Finally, I may add, that, excepting in small details, the prehensile antennæ present no difference throughout the Order: I have minutely examined them in several genera of the Lepadidæ; and in the [Balanidæ], I have seen them in [Coronula], and in several species of [Balanus]. In [B. balanoides] I have examined them carefully; they are smaller and thicker than in Lepas, with the second or main segment bowed outwards, carrying its usual single spine; with the disc excised on its inner margin and apparently without the spoke-like vessels for the cement; and with the ultimate segment proportionably longer, and carrying, I believe, six spines, of which two appeared to be longer and more flexible than the other four shorter and somewhat hooked spines. In [Coronula balænaris], also, the terminal segment is, proportionably to the others, of large size. Not only throughout the order, but throughout the whole Class, the antennæ are singularly uniform in structure, as will be seen, when the last two orders are described.
Eyes.—These present no difference, except in size, throughout the class; and have been sufficiently described in my former volume. The true basal segments of the antennæ (incorrectly designated formerly as sternal plates or segments) are separated from each other by a deep fold; and are separated from the edges of the carapace on each side by a crest and slight fold (Pl. [30], fig. [7], c; and [4]); these folds and crests die out posteriorly, and disappear. The hinder, rounded margins of the basal segments are inflected inwards, and their corners are produced far up into the body, thus forming the curious UU-like apodemes. These apodemes exist throughout the whole class; and the outer arms always carry the great compound eyes. I noticed, in Lepas pectinata, that the two middle arms are proportionably longer than in L. australis. Owing to the presence of these apodemes, and to certain coloured marks on the adjoining corium, the eyes, though enclosed fairly within the carapace, yet deceptively appear pedunculated, so that even J. Vaughan Thompson was thus deceived. I have already described the several muscles attached to these apodemes, and the constant vibratory movement of the eyes. Whilst the pupa remains a freely swimming animal, the eyes are included, not only within the shell or carapace, but (as would naturally happen) within the corium or true skin lining the carapace; but after the pupa has become attached, preparatory to its final metamorphosis (in the state represented at Pl. [30], fig. [2].), not only are the muscles, as before remarked, which are attached to the apodemes, absorbed, but so is the corium investing the apodemes and the immediately adjoining parts of the carapace. Hence it comes that the new corium of the young Cirripede within, is formed in a deep transverse fold across the whole lower half of the animal, and the apodemes with the eyes are thus, as it were, rejected from within the corium, though still remaining within the carapace. Consequently in this final stage, the eyes and apodemes are very conspicuous from the outside, being seen only through the transparent carapace. I presume that the eyes at this period have become functionless, with the optic nerve divided and absorbed. The eyes, apodemes, and carapace soon afterwards are all moulted together.
The eyes of Cirripedes certainly undergo a remarkable series of changes: in the larvæ in the first stage, there is a single eye, perhaps formed by the confluence of two eyes, occupying the normal position in the front of the head: in the second stage, according to Burmeister, the eye has become double, but the two are as yet simple; they are now situated posteriorly to the second pair of antennæ: in the third or pupal stage, they remain in the same situation, but have become compound, of great size, and are attached to the apodemes of the antennæ: in the mature and fourth stage, they have moved someway posteriorly, and again have become simple, of minute size, and are either confluent, as in the Lepadidæ, or tolerably far apart, as in the [Balanidæ]. It must not be supposed that the eye of the mature Cirripede is metamorphosed from the eye of the pupa, for such is not the case; the new eyes and old eyes being formed someway apart, and frequently both can be seen within the pupa (as in [Alcippe], Pl. [23], fig. [16]) at the same time. It is scarcely possible that the eye of the larva in the first stage, can be changed into the double eyes of the second stage; though these latter may possibly be multiplied into the eyes of the pupa, as both continue to occupy nearly the same position.[65]
[65] Zenker, in his ‘Physiological Remarks on the Daphnidæ,’ (‘Journal of the Microscopical Society,’ 1853, p. 274), speaks of a “tripartite azygous eye” as common amongst Crustacea, and as occurring “in conjunction with the aggregated eyes in Artemia, Argulus, &c.; but as appearing regularly in all the Branchiopoda and Siphonostomata as the earliest visual organ.” Hence I conclude that this azygous eye is the homologue of that single eye which appears in the earliest larval stage of Cirripedes; and that the compound eyes of the cirripedial pupa, answer to the aggregated eyes of Artemia and Argulus, &c., with the difference, that in these latter genera the single eye is retained. See, also, Von Siebold, ‘Anatomie Comparée,’ tom. i, p. 435.
Mouth, thorax, limbs, abdomen.—I have nothing to add regarding the mouth, except to confirm my former account; viz., that it is functionless, consisting merely of crests, which project inwardly between the gnathites of the young Cirripede, and of a shrivelled closed tube representing the œsophagus. In fact the mouth is a model of the outside of the mouth of the young Cirripede. I may remark that some little way beneath the membrane answering to the labrum, a pair of ligamentous apodemes, the use of which I do not know, slightly penetrate the body. The degree of prominence of the mouth varies, but it is far less than in the mature animal. On the limbs I have nothing particular to add: the drawing of the first pair of legs (Pl. [30], fig. [5]) is, I think, very accurate: I observed all the spines here figured, on the corresponding leg of the pupa of [Balanus Hameri]. The five posterior pairs of legs differ only in the outer ramus having five plumose spines, instead of four, and one short simple spine at the exterior angle, making six altogether. The legs, in their natural position (fig. [2]), have only the terminal segments of their two rami directed posteriorly; and as a consequence the spine (close to i in fig. [5]), borne on the penultimate segment of the outer ramus, is directed in the same line with that segment and with the pedicel, namely, anteriorly, and at right angles to the natatory plumose spines. This short spine acts, I imagine, as a defensive weapon; it has been omitted in fig. [2]. Of the thorax I need not give, from my notes, any more details. The abdomen (fig. [6]) is similarly constructed, as far as I have seen, throughout the order, with the exception of [Alcippe] (Pl. [23], fig. [17]), in which it is composed of only one segment instead of three. In this genus the caudal appendages likewise consist of only one segment, with very short spines. In the pupa of [Balanus balanoides], the three spines borne on each caudal appendage are very much more unequal in size than in the pupa of Lepas australis, although in the latter (fig. [6]) the inner spine is considerably thicker than the two outer. Whether the three segments of which the abdomen is composed, are the three anterior or three posterior, of the normal seven segments, I know not: on the view that they are the three posterior segments, I presume, according to analogy, that the caudal appendages are borne on the penultimate segment, and that the ultimate segment is here quite aborted.
On the internal viscera I have nothing to add. The cement-duct is represented in Pl. [30], fig. [2], t′, on the near side, running into the antennæ; and I repeatedly traced it, for the duct is very strong, as far as the disc segment; at the other end it joins the cement-gland (t) on the same side of the body. This cement-gland is proved, by the clearest series of facts, to be converted into the incipient ovaria and ovarian cæca. The cement-glands in the older pupæ could be traced as far as the cæca of the stomach, exactly where the ovaria lie in the mature animal; but in some young pupæ, they extended further posteriorly, past the mouth, between the outer and inner membranes of the overlapping carapace. I have faintly shown the course of the stomach, with its two cæca at the upper end; the anus lies between the caudal appendages, at the extremity (above b′) of the abdomen. At this age there is no trace of the vesiculæ seminales, so conspicuous in the mature Cirripede.
Young Cirripede, whilst within the pupa.—I repeatedly succeeded in dissecting the young Lepas australis out of the pupa; and by the previous action of boiling potash, and by a strong light, I was enabled to make a camera sketch (Pl. [30], fig. [2]) of the relative positions of their several parts. The young Cirripede is drawn very faintly, and is best seen by holding the plate in the same position with the mature animal, of which a section is given in my volume on the Lepadidæ, Pl. [9], fig. [4]. I may just notice how complicated are the membranes in a longitudinal section taken at this period: we have, 1st, beginning at the back, the horny tissue of the carapace or bivalve shell of the pupa; 2d, the primordial valve (z, in fig. [3]) of the young Cirripede; 3d and 4th, two folds of corium; 5th, the membrane of the sack of the Cirripede; 6th, the membrane of the sack of the pupa; 7th, the outer tunic of the thorax of the pupa; 8th, the outer tunic of the thorax of the young Cirripede; 9th, the corium lining the latter membrane; and these nine membranes would be repeated on the opposite side of the section, if it were taken through either side of the shell or carapace, bordering the orifice.
After the exuviation of the outer membranes of the pupa, certain pre-existing coloured marks in the corium, such as those round the eyes and round the acoustic orifices, along the ridge of the back and on the borders of the orifice, &c., are still retained by the young Cirripede, either temporarily or permanently; so that the correspondence of part with part of the external surface admits of no doubt. Moreover, the three terminal segments of the antennæ are invariably retained by the young Cirripede, though in a functionless condition, and into them the outer membrane of the body, and an important organ, viz., the cement-ducts are still prolonged; hence these prolongations must be considered as aborted antennæ. Again, we have seen that the mouth of the young Cirripede is formed under the rudimentary mouth of the pupa, with the new œsophagus, round the old œsophagus, leading into the same alimentary canal. The twenty-four extreme tips, likewise, of the six pairs of biramous cirri of the Cirripede are formed within the twenty-four extremities of the six pairs of biramous, natatory legs of the pupa. Consequently, in the Cirripede and pupa, thus far, part corresponds with part, notwithstanding that new eyes are formed posteriorly to the old eyes, and new acoustic organs in a quite different position from the old ones; but now we come to a most important diversity in the metamorphosis, or rather to follow Professor Owen,[66] in the metagenesis, of the young Cirripede. Although, as just stated, the extremities of the cirri are formed within the legs of the pupa, yet, from the great length of the cirri, they occupy more than the whole of the thorax of the pupa; so that the thorax of the young Cirripede is not formed within the pre-existing thorax of the pupa, but within that part of the pupa, (homologically a portion of the first three cephalic segments), which lies anteriorly to the thorax. As a consequence of this, the pedicels and lower portions of the cirri, the segments of the thorax and its dorsal surface, all come to occupy a position at nearly right angles to that of the corresponding parts in the pupa: this is shown in Pl. [30], fig. [2]. And as a further consequence, (and this is the more important point), the sack, which both in the young Cirripede and pupa is formed by the overhanging and produced portion of the carapace, and which is internally lined by a reduplication of the membrane of the thorax, is necessarily, owing to the changed position of the thorax, altered in extent and carried much further; namely, from extending merely parallel to the longitudinal axis of the pupa (from b to b′), it is now in the young Cirripede, in addition, carried (to s′) almost quite across the inside of the animal. Hence it comes that the young Cirripede is, as I have said in my former volume, internally almost intersected; and its body remains attached only by a small space, (see the broken line, round a and b in Pl. [25], fig. [1], of a [Balanus] with the shell, &c., removed from one side), to the sternal or ventral, inner surface of the carapace,—the carapace being modified either into the capitulum and peduncle, or into the shell with its operculum and basis. As a still further consequence of this change of position of the body of the young Cirripede within the body of the pupa, the alimentary canal becomes shortened to fully half its former length. At the same time, the interspace between the mouth and first pair of legs of the pupa, (consisting of the seventh and eighth segments of the archetype), is quite lost in the Cirripede by coalescence. The final cause of the thorax of the young Cirripede not being developed within the thorax of the pupa, probably is, that the cirri may be formed of considerable length, so as to be immediately enabled to seize prey; and that the thorax, which supports the cirri (and this probably is even more important) should be as free as possible within the sack, so as to aid the captorial action of the cirri.
[66] ‘Parthenogenesis,’ pp. 13 and 26.
After these remarks, more especially with regard to the formation of the sack, if any one will look at the sectional drawing of a pedunculated Cirripede in my former volume, or of a sessile Cirripede (Pl. [25], fig. [1]) in this present volume, in which latter the shell adds to the complexity, he will perceive the cause of the extreme difficulty in understanding the relative position of the parts throughout the whole class. Even after I had discovered that the prehensile antennæ of the pupa might always be found in the centre of the basis or surface of attachment, and which fact, it might have been thought, should have convinced me that this was the anterior end of the whole animal, yet still I fancied that the prominent mouth represented the entire head, and that the shell was something quite distinct. It is clear that others have been equally perplexed; for that which is the anterior end in the eyes of one naturalist, is the posterior end in the eyes of another; so with the dorsal and ventral surfaces: one naturalist considers the peduncle of the Lepas as the abdomen; another considers it as a pair of metamorphosed, thoracic limbs, &c.! The probable position of the segments of the body of a mature Cirripede, in relation to the three anterior cephalic segments, or carapace, is shown in the diagram (Pl. [25], fig. [6]) of the supposed position of the mature [Proteolepas] within its pupal envelopes. Here, in the diagram, the two segments immediately succeeding the mouth (c), which are the seventh and eighth of the archetype, (for the mouth consists of three segments, and all in front of the mouth of three other segments), have come to adhere by their dorsal surfaces to the internal surface of the carapace,—that is, of the first three segments, which ought of course to have stood quite in advance of these two segments, and these two segments again ought to have stood in advance of the mouth. The mouth is directed posteriorly, instead of from the body; and the three segments of which it is formed (closed at their anterior end by the labrum), and are very small compared to the relatively monstrously great, three anterior cephalic segments, composing the carapace. To place the segments of the body of [Proteolepas] in proper sequence, in respect to those of the carapace, and in accordance with the sequence of the archetype Crustacean, it would be necessary, by seizing the extremity of the abdomen (a), to tear the two segments succeeding the mouth from their dorsal attachment, as far back as the basal margin of the labrum; and then pull them till they stood posteriorly to (or in the diagram, above) the mouth; which latter part would, by the same movement, be made to project out at right angles to the ventral surface, and would then be preceded only by the first three, great, confluent segments of the head, which being produced backwards, form the carapace. All that has just been said on the position, in [Proteolepas], of the segments of the body in relation to those forming the carapace, I believe to be applicable to all ordinary Cirripedes, with this difference, that in the latter, after the metamorphosis, the two segments succeeding the mouth quite disappear on the ventral surface, and dorsally are either aborted or have coalesced with the adjoining segments.