A Monograph on the Sub-class Cirripedia · Charles Darwin
ORDER II.--ABDOMINALIA.
On each side, within the first two segments of the body, and projecting a little before the great ovarian sack (_e_), two gut-formed organs (_f_) may be seen, even from the outside, owing to their opacity and dark colour. They lie near the external surface; the first pair of latero-ventral oblique muscles passing between them and the ovarian sack. They are formed of a branching, grape-like mass of opaque, orange-coloured cells. They are intimately united, at their posterior extremities, to the ovarian sack, and I believe open into it; but I cannot say that I demonstrated this. From their absolute identity in structure, and similarity in position, namely, on each side of the lower end of the œsophagus, no doubt is left on my mind that these bodies answer to the true ovaria, which are situated within the body of other cirripedes; and that the ovarian sack answers to the inosculating and branching ovarian tubes and cæca, which fill the peduncle, or cover the basis in other cirripedes, but here, from the absence of these parts, necessarily occupying the body.
_Male Organs._--The whole surface of the ovarian sack, the space before it, even to within the lower parts of the mouth, the posterior half of the last thoracic segment, and especially the whole three abdominal segments, are completely netted by branching delicate vessels or ducts terminating in spherical glands about 1/2000th of an inch in diameter. These little glands include a brownish pulpy centre, and sufficiently resemble the testes of other cirripedes in appearance, position, and connecting ducts, to make me believe that such is their nature. I may remark that in the more central parts of the abdomen the glands and ducts seemed to be in process of formation by the confluence of cellular matter, and in some other cirripedes I have suspected that the testes are periodically renewed, or at least redeveloped from an undistinguishable condition. Within the posterior half of the abdomen, some of the ducts become thicker and unite, others joining in laterally, so as together to make a dark chord, 7/2000ths of an inch in diameter. Until dissecting this chord, I thought it was a single vesicula seminalis, but it separated into several rather thick ducts or vesiculæ. I was not able to remove from within them the contained matter, but it appeared very finely and longitudinally flocculent, like spermatozoa not quite matured. In accordance with the immature state of the contents of the ovarian sack, in all probability these ducts would hereafter have become greatly enlarged, and have formed a compound vesicula seminalis of considerable size. The dark chord, formed by their union, contracts as it enters the rudimentary penis, and terminates in a very minute orifice on its apex. The penis consists of a papilla, only 3/4000ths of an inch in length, situated on the extreme point of the abdomen, but rather towards the ventral surface.
_Metamorphosis._--In accordance with the general law of the correlation of parts, it may be inferred, from the description and measurements of the pupal antennæ immediately to be given, that this abnormal creature was developed within a pupa of the same general structure, and of about the size, as the pupæ whence Scalpellum, Alcippe, and many other cirripedes are developed. As the ova are of remarkably small size, indeed I have seen no others quite so small, it is certain that the larvæ, as in the case with all other cirripedes, excepting Cryptophialus, must undergo several metamorphoses, and increase much in size, before attaining their pupal condition. _Attachment._--The animal is attached, as already stated, to the sack of the Alepas by two threads, rising close together from the medio-dorsal line, near the posterior end of the second segment of the body. These threads are attached likewise close together at their further ends, by the antennæ, into which they enter. They are flattened and strong, yet quite flexible, with a somewhat sinuous surface: they were, in this specimen, 42/1000ths of an inch in length, and a little above 3/1000ths in diameter: where joined to the thoracic segment they were a little contracted. Their structure in this specimen could be made out (Pl. 24, fig. 1) with perfect distinctness. Their transparent outer tunic (_e_, fig. 1) is 1/2000ths of an inch in thickness, and is continuous with that (_d_) enveloping the whole body, but is abruptly and considerably thicker than this membrane; and hence a very slight collar is formed outside, round the line of junction of each thread with the body. The delicate corium (_c_) lining the external membrane of the body runs, at least someway, down these threads. It was likewise indisputably evident that the membrane (_b_), for I separated it by dissection, forming the great ovarian sack, together with the cellular contents of this sack (_a_), entered and extended down both threads.
It should, also, be particularly observed, that the coarsely cellular matter within the ovarian sack, immediately that it entered the tube formed by the membrane of the ovarian sack, suddenly changed its appearance into a homogeneous, stiff, pulpy matter, which retained the same appearance all down the threads to within the antennæ. This finer matter readily separated from the coarser cellular matter within the sack, but was not divided from it by any septum or membrane. Some way within the threads, the corium, the membrane of the ovarian sack, and the contents appear (_e′_), as seen from the outside, to become, and perhaps really are, blended together. These threads could not have been originally formed of their present length, and must therefore have been added to during the growth of the animal; but from their entering the not-moulted antennæ, and from the animal being permanently attached by them, they cannot have grown, by means of the moulting of their integuments; hence I conclude that at each period of growth and exuviation they have been added to only at their upper ends, where there is a sort of collar, or line of growth; and where, I may remark, the lining corium is alone well developed. We shall presently see the bearing of these remarks.
These threads contract to about half their former diameter as they enter the old prehensile antennæ of the pupa, within which they are firmly attached. Each thread, with its three tunics apparently blended together, can be traced to the extremity of the disc-segment (_g_), where the included matter seems to have burst forth. The whole disc and the terminal segment of both antennæ are enveloped, close together, in cement, formed into two almost separate little capsules, by which they adhere very firmly to the integuments of the Alepas. The cement required to be removed before the antennæ could be plainly seen. The cement presented all the usual characters, namely, its homogeneous laminated structure and its yellowish colour. The cement in the case of the male Ibla, which is parasitic within the sack of the female Ibla, affects the corium and fibrous matter beneath the chitine-tunic, and causes them to adhere together, and thus prevents the male from being cast off each time that the inner tunic of the sack of the female is moulted: exactly so has the cement of the Proteolepas affected the integuments of the Alepas. The only difference between ordinary cement-ducts and the two threads here described is, that the ducts, in both cases formed by the prolongation of the coat of an ovarian receptacle, are here protected by a thick outer membrane, lined, at least in the upper part, by corium; whereas, in the Lepadidæ the two ducts are included within the peduncle, and are therefore protected by one common membrane, lined of course by corium; and this membrane, we shall presently see, is homologous with that separately investing the two threads.
The antennæ differ remarkably little, considering the anomalous character of the mature animal, from the same organ in other genera; they come nearest, perhaps, to the antennæ of Ibla. The length of the disc (_g_, fig. 1) and great succeeding segment (_f_) together is 40/6000ths of an inch. The lower segment has its basal articulation only slightly oblique, showing that, as in Alcippe and Ibla, it was probably articulated near the anterior end of the pupal shell: it is of nearly the same width throughout.[154] The disc (_g_) is remarkable from its great proportional length; it is hoof-shaped, with the outer side rather protuberant, and the end pointed. The ultimate segment (_h_) is of moderate size: as in Ibla, it has a shoulder or notch on its inner side near its end, bearing two long spines; and probably there were originally three or four spines on the square broad upper end, but these have been broken. This segment is articulated unusually near to the end of the disc. [154] As I have given the measurements of the antennæ in so many genera, I will give these: second (_f_, fig. 1) segment, 24/6000ths of an inch in length, and (8-9)/6000ths in width. Disc, 16/6000ths in length, and 8/6000ths in width. Ultimate segment, 6/6000ths (?) in length, and 10/20,000ths (?) in breadth.
The foregoing remarks on the two threads by which Proteolepas is attached, are, independently of their relation to this individual animal, of considerable interest. In my volume on the Lepadidæ, I have stated, after repeated and rigorous examinations (for I was well aware how singular the facts were), that in _Conchoderma aurita_ and in some other genera, the cement-ducts, which entered the pupal antennæ, could be traced till they joined a gland, the coat of which gland was absolutely continuous with the coats of the adjoining and continuous ovarian tubes, of which it was only a modified portion; and what was still more remarkable, that the matter within the gland was continuous with, and differed only from, the cellular matter within the ovarian tubes and cæca (from which ova were in the act of formation), by being more homogeneous and more coherent. Furthermore, I have shown, that in Ibla an ovarian tube, becomes by a very small change, namely, by a double flexure and slight thickening of its coat, converted into a gland, and thus acquires the power of affecting the cellular ovarian matter and changing it into cement. Now, in Proteolepas, the great ovarian sack replaces the ovarian tubes and cæca; and we here see the very same relations even still more plainly; for the coat of the ovarian sack is indisputably continuous with that investing or forming the two cement-ducts within the two threads; and immediately that the coarse cellular matter, which within the ovarian sack is being converted into ova, enters the upper contracted end of the cement-duct, by some power, we must suppose, inherent in its coat, it is converted into cement, which debouches with all its usual properties through the pupal antennæ. I may venture to reaffirm that nothing could be plainer than this structure, or be in more striking conformity with my previous observations, given in the introduction to the Lepadidæ.
_Thirdly_: the external covering or carapace of every young cirripede, at the period of its metamorphosis, enters, at its lower end, the cemented antennæ, in the form of two short tubular prolongations, by which alone, at first, the cirripede adheres to the surface of attachment; within these prolongations the cement-ducts are included. I have, moreover, seen instances, as in _Conchoderma aurita_ and in the male of Ibla and Alcippe, in which these tubular prolongations, lined internally by corium, were increased a little in length, so as to form a trouser-like termination to the peduncle. That the forked extremity should be a little more developed, and so be converted into a pair of short tubular threads, cannot be considered as very improbable. _Fourthly_: in the male Ibla the capitulum is so much atrophied that it does not enclose the thorax or mouth, but still an elongated support or peduncle is left. But it would be no very violent assumption to imagine the peduncle, which does not essentially differ from the capitulum, to become likewise rudimental,--to grow smaller and smaller, and shorter and shorter, till the merest remnant was left at the spot where it entered the cemented antennæ. And in the last paragraph it has been shown that it would be no violent assumption to imagine this lower end of the peduncle, where it enters the antennæ, developed into two short thread-like prolongations.
_Lastly_: it is certain, from the existence of the prehensile antennæ, that Proteolepas was developed within a pupa, probably differing in no very essential respect from the pupæ of other cirripedes. Therefore, in accordance with all analogy, we may believe that the position[156] of the young Proteolepas (probably much coiled up, with a deep fold close under the mouth) within the pupa, the general form and structure of the latter, and the course of the cement-ducts, did not _essentially_ differ from the imaginary figure given, Pl. 25, fig. 6. Now, at the period of the metamorphosis, let us imagine that no general covering or carapace was formed, except a small portion on the ventral surface, round the cemented antennæ. Let us further suppose this remnant to be specially developed (as in the case of some cirripedes) into a short trouser-like prolongation, entering the antennæ; and subsequently, in accordance with the almost universal laws of growth in cirripedes, that this portion was never moulted, but continued to be added to, during growth, only at its upper end. By this means we should produce every leading peculiarity of the _Proteolepas bivincta_.
As this parasite lives within the sack of another cirripede, and is protected by the capitulum of the latter, we can understand, in accordance with the usual admirable economy of nature, the absence of any general covering for its body. We can now, also, understand the structure and manner of growth of the two threads by which it is bound to its prey; and the connection, at first so strange and perplexing, between the old pupal antennæ and the second segment of the thorax. I am convinced that no other explanation than that here given, will accord with the relations of the several parts and organs of Proteolepas. Consequently, I fully believe that we here see an articulate animal in which the whole of the three anterior segments of the head have been, during the act of metamorphosis, absolutely aborted, with the exception of a mere rudiment on the ventral surface, near the anterior end, round the old antennæ, and which rudiment has been specially developed as a covering for the two cement-ducts. As the pupal antennæ are, homologically, the second pair of antennæ, we may further infer that this modified remnant of the carapace, investing the two threads, belongs to the third cephalic segment.
Text from Project Gutenberg, public domain in the United States.