Part 67

_Body._--This, as already stated, consists of eleven segments, of which the three posterior (abdominal) smaller segments can hardly be distinguished, without dissection, as separate from each other. The body is mainly occupied by a vast ovarian sack (_e_, _e_, fig. 7), filled by innumerable ova: and the three posterior segments by small testes and their vesiculæ seminales (_i_): but I shall return to the internal anatomy. The outer membrane, lined by delicate corium, is thin, transparent, elastic, and covered by groups of excessively minute blunt little points. The segments can be plainly distinguished by their outlines, especially on the ventral surface; but they are rendered unmistakably distinct by the attachment of the muscles; they can also be perceived when the external membrane is perfectly cleaned, by yellowish lines. The muscular system is highly symmetrical and simple: along all eleven segments, there is a narrow, medial, ventral and dorsal clear space; on both sides of which space there is a band of longitudinal muscles, which, though encroaching on the two sides, and rather largely on the dorso-lateral sides, may be called the ventral and dorsal muscles. These muscles are striæ-less, which is the case with the homologous posterior thoracic muscles in some other cirripedes: on the dorsal surface (lower surface in fig. 7) they are more spread out, and consist, on each side of the medial line, of four ribbons: this seems to be the case on the ventral side, but the ribbons are here much more confluent: in the seventh and eighth segments, the ribbons become broader; but in the ninth, tenth, and eleventh, or three posterior segments, they become much narrower, and some of the fasciæ disappear, so that these muscles can hardly be seen from the outside. Each separate ribbon expands a little at its two ends, which are attached to the articulations separating the successive segments: I carefully observed that they did not pass over at either end to the adjoining segments: hence their action must be either simply to shorten and arch each segment separately; or when acting together, to shorten the whole body, or perhaps the ventral or dorsal surface by itself.

In the first segment, and in the three posterior segments, these longitudinal muscles alone occur; but on the seven segments, from the second to the eighth inclusive, there are other oblique latero-ventral muscles. These muscles lie within the longitudinal muscles, and adhere pretty firmly to the coat (_e_, _e_, fig. 7) of the great ovarian sack. At their ventral extremities they are attached, near the anterior margin of each segment, beneath the point of attachment of the longitudinal fasciæ, and thence they run posteriorly in an oblique line to the anterior margin of the next succeeding segment, where they are attached: so that these muscles run obliquely from segment to segment. The first of these oblique muscles, lying chiefly within the second segment of the body, is thinner and longer than the others: those within the third and fourth segments are short: those within the fifth and succeeding segments extend, at their dorsal (or lower in fig. 7) extremities, as far as the outer dorsal longitudinal fasciæ: those within the seventh segment are broad and short, and cross the longitudinal muscles at only a small angle. In the eighth segment, there is an oblique lateral muscle, like that in the seventh segment, running from the ventral surface towards the dorsal surface; but there is in addition a second oblique lateral muscle, rising from the dorsal surface, and running towards the ventral surface. This muscle does not occur in the other segments, but in the fourth segment, at the dorsal end of the oblique latero-ventral muscle, there may be seen a small branch of fibres, at right angles, which seems to represent a muscle homologous with that just mentioned in the eighth segment: obscure traces, moreover, of similar fibres, can be detected in some of the other segments: had these oblique latero-dorsal muscles been as fully developed in the seven anterior segments of the body, as on the eighth segment, the whole muscular system would have been perfectly symmetrical. The oblique latero-ventral muscle in the sixth segment is distinctly striated transversely; but this is not the case with most of the other muscles, if with any of them; I cannot account for this difference. The muscles of the gnathites are the only other voluntary muscles in the animal's body.

_Homologies of the Body._--It will hereafter be, I think, clearly shown, that when the shell and integuments of the pupa of Proteolepas are shed, no carapace or general covering for the body is formed; the three anterior segments of the head, the backward prolongation of which (as has been elsewhere explained) certainly forms the carapace of ordinary cirripedes, being here almost absolutely aborted. In every cirripede the mouth is formed of three pairs of gnathites, which, no one will doubt, rise from the fourth, fifth, and sixth segments of the head: here in Proteolepas, the mouth, even on the view of the mandibular organ on each side being compounded of only two gnathites, sufficiently resembles the ordinary cirripedial type to make it very probable, that if examined in the earliest stage of its development, three pairs of gnathites would be discovered. In accordance with this conclusion, the segment succeeding the mouth (_i. e._, the first segment of the body in fig. 7) homologically is the seventh, or last cephalic segment. The succeeding seven segments, of course, are the seven thoracic segments, and the three posterior segments are abdominal; the latter are not developed in ordinary cirripedes when mature, but are present during their pupal condition. Now this conclusion, which is, in fact, deduced from what we know of the front part of the head in other cirripedes, both larval and mature, appears to me most satisfactorily confirmed by the differences in the muscular system of the segments in Proteolepas. In no other way, I believe, can it be explained, why the last cephalic segment and the three abdominal segments should differ from the seven thoracic segments, in the entire absence of the oblique lateral muscles. The abdominal segments, moreover, differ a little in shape, in the indistinctness of their articulation, in the thinness of the longitudinal muscles, and even in their contents. With respect to the two threads enclosing the cement-ducts, which spring from the second segment of the body (or first of the thorax), and which terminate within the prehensile antennæ of the pupa, we shall hereafter see that their apparently most anomalous position, and even the flattened shape of the dorsal surface of the two anterior segments of the body, all accord perfectly with the homologies just given.

_Alimentary Canal._--The œsophagus is thin, and for a cirripede short, for it extends only half-way down the first segment (_i. e._ last cephalic) of the body; the lower end, which is slightly dilated, nearly touches the anterior end of the great ovarian sack. At its upper end, it is surrounded by delicate, striæ-less constrictor muscles; and there are others radiating outwards, evidently serving to open it: the lower part of the œsophagus, differently from other cirripedes, is destitute of muscles, and is only coated by a thin layer of corium, which would serve to produce a new œsophagus at each exuviation. Strange as the fact may be, I am prepared to assert that there is no stomach, rectum, or anus. As I was able to trace so distinctly the œsophagus, and likewise the generally far smaller orifice and ducts of the male generative organs, I consider it quite impossible that I could have missed the stomach. The rectum and anus are absent in Alcippe: and the absence of a stomach is here in some degree the less surprising, as the structure of the mouth shows that Proteolepas must live on the already elaborated fluids of the Alepas, to which, being a cirripede, it is allied. It is of some importance to observe, that the œsophagus is fitted with muscles simply for shutting and opening it, the wave-like swallowing action of which other cirripedes are capable, being, apparently, here impossible; but the contraction of the body and its subsequent expansion, the œsophagus being opened, would allow the blood of its prey to flow inwards.

The nervous system must be much atrophied, for I could not detect it, and the small size of the animal is not sufficient to account for this: I wish I could have seen this system, for then I should almost certainly have beheld an articulate animal without a trace of a supra-œsophageal ganglion. There is no eye, but such could hardly be expected, as the anterior cephalic segments are aborted. There are no branchiæ. I may state that within the abdomen, along the dorsal surface, there was either a lacuna or a delicate vessel, apparently of a circulatory nature, of considerable diameter, which, near the extreme posterior end of the body, gave out branches.

_Female Reproductive Organs._--The eight anterior segments of the body, with the exception of a small space at the two ends, are occupied by an immense (_e_, _e_), opaque, ovarian sack. The tissue forming it is delicate, and presents a peculiar cellular aspect: it is slightly attached to the corium on the ventral surface of the body, and to the oblique latero-ventral muscles. Internally, at the anterior end, it is thickly coated by cellular matter, the cells varying from 4/6000ths to less than 1/6000th of an inch in diameter, becoming in parts confluent, and the whole forming a dark orange-coloured mass. In the more central parts of the sack this cellular matter became aggregated into little pellets, which, in proceeding towards the posterior end of the sack, gradually increased in size, from about (4 to 6)/1000ths of an inch in diameter, and at last appeared as almost mature and perfect ova of a broadly oval figure. Their size, as we see, is small, and their number almost infinite. I carefully examined all round this ovarian sack, and could detect no oviducts; nor from analogy could they be expected: I have no doubt that the ova burst forth by the rupture, probably, of the posterior end of the sack and of the overlying corium; and that they accumulate beneath the external membrane of the body, until this is moulted, the rupture beneath being in the meantime healed, when they are freed, or perhaps temporarily protected in the old moulted envelope of the body.

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.

* * * * *

I can hardly express the perplexity which I felt when I first examined Proteolepas, and when I naturally mistook the mouth for the entire head, for I saw, as I thought, the antennæ in direct connection with the second segment of the body, posteriorly to the mouth! It was quite as monstrous and incredible an inversion of the laws of nature, as those fabulous half-human monsters, with an eye seated in the middle of their stomachs. After a time, I perceived that the following considerations removed all difficulty, and brought Proteolepas into the type of other cirripedes.

_Firstly_: in ordinary cirripedes, the two cement-ducts can be traced up from the cemented antennæ to the glands, formed by a part of the ovarian branching cæca; and the latter can be traced to where they enter, as two simple tubes, the body of the animal, at a medio-dorsal point, a little anteriorly to the prosoma, or second thoracic segment.[155] From what is actually seen in the complemental male of _Scalpellum Peronii_, and from what may be inferred from the structure of these parts in the pupæ of all cirripedes, there can be no doubt that if the ovarian cæca were in any case not developed, the cement-ducts would enter the body at the spot where the two simple ovarian tubes, which serve to unite the ovarian cæca with the true ovaria, do enter. Now if we look at the drawing (Pl. 25, fig. 7) of Proteolepas, we shall see that the cement-ducts enter the body at a medio-dorsal point, a little anteriorly to the second thoracic segment, and therefore in the normal position.