A Monograph on the Sub-class Cirripedia · Charles Darwin
ORDER II.--ABDOMINALIA.
_Cirripedia, having a flask-shaped carapace; body consisting of one cephalic, seven thoracic, and three abdominal segments; the latter bearing three pairs of cirri; the thoracic segments without limbs; mouth with the labrum greatly produced, and capable of independent movements; œsophagus armed with teeth at its lower end: larva, firstly egg-like, without external limbs or an eye; lastly binocular, without thoracic legs, but with abdominal appendages._ I feel compelled to form an Order for the one genus and species, namely, _Cryptophialus minutus_, to be here described. We must, I conceive, attribute much greater value, in classification, to internal parts and organs, at least where such are not known to vary, than to external structure. Now in Cryptophialus, the body consists of eight segments, of which the first two are not developed in any cirripede hitherto described. Of the eight, the seven posterior or thoracic segments are quite free, or detached from the carapace, and do not bear any appendages; whereas in all the foregoing cirripedes of the order Thoracica there are (at least in the normal sex) six pairs of cirri; Alcippe alone must be excepted, in which there are only two pairs. Again, in the Thoracica there are no abdominal appendages, excepting the terminal or caudal, whereas in Cryptophialus the abdomen bears three pairs of biramous cirri. In the crest of the labrum, being produced into a special, lancet-formed organ, articulated at its base and capable of movement, and in the palpi projecting straight upwards, we have a great difference from all other cirripedes; and these organs, we have every reason to believe, possess a high classificatory importance. The œsophagus in Cryptophialus, where it enters the stomach, is armed with teeth and hairs, moved by muscles, forming a beautiful structure, of which we have not a trace in any other cirripede. Lastly, and perhaps most importantly of all, the metamorphosis is different; for the early larval stages are passed under an egg-like condition within the sack of the parent; and the pupa differs from the pupæ of all other cirripedes, in not having natatory thoracic limbs, and is therefore only able to crawl about by the aid of its great prehensile antennæ.
Thus far the evidence is decisive in favour of Cryptophialus being placed in a separate Order; but if we were to trust to the characters derived from the external covering or carapace,--and such characters are of high importance, as we may safely infer from the natural arrangement of the foregoing families which depends on the structure of the carapace,--we should place Cryptophialus close to Alcippe amongst the Lepadidæ. These genera agree in their burrowing habits,--in their attachment by a horny rostral disc,--in the external membrane being covered with triturating points,--in the spinose and notched orifice, with an external lateral bar on each side, and in the inner tunic of the sack being protected by hairs and spines. They agree to a considerable extent in shape, and in the peculiar arrangement of the muscles of the whole external covering of the animal: they agree, also, in their manner of growth, and in the sack extending down to their basal extremity. Some of these resemblances may possibly be analogical, and due to similarity of habits, and not to affinity; and we must attribute to mere similarity in function, a certain amount of resemblance in their labrums, for this part is essentially different in the two genera; and to the same cause, the resemblance between the brush formed by the two pairs of thoracic cirri and caudal appendages at the end of part of the thorax in Alcippe, with the three pairs of abdominal cirri at the end of the whole thorax in Cryptophialus. I allude to this latter resemblance, for it was owing to it, and to the similarity in the habits of Cryptophialus and Alcippe, that I stated, in the introduction to my former volume on the Lepadidæ, that the two genera would probably fall into the same order. In the structure of all the parts of the mouth and of the cirri, in the digestive organs and in the metamorphoses, Cryptophialus is not more closely related to Alcippe than to any other genus whatever amongst the Lepadidæ.
Nevertheless I am confirmed in the view that the external resemblances between these two genera are due to some real affinity, and are not merely analogical, by a very remarkable fact,--namely, that Alcippe and Cryptophialus are both bisexual, and have males, several in number, attached exactly in the same position, and which males are so closely similar that, considered by themselves, they might absolutely be almost classed as species of the same genus! For they agree in the absence of all internal organs and parts, excepting the single testis, vesicula seminalis, and immensely long probosciformed penis; and they agree, also in manner of growth, in the arrangement of the muscles, and even in shape. The whole case seems to me very singular, and, as far as my knowledge extends, unique: we have two animals, of which the females, if classed by their external parts (homologically consisting of the three anterior segments of the head), would be placed alongside each other in the same family; but when classed by the whole rest of their organisation, certainly must be ranked in distinct orders;[150] yet the males of these very same animals might almost stand in the same genus.
If it be asked why the position of Cryptophialus in the system should not be determined by the male, instead of by the female, the answer would be that the male is here abnormal and rudimentary in its whole structure; and I believe systematists are agreed that less perfect parts (and therefore a less perfect whole) offer less valuable characters than the more perfect parts or whole. We see this conclusion plainly verified in the case of the hermaphrodite _Scalpellum vulgare_ and _Ibla quadrivalvis_, for there can be no doubt where these species should be arranged, yet if we attempted to place them by their complemental males, we should utterly fail: exactly in the same manner, if _Ibla Cumingii_ and _Scalpellum ornatum_ were ranked by their males, they would be quite misplaced. So again, if we were to attempt to class the six species of Scalpellum by their males and complemental males, undoubtedly the first three and last three species of the genus would have to stand in distinct orders! Hence we may reject the males as a foundation for classification, but no doubt they serve to show that the resemblances in the carapaces of Alcippe and Cryptophialus, are not merely analogical or functional, but evince a true affinity, though these genera differ so greatly, in mouth, body, œsophagus, cirri, and especially in their metamorphoses.
_General Appearance._[151]--This, the smallest known cirripede, is flask-shaped and compressed, with a small orifice, on a more or less produced neck, placed at one corner: one of the narrow sides of this carapace is somewhat flattened or depressed, with its superior edge prolonged a little upwards; by this side, the animal is attached to the cavity in the shell, within which it is lodged. There is here no distinction between a peduncle and capitulum, that is between the lower or anterior, and the upper or posterior end of the animal, as seen externally. The small orifice is toothed and hairy: it is coloured purple, as is likewise the projecting labrum; the rest of the animal being tinted only by the muscles and internal parts seen through the outer integuments. The largest specimen (fig. 2) which I have measured did not quite attain the length of one tenth of an inch. This cirripede inhabits, in vast numbers, the shells of the living Concholepas Peruviana, amongst the Chonos islands; the whole outside of the shell being sometimes completely drilled by its cavities, almost touching each other, as happens in the case of Alcippe with the shells of Buccinum. The oval aperture leading into the shell-cavity, in full-sized specimens, is between (2-3)/100 of an inch in length: it is generally surrounded by a narrow, internal, calcareous rim, which apparently has the same inorganic origin, as in Alcippe. The toothed orifice of the carapace leading into the sack, fills up the orifice of the shell-cavity; but it can be voluntarily withdrawn a little: when opened, and the animal is in action, the lancet-formed, moveable crest of the labrum, and the abdominal cirri, are exserted.
The orifice is formed on each side by a toothed rim of hard chitine (fig. 3, 4), which can be opened and shut, owing to its being flexible at the rostral end, and folded inwards at the carinal or posterior end. The teeth vary in arrangement and sharpness: generally they form, taking a rim on one side (fig. 3), two prominences or groups of points at the rostral or anterior end, always separated by a broad notch, from the bottom of which the lateral bar extends downwards, from the posterior, larger and less regularly toothed half of the rim. Of these teeth the larger ones project nearly straight up, and the smaller and lower teeth outwards, graduating into the smaller teeth, just below the rim, which again graduate into the minute points, studded over the whole surface. These outer teeth probably serve to prevent any creature crawling into the cavity, between the shell and animal. Scattered bristles rise from all over the rim. The lateral bar, above alluded to, consists of the general membrane of the body, thickened, hardened, rendered elastic, coloured yellow, and apparently formed into a fold: where attached to the under side of the rim, at the above-mentioned notch, the bar is thinner and more flexible than elsewhere: it runs half way down the animal, first straight and then curved towards and closely approaching the disc or surface of attachment. At the lower end, the bar, or more strictly the thickened membranous margin of the bar (_b′_, fig. 3), expands into an oblong, slightly rigid plate, studded with from three or four to ten or twelve points, which have their ends expanded and truncated, or even slightly bifid. The extremity of this plate projects freely from the general surface of the body. We shall afterwards refer to the use of this extremely peculiar bar and plate.
The two rims forming the orifice cannot be quite closed; but ingress by any foreign object into the sack is beautifully prevented by an internal membranous lip on each side (_d′_, fig. 3, 4), and by a third inwardly folded lip (_d′_) at the posterior and broad end of the orifice. These three lips can be brought together, and form a valve. The lateral lips are very narrow at the mouth or rostral end of the orifice, where the hairy lancet-shaped crest of the labrum closes the orifice, and largely expand towards the posterior end: they are produced from the inner tunic of the sack: they appear formed of the finest hairs, placed parallel and approximate, but when examined under the highest powers, these hairs (for they still appear such) are found to be united by delicate membrane, which has its extreme edge fimbriated. The third, or posterior and inwardly folded lip, differs in being composed of much coarser, flattened hairs, which are united towards their bases, and are free at their extremities, where they are serrated or coarsely plumose on both sides.
The sack extends down to the lower end of the animal. It is lined by delicate membrane. At the orifice on each side, a little posteriorly to the lateral external bars, this inner membrane is strengthened by a pair of thin yellowish bars (_c′_, fig. 3), which run parallel to the straight portions of the external bars. These inner bars at their lower ends become pointed and die out: at their upper ends, and close to the rim, they are broader, but more flexible, and so transparent as hardly to be distinguished from the rest of the membrane. At the rostral end of the orifice, in a medial line, this same inner tunic of the sack is thickened for a short space downwards, so as to form a fifth bar (_c′′_); which separating from the inner tunic, runs inwards between the outer and inner membranes of the carapace (_i. e._ between _b_ and _c_, fig. 3, 5), behind the mouth, as far down as opposite to the lower end of the œsophagus, and there becoming thinner and ligamentous, gives attachment to some powerful muscles.
At each exuviation, the external membrane with the dentated hardened orifice, the lateral bars, the inner tunic of the sack with its bars, are all moulted, together with the usual integuments of the animal's body. New and sharp triturating points are thus periodically formed for the work of excavation. The whole animal increases during growth in every direction, and therefore, at its lower or basal end, as was the case with Alcippe. The disc or surface of attachment, is added to by new underlying layers, extending beyond the old layers at the lower end and on the sides, but not at the upper end, where, as in the case of the calcareous discs of Lithotrya, the old layers are deserted and worn away. I saw what I believed to be little globules or patches of cement; but I was not able to discover any cement-ducts. _Muscles of Sack and Orifice._--The animal is surrounded by rather strong longitudinal muscles, not running up close to the orifice: these muscles exhibited, to my surprise, distinct traces of transverse striæ: there are no external transverse muscles, as in all the Lepadidæ. Attached to both sides of the inward fold or hinge, at the posterior end of the orifice, some striated or voluntary muscles run for a short distance downwards, diverging like a fan: their contraction would cause the dentated rim to open: a strictly homologous muscle occurs only in Alcippe. At the opposite end of the orifice, a remarkably powerful voluntary muscle is attached to the ligamentous bar above described as proceeding from the rostro-medial end of the toothed rim (_c′′_); and at its lower expanded end, it is attached under rather above the middle of the disc: this muscle corresponds with a similar one in Alcippe, and with some much weaker muscles in other Lepadidæ.
Its action would be to draw down within the shell-cavity the whole dentated rim, and likewise to close the orifice; and here, I believe, come into use the lateral elastic horny bars with their curious basal projecting plates, furnished with expanded points, for much friction would thus be caused by, yet some play be allowed for, the several movements; the elasticity of the bar bringing up the dentated orifice, when the powerful muscles attached to the rostral end of the latter became relaxed. Round the space where the just-mentioned muscles are attached to the horny disc, a sheet of other muscles radiate, a few on both sides obliquely upwards, but the greater number transversely and within the first-described longitudinal muscles; they extend on both sides about half round the animal. There are similar muscles in Alcippe, but not extending so far round the animal. Their action must be to draw the whole carapace towards the surface of attachment; the action of the longitudinal muscles being to shorten it; the orifice supported by the lateral horny bars, serving as the fulcrum for the contraction of the longitudinal muscles. I could not see any adductor scutorum muscle, although I looked particularly under the expanded plates at the ends of the lateral external horny bars.
_Body._--This is laterally compressed: it is widest and thickest at the upper end, and thence tapers to the lower or posterior end. The last three or four thoracic segments are bent under the anterior segments, giving the whole something of the appearance of certain crustaceans, divested of their legs. The somewhat conical mouth, with its singular labrum, is very large. The body consists of eight segments. The first segment (fig. 5, 1), or that succeeding the mouth, is the seventh or last cephalic segment of the archetype crustacean; it is the largest of all eight segments; it is joined by its dorsal surface to the carapace or external covering of the animal, and the membrane with which it is invested is prolonged upwards and downwards (_c_, _c_, fig. 5), and so forms the inner tunic of the sack. The succeeding seven segments are thoracic; they are free, and are destitute of limbs; the articulations separating them are transverse. The first and second thoracic segments give rise, on their medial dorsal surfaces, each to a remarkable tapering curved appendage, presently to be described. At the end of the last thoracic segment, there is a minute abdomen, bearing three pairs of biramous cirri.
The _Mouth_ consists of three pairs of organs, namely, the outer maxillæ, maxillæ, mandibles with their palpi, and of a great and very curious labrum. These organs, by the fusion (as in other cirripedes) of their lower segments, form a large, somewhat conical, projecting mouth, which is separated on the ventral surface from the rest of the body by a distinct fold or articulation, where the muscles proceeding to the above gnathites are attached. The labrum (_e_, fig. 5, 9) is of large size; and the crest close over the opening of the œsophagus is produced into a great, lancet-shaped, moveable organ, wholly unlike anything occurring in any other cirripede: it is coloured purple, and is thickly fringed in the upper part by very fine hairs: it is bowed a little backwards from the mouth: the base, which rather overhangs the œsophagus, is a little contracted, and is transversely marked by an articulation: two small, parallel, voluntary muscles (with transverse striæ) are attached at their lower ends close beneath the articulation, and extend about one third up the organ: their contraction would serve to erect it; and their relaxation would, apparently, allow it to fall backwards on a little knob (_e′_, fig. 5) behind. This little knob resembles a similar projection in many of the Lepadidæ. As the labrum is formed of similar membrane with that of the succeeding segment of the body, its limit downward, beneath the knob, can be told only by a small apodeme which projects inwards, at a little distance within the line where the membrane of the body is reflexed upwards (_c_), so as to form the inner tunic of the sack.
The mandibles, palpi, and maxillæ, all project more than is usual. The _Palpi_ (_f_, fig. 9, 5) are narrow, flattened, and taper a little; they support a few long bristles on their tips, and on one of their sides. In every other cirripede (in which the palpi are developed) they are directed transversely across the mouth, one towards the other, and are for a considerable space united to the labrum: here they project straight up, and seem to rise exteriorly to the bases of the mandibles; they are, however, united to the basal lateral edges of the labrum, and when the latter is torn from the rest of the mouth, the palpi separate with it. I could not distinguish the knob on which, in every other cirripede, the palpi are articulated. The _Mandibles_ (fig. 8) have an upper, lower, and middle tooth, with some finer intermediate points and hairs. The _Maxillæ_ (fig. 7) are narrowed in at their spinose edge, where there are three large spines and several finer bristles, together forming a flattened brush: this organ is remarkable from the apodeme (_a_) being bent into the shape of a scythe, with the terminal or blade-portion a little expanded, and directed backwards and inwards. The _Outer maxillæ_ are sub-triangular in outline, with several bristles on their summits and along their outer surfaces.
_Segments of Body._--I have stated that the mouth is succeeded by eight segments. As in all the cirripedes hitherto described, the body consists of only six segments, the number eight at first seems very improbable, and therefore I may be permitted to state that both Dr. Hooker and myself, when first examining this animal, and having no notion whatever regarding its homologies or the structure of other cirripedes, came to the conclusion, judging only from external appearances, that is, from the transverse folds, and from the lines of movement when the body was bent by a needle, that there were really eight segments. I have since carefully looked to this point: when the outer membrane is cleaned and examined, the four posterior segments are very plain, owing to a dorsal medial line, being alternately either thickened and coloured yellowish, or thin and colourless: the four anterior segments are less plain, but yet the membrane on the dorsal surface, on the line of each fold or articulation, does present some difference, from being destitute of the fine, transverse, toothed scales which occur on other parts. But I lay most stress on the fact, that all these eight articulations were used for the attachment of muscles. Hence I conclude that the eight segments are real; and we shall see, in the next order, that the very same eight segments are as plain in Proteolepas, as in the larva of an insect or as in an annelid. There is good reason to believe that the general covering or carapace consists in all cirripedes of the three anterior segments, and that the mouth (judging from its appendages) also consists of three segments, consequently the first segment of the body in Cryptophialus must be the seventh or last cephalic segment, and the seven next free segments must be the normal seven thoracic segments.
The first segment of the body (_i. e._ last cephalic, fig. 5, 1) is, as stated, the largest, and is attached dorsally to the carapace: its ventral surface is flattened, and is formed of somewhat thickened membrane: on each side, a little below the articulation separating this segment from the mouth, there is a small blunt projection, with the free part only 1/500th of an inch in length. Each of these appendages bears four or five bristles on one side near the summit, and a few on the other side, lower down: from their position I believe them to be rudiments of a first pair of maxillipeds (tetartognathites of Milne Edwards), of which no trace occurs in any other cirripede. The differences between this segment and the seven succeeding segments, is of interest, as offering some confirmation of the belief, lately disputed by some naturalists, that the cephalic and thoracic segments in the class Crustacea, do differ in their nature,--a conclusion which we shall see further confirmed under Proteolepas. The second and third (_i. e._ first and second thoracic) segments (2, 3, fig. 5) are the next largest, and are remarkable from supporting singular appendages, already alluded to. The sides of the second segment are formed of thickened yellowish membrane. The fifth and sixth segments are the smallest, and mark the point of chief flexure of the body. The eighth segment is a little elongated, formed of thicker membrane than the other segments, and dorsally is indented by the anus.
The singular tapering appendages (fig. 5, _i_, _k_) arising dorsally from the second and third segments, differ from each other only by the upper one being smaller, less curled, and perhaps rather smoother. When first examining this animal, not knowing that it was female, and not finding a probosciformed penis, I concluded that these organs were of this nature,--an excusable mistake, considering their almost ringed structure, their somewhat constricted bases, the direction of their curvature, and their position in the midst of the ova within the sack. On careful examination, however, these appendages are seen not to be truly ringed or articulated, but are covered with transverse lines of scales, hirsute on their edges; these scales being less distinct, or even quite absent on the smooth upper portion; they do not include any muscles; they are imperforate at the apex, which is not furnished with bristles (as seems always to be the case with the probosciformed penis); and, lastly, they are lined by corium, but are not occupied by any vessel, gland, or organ of any kind.
Text from Project Gutenberg, public domain in the United States.