Chapter XXXII: Introduction (28)

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. The only function which I can assign to these appendages, is that of aiding the retention of the ova within the sack: for the ova, when first produced, are aggregated round them; at this period I several times observed long, somewhat curled, very thin fibres, not tapering like hairs, adhering to both appendages, the nature of which fibres I cannot explain. In very many cirripedes there seems a strong tendency to the production of tapering, filamentary appendages, somewhat like the two (_i_, _k_) here described,--namely, at the bottom of the sack in some Balaninæ, at the bases of the anterior cirri in Lepas and in some other genera, and on the dorsal surface of the prosoma in certain species of Pollicipes; in this latter case some of these appendages were covered by scales; and the prosoma whence they arose answers to the third segment of the body in Cryptophialus, or that supporting the lower and larger appendage. Appendages of this nature, in several cirripedes, serve for the lodgment of the testes, but in some cases they are of no apparent use, excepting, perhaps, in aiding respiration by the expansion of corium thus exposed, and this partially may be their function here, for there are no proper branchiæ.

_Cirri._--There are three pairs, together forming a nearly straight brush, of considerable length, projecting in a line with the last thoracic segment. Each cirrus is biramous. In a moderately large specimen there were twenty segments in one of the longest rami. Each segment (fig. 14, _a_) is strengthened by a dorsal or posterior shield of thickened yellowish membrane, from the upper edge of which a single smooth spine projects; the anterior surface is likewise strengthened on the two edges by thickened membrane, and supports two pairs of long spines, which are plumose, or rather hirsute, on both sides. In the lower segments of both rami of the several cirri, the inner spine of each pair is considerably shorter than the outer spine,--evidently in relation to the little power of divergence of the two rami. All the cirri resemble each other, excepting that the rami of the anterior pair, are rather shorter than those of the other cirri, but the dorsal spines on their segments are longer. In all ordinary cirripedes the two rami are equally supported by the upper segment of the pedicel, which latter is very short compared with the lower segment, and is separated from it by a transverse articulation. Here (fig. 13) the exterior or anterior ramus is articulated on the pedicel, almost like a branch, in front of the other ramus, which seems more directly continuous with the pedicel. The upper segment of the latter is separated from the lower segment, both being of about the same size, by a very oblique articulation. On the front surfaces of the pedicels there are two or three pairs of spines.

_Abdomen._--The three cirri on one side are separated from their opposite pairs by a prominent, longitudinal fold, formed of thickened yellowish membrane, which, when foreshortened by being viewed dorsally, looks like a style projecting immediately beneath the anus. The transverse folds separating the three pairs, are exceedingly slight. The inner basal edges of the pedicels of the cirri project slightly inwards as apodemes, giving, I believe, attachment to some muscles. The anus lies between the posterior pair of cirri, and deeply indents the last thoracic segment. The transverse folds separating the three pairs of cirri, little developed as they are, must, according to all analogy, be considered as representing three segments of the body, and as we have accounted for seven cephalic and seven thoracic segments, we must conclude that these are three abdominal segments. We know, moreover, that the abdomen in the pupæ of the Thoracica, with a single exception, does actually consist of three segments.

_Movements of the Mouth, Thorax, and Cirri, &c._--Judging from the fact of the lancet-shaped appendage of the labrum being more or less exserted in dead specimens out of the shell-cavity, and from the analogy of other cirripedes, I do not doubt that the whole mouth can be considerably raised and depressed; we have seen, also, that the rostral end of the toothed orifice of the sack can be pulled down, which would aid in exposing the mouth and labrum. The well-articulated thorax, by the aid of the muscles attached to every segment, can certainly be doubled up and contracted, so that the cirri with their pedicels (coloured purple like the exposed labrum) can be wholly protruded out of the sack and shell-cavity. The three cirri no doubt can be separated a little from each other, both transversely and longitudinally; and according to analogy, the two rami of each cirrus can likewise be separated: there are, also, muscles for moving the two segments of the pedicel of each cirrus; and other muscles run up the many segments of the rami. We have seen that the great lancet-formed appendage of the labrum, laterally fringed with fine hairs, can be erected; and I do not doubt that the prey when entangled by the expanded cirri, is borne against this appendage, and is then, by the retraction of the thorax, dragged down its smooth surface to the mouth, where it is seized by the mandibles and maxillæ, which lie like a trap at the bottom of an inclined and moveable plane.

_Alimentary Canal._--The œsophagus is long; it runs backwards from the mouth and then downwards; at its lower end, where it enters the stomach, the part, which in other cirripedes is expanded and bell-shaped, is modified in a most singular and quite peculiar manner; for the lower part of the œsophagus, after widening a little, becomes converted into what appears at first like a square box, 8/1000ths of an inch across. This box is really deeply folded (see diagram, Pl. 24, fig. 11) into six longitudinal ridges and hollows: two of these hollows, facing each other, are wider than the others, and when the organ is dissected out of the body, it generally lies (fig. 10) with one of these faces exactly over the other, the narrower lateral folds being thus hidden, and the whole consequently appearing like a simple square box with concave sides. But when a section is made, or the lower open end is turned upwards, we see that the organ is six-rayed and elongated, with the longer axis standing parallel to the flattened sides of the animal's body. The edges of the folds are formed of yellowish, thickened membrane, with a sinuous or beaded outline, which serve to strengthen the organ. Internally, the two broad concave sides are armed, in their upper inwardly prominent (fig. 12) part, each with a disc, 2/1000ths of an inch in diameter, crowded with short, thick, brownish, inwardly projecting teeth. The two discs stand exactly opposed to each other. The bases of the teeth, seen from the outside (Pl. 24, fig. 10), seem like brown little circles, with a smaller circle within. The disc obscurely appears to be formed by the confluence of two smaller discs which lie, I believe, at a very small angle to each other: beneath each of these half discs there is a longitudinal band of very fine hairs, the two bands uniting into one, lower down within the organ. The internal longitudinal edges, also, of the four lateral smaller folds are likewise clothed with fine hairs; hence we have six parallel longitudinal rows of very fine but stiff hairs, or eight, if the united bands on the two broader faces under the discs be counted each as two. These bands of hairs, and the opposed discs, armed with very strong teeth, can be separated and brought together with force, by the action of strong constrictor and diverging muscles. Hence any prey carried down the œsophagus, before entering the stomach, would have to pass, as it were through a mill, and be subjected to a severe trituration by the discs of teeth, and immediately beneath to a brushing by the six longitudinal bands of hairs. This curious and unique structure answers, I believe, the same purpose as the four convex, hardish, crenated buttons on the posterior thoracic cirri in Alcippe, which are likewise unique in that genus. I observed that in some specimens the teeth had been worn quite blunt, but the teeth and hairs are periodically moulted and renewed, together with the whole œsophagus.

The stomach is broadest at the upper end, and extends from a little beneath the mouth down to the fifth segment of the body, where it becomes narrow. It presented an irregularly contracted appearance, and was covered by a pulpy hepatic layer. The rectum is of rather large diameter; it extends from the middle of the fifth segment to the end of the eighth segment of the body, or seventh of the thorax, where the large anus is situated, lying between the posterior abdominal cirri, and partly hollowed out in this seventh segment. The rectum, as in all other cirripedes, is periodically moulted. The food is of a bright green colour, as if of a confervoid nature; the triturating and brushing action of the œsophagus seems to roll this matter into pellets, which apparently retain this form until finally expelled as excrement: certainly the excrement is in pellets, and I have several times seen pellets within the stomach.

_Organs of Generation._--The specimens as yet described are exclusively female, there being certainly no testes or vesiculæ seminales. As in every specimen collected (early in January) there were within the sack either nearly mature eggs, or young larvæ, it was the worst period for seeing the ovarian cæca, and I have failed to discover them in the specimens now long kept in spirits of wine; but I cannot doubt they would be found, between the inner and outer tunics of the carapace or general covering, near the disc. I have stated in my original notes, made when the specimens were alive, that the ova are at first perfectly detached; but some appearances make me believe that I overlooked (as might easily happen) the often _excessively_ fine membrane which in other cirripedes unites the ova together, and so forms the ovigerous lamellæ. The ova are much less numerous than in other genera, varying from only nineteen to about sixty. In the same individual all the eggs were always in the same state of development.

_Metamorphoses._--The true ova, in their earliest condition, when in the sack, are ovate (Pl. 24, fig. 15), orange-coloured, quite smooth, and barely 10/1000th of an inch in their longer axes. They soon become broader at one end than at the other; and by degrees the narrow posterior pointed end becomes developed into a slightly club-shaped, almost transparent (fig. 16) horn, and the broader anterior end, into two rather longer horns. The length of the oval part, not measuring the horns, is nearly the same as in the primary true egg condition. There is as yet no trace of internal organs, the whole contents consisting of pulpy granular matter. How far the above changes are effected by moulting, either of the whole or of part of the integuments of the egg-like body, I cannot say; but the pulpy matter within the ovum, even in its earliest stage, was included within an inner envelope or case.

In the next distinct stage (there being, however, slighter intermediate changes) the posterior horn has shrunk, and become converted into a bluntly-pointed conical termination for the whole body (fig. 17), whereas the two anterior horns have approached each other on the future ventral surface, and have increased considerably in length and thickness, and contain within them the prehensile antennæ, which can be externally seen, and which I dissected out of these horn-like cases. The oval part of the egg-like larva (for I hardly know what to call it) is now very slightly shrunk, being hardly more than 9/1000ths of an inch in length. At this stage, these bodies adhere by the tips of their anterior horns, containing the antennæ in process of formation, to the inner tunic of the sack, and likewise in little groups one to another: as the included prehensile antennæ ultimately become attached by cement (proceeding, no doubt, as usual, from a modified portion of the ovarian tubes), it seems probable that some cement may at this early period be excreted, but I could not make out the exact means of attachment. The egg-like larvæ are, also, aggregated round the tapering curled dorsal appendages of the second and third segments of the body, and it is possible that at this, or at an earlier period, these appendages may act like the ovigerous fræna in the Lepadidæ, and serve to retain the egg-like larvæ within the sack.

We come now to the last larval, or pupal condition, before the final metamorphosis into the mature animal; the changes above described have been, at least to a great degree, if not absolutely gradual; but the pupa suddenly appears perfectly developed, from the moulting of the last-described horned, egg-like larva. It is now a free animal crawling about the sack of its female parent. It has increased a little in length, as compared with the oval part of the egg-like larva in its second stage, namely, from a little above 9/1000ths to 16/1000ths of an inch: from the position of the prehensile antennæ in the two states, I have no doubt that this increase of size is entirely due to the anterior part of the pupa being doubled up whilst within the egg-like larval envelope. The pupa in shape (fig. 18) somewhat resembles a coffin, and is far less laterally compressed than other pupæ, and hence can easily be placed either on its dorsal or ventral surface. The prehensile antennæ are of large size: when the animal was alive, they were concealed under and partially included within, the front part of the carapace or shell, which in this condition was not so much truncated as in the drawing given (fig. 18) of a specimen lying on its back, with its antennæ protruded. Some specimens formerly examined for me by Dr. Hooker, had their antennæ and whole ventral surface forced outwards, apparently from the endosmose of the spirits of wine. The whole dorsal surface, and the overlapping sides of the carapace are elegantly punctured, and are formed of a rather brittle substance, here and there supporting, especially at the front end, some fine and rather long bristles,--which latter I have not seen on the pupæ of other cirripedes. The ventral surface is very narrow towards the posterior end of the animal; it is formed of thin, structureless membrane. On this surface, close to the posterior end, there is a minute orifice, through which three pairs of bristles are protruded, attached to (as I believe) the rudimentary abdomen; the bristles apparently cannot be withdrawn.

The antennæ, (fig. 18) as stated, are of large size compared to the whole animal: they resemble, in all essential respects, the same organ in other cirripedes. The ultimate segment is unusually thick;[152] it is terminated by five bristles, one of which is longer than the others, and stands rather separated from them. The disc-segment is large, nearly circular, with the broad edges transparent and membranous; on its posterior edge there is a single small spine. The second or main segment, counting from the base, has a single spine on its upper margin, close beneath the spine on the disc; it is articulated to the disc-segment, a little way from the disc itself,--which is a peculiarity I have not elsewhere noticed. The basal segment is thick and not so short as usual. These organs are furnished with powerful muscles. They are generally protruded alternately; and by the adhesion of the sucker-like disc, the animal drags itself along. The sucker-disc has great play, and when observing specimens alive, I compared its action to that of a wrist-joint. The antennæ, when retracted within the carapace, lie parallel to each other.

[152] As I have given, in my former volume on the Lepadidæ, p.
286, so many measurements of the antennæ, I may here add those of
Cryptophialus,--the length from the end of the disc to the end
of the second segment, (formerly called by me, erroneously, the
basal,) is 26/6000ths of an inch; the greatest width of the second
segment, 9/6000ths; the length of the little ultimate segment,
3/6000ths, and its width under 3/20000ths of an inch.

Posteriorly to the antenna, I distinctly saw the apodemes to which the eyes are attached: I was not able to distinguish any middle fork to the apodeme, which consequently does not resemble a UU, but U. The eyes are dark purple, and, as usual, compound: in one specimen I counted twelve ocelli within the common spherical envelope.

I could not distinguish any thorax, and certainly there is no mouth; nor, from analogy, could the latter be expected, excepting as forming part of the young cirripede: there are no natatory legs, which the pupæ in all other cirripedes possess. Of the three postero-ventral pairs of bristles, the most posterior or dorsal pair, differs from the other two pairs in being considerably smaller, and in being mounted on elongated pedicels: the two anterior pairs of bristles are strong: the three pairs are articulated, one behind the other, on a small body, apparently enclosed in a minute sack, and certainly attached all round by membrane to the internal edges of the orifice, through which the bristles are protruded. These bristles, when the pupa was alive, were often moved, and served apparently to steady the body during the alternate protraction and retraction of the prehensile antennæ. From the fact of the pupa of other cirripedes having an abdomen, formed of three segments, placed exactly in the same position as the minute body here supporting the three pairs of spines, I believe this body to be the abdomen. In other cirripedes only the posterior segment of the abdomen bears spines, which are supported on little limbs or pedicels, namely, the caudal appendages, the other segments being naked. But as the mature Cryptophialus, unlike other cirripedes, has abdominal cirri, the presence of spines on the corresponding abdominal segments in the pupa, is explained and rendered probable: there can, I think, be little doubt that the small terminal pair of spines, supported on elongated pedicels or limbs, answers to the caudal appendages found in many cirripedes.

The whole course of the metamorphosis is very peculiar. The gradual changes in the egg-like larvæ (for I suppose they must be called larvæ) from a simple oval egg, to pointed oval, to oval with three horns, and lastly to oval with the two anterior horns larger, and the posterior horn reduced to a mere point, seems to me very curious; and offers, as far as I know, a unique case. It is interesting to reflect how perfect a series, in the development of an animal, we have, in different members of the Articulata,--from an ordinary egg, in which all the changes go on unperceived, and whence a perfect animal is matured,--to an egg-like larva which undergoes the changes just described, and which turns into a pupa that does not eat or increase in size,--to a larva which eats and increases in size, but undergoes only one great change, as in most insects,--to a larva undergoing several great changes, as in the case of ordinary cirripedes, before its final metamorphosis into the mature animal. The first larval condition of other cirripedes, in which there is a single eye, three pairs of thoracic limbs, and a much elongated pointed body, covered by a prolongation of the carapace, is here not fully developed or matured; but this stage is, I think, clearly and very curiously indicated by the posterior horn of the egg-like larva, which we may suppose represents the posterior pointed end of the body, for it disappears in the succeeding stage, just as it does in the second larval condition of other cirripedes. In the first stage of ordinary cirripedial larvæ, the anterior horns are always present, serving, as in the case of these egg-like bodies, to enclose and protect the antennæ during their formation. The second egg-like stage answers to the second larval condition of ordinary cirripedes, as described (and figured, Pl. 30, fig. 1) in the introduction to the Balanidæ. The third or pupal state is fully developed in all cases.

Finally, the pupa of Cryptophialus is peculiar in its punctured, hairy surface, and in its shape, which, in being so much more depressed than usual, retains an earlier larval condition; but its chief and highly remarkable character consists in the entire absence of natatory legs; and, in consequence, instead of there being a large sack within the carapace, with an elongated orifice on the ventral surface, there is only a quite minute orifice at the extreme posterior end of the animal, through which the bristles, borne apparently on all three segments of the minute abdomen, are protruded.

The pupæ of the male and female are exactly alike in all their general characters, and probably in every point of detail; but my later and more minute observations were made only on pupæ, which, from their place of attachment, would certainly have turned into males. As these pupæ, without any further metamorphosis, were developed into males, we may, I think, safely infer that such is the case with the females: and, consequently, that the whole course of the metamorphosis has been, in this cirripede, seen and described. During this whole course, no food could possibly have been obtained, for the pupa is destitute of a mouth or organs of prehension, and the stock of cellular matter, enclosed within the ovum, has been sufficient for all the above changes, and for the final metamorphosis. We shall, moreover, immediately see, in the case of the male, that the stock of cellular matter has also sufficed for the development of testes, spermatozoa, and a wonderfully elongated probosciformed penis.

MALE. Pl. 24, fig. 19.

By throwing pieces of the perforated shell of a Concholepas into acid, I examined several scores of specimens of the Cryptophialus, and on all, with the exception of a few young individuals, males were attached. They were attached by cement, proceeding in the usual manner from the prehensile antennæ, outside, to the edges of the upper half of the disc formed of the thicker not-moulted membrane, by which the female adheres in her chamber: hence the males are included in the upper part of the same cavity with the female, into which they must have crawled as pupæ. I found from one or two up, in one case, to seven males, attached to the same female; four or five being the most usual number. In the early part of January, when all my specimens were taken, many of the males had not shed their pupal integuments, and of those that had, the majority were immature, a few only having spermatozoa: all the females had within their sacks, either ova including almost perfect pupæ, or fully developed pupæ: we may, consequently, conclude that these young males were maturing in order to impregnate the next set of eggs.

The male, immediately after its metamorphosis from its pupal condition, which has been fully described, is almost globular, but slightly bilobed, and is formed of strong, structureless, transparent membrane, including a mass of cellular matter, apparently without any included organs: it is attached by about the middle, between the anterior and posterior lobes, by the not-moulted prehensile antennæ. When the male is mature, its greatest length, measured from the posterior end, where the orifice is seated, to the anterior and blunter end, is about 13/1000 of an inch, and therefore rather less than the pupa, which was 16/1000ths in length. Relatively to a full grown female, the male slightly exceeds half the diameter of the toothed orifice leading into her sack, see (_z_) fig. 1, Pl. 23. In the mature condition, (fig. 19), one lobe, namely, the upper or posterior, has become more pointed, and is terminated by a minute orifice, 8/6000ths of an inch in diameter. This orifice is formed by a rim of thickened brownish membrane, which, on what was the ventral surface, has a few very minute, but strong, sometimes bifid spines;--in this one character, the male resembling the female. The other and lower (homologically anterior) end or lobe is broader, and contains a mass of cellular matter, which, from its close resemblance in appearance and position to similar matter within the male Alcippe, I have no doubt forms the contents of the testis. In one single specimen, I succeeded in isolating a vesicula seminalis of small size, containing perfectly distinct spermatozoa. Across the middle, between the two lobes, close under the outer integument, there is a broad layer of rather strong transverse muscular fasciæ. I did not observe any eye, the presence of which I should have expected from analogy. Internally there is no mouth, thorax, cirri, or other organs, excepting the testis and vesicula seminalis just mentioned, and an immensely elongated probosciformed penis, coiled up and filling the rest of the inside of the sack down to the testis, which latter occupies the whole anterior, and generally lower end of the animal. This penis is plainly articulated, and includes fine transversely-striated muscles: no doubt it can be protruded through the minute orifice, and voluntarily moved about. Out of a male, 12/1000ths of an inch in length, I dissected a penis, which, when not stretched, measured 50/1000ths of an inch in length; when a portion was pulled between two needles, it could be stretched to apparently three times its former length, and I should think that this organ could be extended by the animal to, perhaps, even the 100/1000ths of an inch,--that is, to between eight and nine times its own entire length! The use of this enormously elongated penis obviously is, that the spermatozoa of these males, which are so extremely small in size, compared to the female, should all be conveyed within the sack, and none be lost. It should be borne in mind, that the whole male, including every part, is scarcely larger than a single ovum, of which sometimes sixty have to be impregnated by only two or three males. In a full-grown female, the distance from one of the attached males to the middle of the orifice leading into the sack, is about the 5/100ths of an inch, equal to the length of the coiled up, not-extended penis: the further distance from the orifice of the sack to an ovum lying at the bottom of the sack, would be almost 10/100ths of an inch, so that the spermatozoa have to pass a distance of 15/100ths of an inch from the testis of the male to the lower ova. I believe two thirds of this distance would be passed safely along the probosciformed penis.

The resemblance between the male of Cryptophialus and of Alcippe is truly surprising; and is the more wonderful, considering the great dissimilarity of their pupæ. Hardly any characters can be pointed out in which these males differ, excepting such as might have been thought of only specific value, namely the relative proportions of the different parts, and mere external shape. The peduncle growing a little after the metamorphosis, in the male of Alcippe, and the prolongation of its capitulum with the included oblique ligamentous fibres, are the greatest differences. Having fully remarked, under Alcippe, on the wonderfully rudimentary condition of these males, destitute as they are of so many parts and organs, I will here say nothing further on these singular creatures, destined to discharge their spermatozoa, die, and be succeeded by a fresh set of short-lived male successors.

Order III.--APODA.

_Cirripedia, with the carapace reduced to two separate threads, serving for attachment: body consisting of one cephalic, seven thoracic, and three abdominal segments, all destitute of cirri. Mouth suctorial, with the mandibles and maxillæ placed back to back, enclosed in a hood, formed by the union of the labrum and palpi. Metamorphoses unknown._

The characters above given fully justify, I think, the formation of this order; though it contains only one species, the _Proteolepas bivincta_. The mere external appearance (Pl. 25, fig. 7), so wonderfully different from that of every other cirripede, would by itself prompt to this same conclusion. At first sight the Proteolepas, if of fresh-water origin, might even have been mistaken for the larva of some insect, fastened by two threads to its prey. The entire absence of the three anterior segments of the head and therefore of the carapace, or, speaking strictly, the mere rudiment of these parts, forming an envelope to the two cement-ducts,--the absence of a stomach, rectum, and anus,--the entire absence of thoracic and abdominal appendages or cirri,--the absence of a probosciformed penis,--are all negative characters, which might ensue from degradation, so common with parasites; and which might, therefore, have been esteemed of not high classificatory value. But the suctorial mouth, with the palpi and labrum united into a hood, and with the mandibles and maxillæ reversed or turned back to back, so as to be utterly incapable of prehension, is a type of structure not hitherto met with, I believe, in any other animal, and cannot be explained away by degradation. The formation of the ova within the segments of the body, a peculiarity confined to this one cirripede, evidently results from the non-development of the anterior part of the head, within which the ova are usually formed; but the compound structure of the vesicula seminalis is a peculiarity which cannot be thus explained. Proteolepas has no particular affinity to any other cirripede; it resembles, indeed, Cryptophialus in one important point, but only in one point, namely, in the number of the segments of its body. It is really beautiful to see how the homologies of the archetype cirripede, as deduced from the metamorphoses of other cirripedes, are plainly illustrated during the maturity of this degraded creature, and are demonstrated to be identical with those of the archetype Crustacean. I was at first inclined to rank Proteolepas in one division, and all other cirripedes in another division of equal value; but as it may be inferred from the characters of the prehensile antennæ, that the pupa did not differ much, if at all in any important character, from the pupæ of other cirripedes, I have thought the three orders, which I have instituted, would be the most natural arrangement. As any one looking at the drawing given of Proteolepas, might very naturally feel inclined to protest against its being ranked as a cirripede, I must reurge the importance of the pupal antennæ being constituted on the common type, for from their structure, by the law of correlation, that of the whole pupa may be inferred; and even still more I must insist on the importance of the one great character of the antennæ being cemented to the surface of attachment by matter proceeding, as we shall see, in a modified state, from the great ovarian sack. The structure, also, of the mouth (to a certain extent), the segmentation of the body, though in appearance so peculiar, the hermaphrodite condition, the single penis, the absence of oviducts, all accord with, and taken together demonstrate, its cirripedial nature.

PROTEOLEPAS BIVINCTA. Pl. 24, 25, figs. 1-7.

_Hab._--Parasitic within the sack of the _Alepas cornuta_, from St.
Vincent's, West Indies, Brit. Mus.

_General Appearance._[153]--The entire animal, as already remarked, curiously resembles, at the first glance, the larva of some insect. It is rounded, but somewhat compressed, and tapers gently towards the posterior end. It lies curved in an arc, the ventral surface being concave, and the dorsal convex, but a little flattened dorsally at the anterior and blunter end. Its length, if straightened, would rather exceed one fifth of an inch. The body consists of eleven segments, which, excepting the three terminal, are conspicuously plain. The first segment is surmounted by a rather small mouth, which any one would, assuredly, at first consider as the entire head, though he would in vain search here for eyes, antennæ, or other parts of the three anterior cephalic segments. On the dorsal surface, low down on the second segment of the body, two, quite flexible, thin, but strong, flattened threads arise, which terminate in a pair of prehensile antennæ, having the usual cirripedial structure. From the penultimate or disc segment of these antennæ, cement has been excreted, by which the antennæ are firmly cemented low down to the rostral end of the sack of the cirripede, the _Alepas cornuta_, on which it is parasitic: hence Proteolepas lies with its back downwards, and with its ventral concave surface fitting the convex body of the Alepas: its mouth lies under the middle of the soft prosoma of the latter cirripede, which I cannot doubt that it lacerates and sucks.

[153] I may be permitted to premise, that though I procured only a
single specimen, yet perceiving its very singular nature, I took
such care and length of time in the dissection, and repeated every
observation so many times that I think reliance may be placed
on the description here given. Fortunately I had acquired, from
dissecting many much smaller specimens of various cirripedes, all
the advantage which full experience could give me, when I commenced
on Proteolepas.

_Mouth._--The mouth is suctorial, and is constructed on a different plan from that in any other cirripede, or, indeed, in any other, as far as I know, articulated animal. It is narrower, in both a longitudinal and transverse plane, than the first segment of the body, and is distinctly separated from it. The lower part on the ventral side, is protuberant and rounded. The summit is square, and is formed by the crest of the labrum, with two large palpi (_d_, fig. 3), having nearly the usual form amongst the Balaninæ, and pointing towards each other, but differently from in any other Cirripede, they are united for their whole length to the labrum, and by their extremities to each other. These parts together thus form an arch or hood, within which stand the other gnathites. The palpi are roughened by groups of very minute spines. At their bases they can be obscurely seen to be separated from the rest of the mouth by an oblique joining or articulation. The back of the mouth is formed entirely of the labrum, which becomes narrow towards its base: it is, from top to bottom, 20/1000ths of an inch in height. Within the hood formed by the palpi and labrum, a pair (_c_, fig. 3), of very singular, compounded, mandibular organs project freely, straight up, with their convex outer edges placed parallel and close together, and their teeth pointing directly from each other, so that they stand in a reversed position compared with the jaws of all other cirripedes, and are absolutely incapable of prehension.

This compounded organ is singularly small compared with the palpi and labrum: it is narrow, being about 5/3000ths of an inch in width, but is produced upwards, so that a considerable length projects freely, and the rounded, properly external, margin can be traced down for a length of about 20/3000ths of an inch. In a lateral view of the mouth, the extreme tip of the mandibular organ could sometimes be seen just projecting out of the hood. The mandibular organ, when separated and carefully examined, presents the appearance, represented from a camera drawing, in (Pl. 24, fig. 2): we here see three groups of teeth; of these the lower set (_c_) consists of blunter teeth, placed more transversely, and easily separated from the others, and altogether clearly appears like a distinct organ. I do not feel nearly so sure regarding the other two sets; my first impression was strongly that they were distinct organs, closely united laterally together,--one (_a_) probably representing the mandibles, and formed into a single large tooth; the other (_b_) formed of three teeth, and probably representing the outer maxillæ; the first-mentioned set of teeth, which seemed to me to arise from between the other two sets, being the inner maxillæ. If this view (and it must be remembered how excessively minute the parts are) be not, as I now suspect, correct, we must suppose that the outer maxillæ are aborted, and we have seen some tendency towards this in other cirripedes; the compounded organ being formed only of the mandibles (having on this view four teeth) and the inner maxillæ. As far as the mandibles are concerned, their existence, I may remark, is plainly shown by the presence of the palpi, which in all cases belong to and form part of the mandibles. The ventral surface of the mouth, immediately beneath the free portion of the compounded mandibular organ, consists of a triangular projection, but I could see no appearances to make me suppose that this part represented the outer maxillæ. The compound organ--in general shape, and in the oblique manner in which the front part is cut off and terminates in ligamentous apodemes, to which muscles are attached,--presents an unmistakable likeness to a mandible. It is hollow within, and muscles appear to extend some way up, perhaps to the transversely toothed portion, which represents, as I believe, the inner maxillæ: these two groups of teeth, anyhow, seemed to have some power of sliding over each other, and altered their positions during the course of dissection. On each side of the mouth, there is a muscle attached by its lower end to the basal edge of the labrum, and two others, one above the other, attached by their lower ends to about the middle of the labrum; these muscles, which are distinctly striated or voluntary, I infer, from analogy, run up to the ligamentous apodemes of the compound mandibles. There appeared to be other more delicate muscles attached to the basal articulation of the mouth on the ventral face, and these, I presume, would run to the supposed inner maxillæ.

The mouth in forming a prominence separated by a distinct articulation from the body, and in the union of the palpi and labrum (though here carried to excess), is constructed so far on the cirripedial type; but how are we to account for the extraordinary reversed position of the united mandibles and maxillæ, with their backs almost touching each other, and their toothed edges twisted round so as to face outwards in a manner unexampled, I believe, in any other articulate animal? It might, perhaps, be at first suspected, that the compounded mandible had not really been twisted round, but that the teeth had been abnormally developed on the outer convex margin: this view, however, certainly cannot be admitted, for the properly outer convex margin can be traced running far down the mouth, in a manner utterly inexplicable, if this were really the inner side; and equally inexplicable on this view would be the position of the ligamentous apodemes. Hence I cannot doubt that this compounded mandibular organ has really rotated on its axis; and if the course of development could be followed, I suspect that the twisting would be seen to be effected as follows: we know in all cirripedes that the outer and inner maxillæ, and to a certain extent the mandibles, instead of facing each other, are directed towards the labrum; they therefore have already been twisted round a quarter of a circle, as may be seen in the diagram (Pl. 24, fig. 4), copied from the mouth of Ibla. Now let us drive inwards the front of the mouth, along a narrow medial line; these organs would then (fig. 5) be compelled to turn round a quarter of a circle more, and so face directly outwards. In this process, the integument between the lower and outer part of the mandible and the base of the palpus, which normally are in close contact, would have to be greatly stretched. By a movement of this order, the mandibles would come to stand posteriorly or exteriorly to the other gnathites; and as far as I could make out (previously to my having any theory) the large single toothed portion of the compound organ which most resembles a mandible, did really stand outside the other toothed portion.

With respect to the action of this singularly constructed mouth; if its ventral and oblique surface were applied to any yielding object, as the adjoining soft prosoma of the Alepas, the compound mandibles would be worked within an absolutely closed chamber. The action of these mandibles would be to make a transverse slit, and subsequently to serve as a grapnel to keep the mouth closely adpressed to its prey: the other teeth might act in keeping the wound open. When the mouth was thus closely adpressed over a wound, the great power of shortening the whole body which the animal possesses (the œsophagus being closed), would, by the subsequent action of the elasticity of the outer membrane, almost certainly create suction, and thus cause the nutritious juices of the Alepas to flow into the body of the parasite. Hence I have called the mouth suctorial.

_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.