animal's body; as the young shell increases in size the minute rostrum and rostro-lateral compartments blend together (Pl. 19, fig. 5 _b_), without even traces of sutures being left.
Text from Project Gutenberg, public domain in the United States.