A Monograph on the Sub-class Cirripedia · Charles Darwin
LEPAS ET BALANUS AUCTORUM.
_External Structure._--The orifice leading into the sack is about one third of the total length of the animal: its edges or lips are thickened, horny, and brownish: at the lower end, exactly where the orifice ends, the lips are formed, from being deeply notched, into two sharp projections (_a_, figs. 1, 5, 6), unlike anything occurring in any other Cirripede. The external membrane (_c_, fig. 6) of the lip supports an irregular but nearly straight band of sharp, thick spines of chitine, about 1/1000th of an inch in length, together with a few hairs: at the carinal or upper end of the orifice the spines are largest and most numerous; at the other and lower end, they decrease in size; and on the two projections (_a_, fig. 6), and on the adjoining parts of the external membrane, they graduate into the small dentated points which cover the whole surface of the animal. The inner tunic of the sack (_b_), on each side along the upper half of the orifice, is remarkable from having a moderately broad, curved band of short, sharp spines, not quite so thick as those on the external surface, closely adpressed together and pointing upwards, like the javelins of an ancient phalanx, thus probably preventing the ingress of any intruding animal. This band of spines curves at the upper end, conformably with the shape of the orifice. The inner tunic of the sack in this upper part is yellowish, and, what is very unusual, is thicker than the external membrane. A little way down, within the orifice, and more especially in front of an elegant row of hairs on the two sides of the great labrum, there is a band of very fine but stiff hairs (5/1000ths of an inch in length), pointing upwards, and making together with those on the labrum a hedge, barring ingress into the sack.
The external membrane over the whole animal, excepting the horny disc which covers the rostral face of the peduncle, is very thin and transparent; it is periodically and often moulted, as may be inferred from the many old lines of junction round the edges of the horny disc: it is irregularly and pretty thickly (but not so thickly as in fig. 7) studded with star-headed, minute points, from 2 to 5/10,000ths of an inch in diameter, composed of hard chitine, seated on a short footstall, and this on a circular, yellowish, slightly thickened disc of the general investing membrane, appearing like a halo surrounding each little point. These points are directed obliquely upwards. There are none on the horny disc, though particularly numerous close to its margin. Their state varies much: just after a moult, when newly formed, the spines are regularly star-headed, with quite sharp rays, from two to six in number, with some of them occasionally bifid; but these points or rays soon become blunted, and ultimately half the star is worn away, so that the appearance then presented is that of a crescent with a few blunt points on its convex side.
At each exuviation, the thickened membrane of the orifice with its strong external spines (the condition of which also varies according to the period elapsed since the last moult), and of course the whole internal tunic of the sack, with its spines and hairs, are all moulted, together with the external membrane and the little star-shaped points. In most specimens a barely distinguishable band or bar of yellowish, slightly thickened membrane, runs from the points (_a_), at the lower end of the orifice, for some way obliquely downwards; and at the lower end of this bar the weak adductor scutorum muscle (having transverse striæ) is attached. This bar is often strengthened by a prominent external fold of membrane, but yet it is so flexible, and as it is united only to the lower end of the orifice, I can hardly believe that it can, by means of the adductor muscle attached to its opposite extremity, have much power in closing the orifice. I believe that this muscle acts simply in narrowing the whole animal, so as to favour its movement within the cavity in which it is imbedded. Owing to this position of the adductor muscle, and its consequent little power in closing the orifice, we can understand the necessity for the defence afforded by the bands of spines and hairs on the inner tunic of the sack and on the labrum, which do not occur in other Cirripedes.
_Horny disc._--The general shape of the disc, its irregularity and position, have been already described. It never extends, as remarked by Mr. Hancock, to the extreme lower point of the peduncle; upwards it reaches to a little below the lower end of the orifice. It consists of successive layers of membrane, either moderately thick and opaque, or only a little thicker than the general membrane of the body, but never furnished with the little sharp points; it increases in size, in like manner as the calcareous valves of other Cirripedes, the undermost and last formed layer extending beyond the others, with its edge united, till the next exuviation, to the general membrane of the body. The disc is attached, at its upper end, apparently in the usual way, by cement, to the roof of the cavity of the shell in which it is imbedded; but the lower parts of the disc are also slightly and partially attached, chiefly along the lines of growth or exuviation; and this, I suspect, is effected by an inorganic calcareous deposit; anyhow I could not perceive here any cement or cement-ducts. Beyond the circumference of the disc the whole animal lies free in its cavity. The lines of growth in the middle part of the disc are generally obliterated by the decay of the older and outer layers. These lines, though of course ordinarily conformable with the general outline of the disc, are not always so, for the disc sometimes becomes during growth slightly changed in form, and the animal, consequently, slightly changed in position; sometimes either one or the other side or the upper end of the disc is left deserted by the new layers of the growing disc; these being formed on the deserted side of less size or extension, instead of larger size, as they normally should be all round the disc.
The upper end of the disc is always produced into a projection of not regular shape, but generally hollowed out or embayed in front (fig. 1), and almost always hollowed out on the two sides. This projection stands directly over the adductor muscle (_b_ in fig. 5), and on the exterior surface is generally convex, being concave on the under side for the attachment of several muscles presently to be described. The horny layers are in this part usually thicker than elsewhere. The disc is thus upwardly produced, owing apparently to the fissure which leads into the cavity of the shell of the mollusc becoming, during the process of excavation, considerably longer than is necessary,--that is longer than the orifice leading into the sack; and consequently, for the protection of the imbedded animal, the lower and narrow end of the fissure is closed on its under side by this upward production of the horny disc, formed of layers of membrane of unusual thickness. A deposition, also, of lime, hereafter to be mentioned, gives further protection. In the bays on each side of the upward production of the horny disc, and likewise a little lower down on its edges, and therefore somewhat protected by lying within the narrow, pointed, lower end of the fissure in the shell of the mollusc, the short-lived Males (Pl. 22, fig. 1, _m_) are attached often in groups of two, three, or more together.
It may be asked to what part, in other Cirripedes, does the horny disc answer? Not considering the upward prolongation, which has been developed for a special purpose, the disc is irregularly circular,--is added to all round,--serves for the attachment of the whole animal to the supporting surface,--is covered on the under surface by a conformable and parallel mass of ovarian cæca, and the latter by the inner tunic of the sack; therefore in every character, and in its relation to the other parts of the animal, the disc answers to the end of the peduncle, or to the basal cup in Lithotrya, or still more closely to the basis in sessile cirripedes, with the important exception that it lies in a line with the longitudinal axis of the whole animal instead of at right angles to this axis. We know that all ordinary cirripedes become first permanently attached in their pupal state by their antennæ, which are seated on the ventral or rostral surface, near to the anterior end of the body; and that from the young cirripede, after the act of metamorphosis, being turned vertically upwards, and from the extreme anterior, now lower, end of the body not being rapidly developed, the surface cemented down, or the basis, encroaches almost equally on the dorsal, lateral, and ventral surfaces.
But if we were to suppose the extreme anterior point of the body to be rapidly developed, the surface of attachment or basis, without it grew still more rapidly, could not possibly reach the dorsal surface, and would, consequently, be confined to the ventral or rostral surface. I have not seen the young of the ordinary or female Alcippe soon after its metamorphosis, but in the male the development of the extreme anterior end of the body is extraordinarily rapid, and from analogy we may fairly conclude that this is likewise the case with the female. Hence I believe that the horny disc answers to the cemented down, lower end of the peduncle, in other members of the Lepadidæ, and to the basis in the Balanidæ, and that it is confined to the ventral or rostral surface, owing to the anterior or lower end of the body having been rapidly developed. To make all the parts, internal and external, of Alcippe, correspond with those of other cirripedes, the main circular part of the horny disc must be turned up at nearly right angles to its present position (the dorsal or carinal integuments, to the right-hand in fig. 5, being shortened), and then we should have a peduncle, certainly very short and broad, but holding its proper relative position.[143]
_Sack and its Muscles._--I have already described the curious phalanx of spines, the long fine hairs, and thickened condition of the inner tunic of the sack along the sides of the orifice. This inner tunic is a reflexion from that enveloping the body of the animal, in the usual manner, as may be seen in the section (Pl. 22, fig. 5). Between the external membrane and the inner tunic of the sack (_e_), there is of course the usual double fold of corium, these two folds being united by minute, transverse, ligamentous fibres, branched at the two ends, as in other Lepadidæ. Imbedded in the corium there are numerous, longitudinal, striæ-less muscles, which do not run quite up to the orifice, but to an oblique line beneath it. Externally to these muscles there are, as in the other Lepadidæ, fine transverse muscles, confined to the middle part of the animal, and running from the carinal margin more than half way round both sides. Attached to the upper notched or folded end of the orifice (_g_, fig. 5, above the upper ends of the longitudinal muscles), there is a fan of rather strong, striæ-less muscles, expanding downwards, with their lower extremities attached to the outer membrane of the capitulum; these muscles apparently serve to open the orifice: there is a somewhat analogous muscle in Lithotrya, but in no other member of the Family: in Cryptophialus, however, there is a closely similar muscle. Owing to the action of these several muscles, the tissues forming the capitulum and peduncle are, according to Mr. Hancock, highly contractile.
I have stated that the under surface of the upper produced end of the horny disc is concave, and serves for the attachment of several muscles. Of these some run to the basal margin of the great labrum, and no doubt, as usual, move the whole mouth; others, as usual, run to the skin between the labrum and the lower end of the orifice,--_i. e._ in fig. 5, between the lower end of the row of fine hairs (see fig. 11), which shows where the basal margin of the labrum is situated, and the lower side of the point (_a_), where the orifice terminates: others run obliquely on both sides towards the point of attachment of the small adductor scutorum muscle (_b_, fig. 5): others, of considerable strength, and these are more peculiar, run and are attached to the lower end of the orifice (_a_), and serve apparently to draw up the orifice from within the fissure-like cavity, in which it lies lodged: others, again, extend transversely on both sides, close beneath the inner tunic of the sack, a little beyond the line whence the ovigerous fræna or branchiæ arise. These transverse muscles lie within the longitudinal muscles, and therefore are quite different from the exterior transverse muscles, which are situated more towards the carinal portion of the peduncle and capitulum, and which are common to most Lepadidæ. The internal transverse muscles, and those running to the lower end of the orifice, are peculiar, but we shall hereafter meet with them even more developed in Cryptophialus.
Along the medial carinal line there is, between the two layers of corium, the usual circulatory channel. On each side of this line, on the inside of the sack, there are generally some slight irregular swellings, and sometimes a large extent of the inner surface is irregularly carunculated with little knobs. The sack (_e_ in fig. 5) extends down almost to the basal point of the peduncle (_d_), more especially when the ovarian cæca are not gorged with ova. _Branchiæ, or ovigerous Fræna._--Within the sack, on each side of the body, rising not far from the ends of the adductor muscle (_b_), there is a large fillet or fold; the two occupy so exactly the position of the ovigerous fræna that I cannot doubt such is their nature, though, as happens in the case of some species of Pollicipes, they are destitute of their proper glands, and so do not serve for the attachment of the ovigerous lamellæ; this attachment probably is not required, owing to the protected situation which the lamellæ hold in the sack, under the animal's body, and over the ovarian cæca. From the unusually large size of these so-called fræna, I cannot doubt that they serve as branchiæ, equally well with the plicated folds of membrane, believed to be homologous with the fræna, in the Balanidæ, which have by every one been considered as branchiæ. The fræna are broad and truncated at their upper ends; their margins are sinuous, and their outer surfaces papillose; they run longitudinally down the sack, narrowing as they extend, almost to the basal point of the peduncle, and hence are of considerable length; they are hidden in the section (fig. 5) by the medial, somewhat protuberant mass (_c_) of ovarian cæca, and partly by the (_i_) prosoma.
_Body._--The body is constructed on the usual type, and indeed does not differ greatly from that of Ibla. The labrum is very large, its lower or basal margin is separated by an unusual space (capable of being contracted or folded) from the lower end of the orifice of the sack; hence the labrum and whole mouth is placed quite remarkably near the upper (or carinal) end of the orifice. This upper end of the orifice, I may remind the reader, is homologically the posterior end of the general covering or carapace, and all that portion of the whole animal (as the sectional figure, 5, stands) below the lower margin of the labrum, on the rostral or ventral surface, is formed by the three anterior segments of the head. The main part of the body, carrying the mouth, is formed by the great development of that segment of the thorax which bears the first pair of cirri (_h_), here closely adpressed, as usual, to the sides of the mouth. The lower portion of this segment forms the prosoma (_i_), and has the characteristic outline, but is not much developed. On each side of the prosoma an oval space of membrane is yellowish and is thickened, and so gives support to this part of the body.
The five succeeding thoracic segments, which ought to carry the five succeeding and posterior pairs of cirri, are together of very small size (as in Ibla), in comparison with either the prosoma, or the whole anterior part of the animal. The segment (_k_) which should have borne the second pair of cirri, is considerably longer than the following segments, and is at the same time less distinct, owing to an oval convex shield of thickened membrane on the sides, not extending the whole length of the segment, thus causing two transverse creases, which, when the thorax is contracted, appear like two additional segments. Had this segment borne cirri, they would have stood, as in Ibla, at a considerable distance from the first pair. The segments (_l_, _m_) which should have borne the third and fourth pairs of cirri are like each other, except that the former is rather the longest. The membrane covering all the thorax is surprisingly thin; and at the articulations, which are straight and transverse, is deeply folded, so that the thorax must be highly extensible, to a degree which I have not seen equalled in any Cirripede except in the males of _Scalpellum vulgare_ and _ornatum_.
The thorax is represented as somewhat extended in fig. 5. This part of the thorax is amply furnished with striated and striæ-less muscles for its retraction and protrusion, and for lateral movements. The segment (_m_) which should have borne the fourth pair of cirri, at first sight falsely appears like the terminal segment of the thorax: in one monstrous specimen it bore a single cirrus, showing (if there had been any doubt) that it was a true segment. The three terminal pairs of articulated appendages, form together a brush; they consist of the fifth and sixth pairs of cirri and of the caudal appendages: my reasons for considering the last-named organs as of caudal origin will be given hereafter. A moderately careful inspection, especially of the ventral surface, will show that the fifth pair of cirri are borne on a small segment (_n_, fig. 5, but plainer in fig. 13), which is quite distinct from, but partially concealed by, that which ought to bear (and did bear in the monstrous case) the fourth pair of cirri: this segment is oblique, and cannot be traced distinctly all round the dorsal surface.
The segment (_o_, fig. 13) bearing the sixth pair is much less distinct, and can only be seen by a longitudinal section, or when the cirri are a little separated, but it certainly exists,[144] as is likewise shown by the presence of small apodemes dipping in amongst the muscles, between this and the last segment. The posterior or caudal appendages are closely approximated; they are not separated by any fold from the sixth thoracic segment; but _appear_ as if they were articulated on the dorsal surface of the sixth pair of cirri, in exactly the manner usual in the other Lepadidæ. The segments bearing the fifth and sixth pairs of cirri are highly oblique to the preceding segments, and consequently the cirri, which they support, instead of projecting inwards, lie like a brush in a line with the longitudinal axis of the main part of the thorax. The membrane forming the two small oblique terminal segments of the thorax is strengthened by irregularly shaped plates of thicker and yellowish membrane. [144] In the middle, the fold is slightly prominent and pointed, and being most finely villose, I for some time looked at this projection as a rudiment of the probosciformed penis.
_Mouth._--The mouth is constructed on the strictly normal type of the Family, but is peculiar in every part; it is remarkable from being situated so near the upper (or posterior) end of the capitulum, this being caused by the great length of the labrum, and of the space of body between the latter and the lower end of the orifice. The _labrum_ is a very singular part of the mouth from its vast size and outline: in fig. 8, we have a front view of the mouth, of which the whole upper pointed part consists of the labrum, and _h h_ is the first pair of cirri; in fig. 11, we have a lateral view of the labrum, with the surrounding thin membrane of the body, _a_--_a_, still adhering to its edges; _h_ is the first cirrus on the near side; _m_ the mandible, a little distorted in order to show its tooth, marking the position of the transverse crest of the labrum and of the orifice of the œsophagus; _b b_ is the medial longitudinal ridge of the labrum.
In the Balaninæ the labrum forms a mere rim to the back of the mouth, consisting of an inner fold running down the œsophagus, and of an outer fold, both close together: in the Lepadidæ the folds are separated, the outer one being swollen or bullate; and here this structure is carried to even a greater extreme than in Ibla and its allies. The distance between the transverse crest over the œsophagus and the blunt projecting point on the summit of the medial ridge, _b b_, equals twice the longitudinal diameter of the rest of the mouth. The lower margin on each side of the labrum is produced into two projections (fig. 11), the longer one curling round to a point beneath the jaws, with its extremity imbedded as an apodeme. Another very peculiar character in the labrum, prominently noticed by Mr. Hancock, is caused by a longitudinal row, on each side, of closely approximate, long, very finely pointed hairs, which, as already stated, are fronted on the opposed internal surface of the sack by an irregular band of still finer hairs. The surface of the labrum is partially covered by minute toothed scales, and these, seen on the longitudinal medial ridge, _b b_, give it a finely denticulated structure. At each end of the transverse crest which overhangs the œsophagus, there is a knob, such as occurs on the labrum of every Cirripede. United to these two knobs, which are formed of thick and yellowish membrane, and springing from the adjoining sides of the mandibles, there are two swellings formed of thin membrane (fig. 8), which occupy the exact position of the palpi, and may be considered as these organs in a rudimentary condition and destitute of bristles.
The _mandibles_ are simpler than in any other Cirripede; they are minute; they consist of an oblong plate, with only one very strong tooth at the upper end: the face towards the labrum is swollen: beneath the upper free part there is a small, sub-triangular piece of thickened membrane, let in and forming part of the general outer surface of the mouth, and representing the large square plate found in other Cirripedes. The _maxillæ_ (fig. 15) are smaller but broader than the mandibles; they have an upper tooth and a smaller lower one, lying not quite in the same plane with the upper one, but nearer the mandibles. The apodeme (fig. 8, 15) is of remarkable length, extending beneath the basal fold of the mouth: it does not arise from the ridge or outer edge of the maxilla, but a little on one side, from the face directed towards the mandible. Between maxillæ and mandibles there is a very singular prominent fold of membrane (fig. 8, 15), which resembles, but probably falsely, the supposed rudimentary palpus attached to the mandible. Altogether the maxillæ differ considerably from the same part in other Cirripedes. In structure they seem adapted to assume the function of mandibles; but they do not stand directly over the œsophagus. The _outer maxillæ_ (fig. 8) appear like a minute, deeply notched lower lip: each consists of a simple, oblong, rounded plate, with a few small bristles at its upper end. The basal fold of the mouth in front, beneath the outer maxillæ is distinct, and runs in a line with the basal articulation of the first pair of cirri. In the rudimentary palpi, minute and little developed outer maxillæ; and in the inner maxillæ, taking the function of the mandibles, the mouth of Alcippe presents some resemblance with that of Anelasma.
Ideas discussed in this chapter
Text from Project Gutenberg, public domain in the United States.