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.