In the bays on each side of the upward production of the horny disc, and likewise a little lower down on its edges, and therefore somewhat protected by lying within the narrow, pointed, lower end of the fissure in the shell of the mollusc, the short-lived Males (Pl. [22], fig. [1], m) are attached often in groups of two, three, or more together.

It may be asked to what part, in other Cirripedes, does the horny disc answer? Not considering the upward prolongation, which has been developed for a special purpose, the disc is irregularly circular,—is added to all round,—serves for the attachment of the whole animal to the supporting surface,—is covered on the under surface by a conformable and parallel mass of ovarian cæca, and the latter by the inner tunic of the sack; therefore in every character, and in its relation to the other parts of the animal, the disc answers to the end of the peduncle, or to the basal cup in Lithotrya, or still more closely to the basis in sessile cirripedes, with the important exception that it lies in a line with the longitudinal axis of the whole animal instead of at right angles to this axis. We know that all ordinary cirripedes become first permanently attached in their pupal state by their antennæ, which are seated on the ventral or rostral surface, near to the anterior end of the body; and that from the young cirripede, after the act of metamorphosis, being turned vertically upwards, and from the extreme anterior, now lower, end of the body not being rapidly developed, the surface cemented down, or the basis, encroaches almost equally on the dorsal, lateral, and ventral surfaces. But if we were to suppose the extreme anterior point of the body to be rapidly developed, the surface of attachment or basis, without it grew still more rapidly, could not possibly reach the dorsal surface, and would, consequently, be confined to the ventral or rostral surface. I have not seen the young of the ordinary or female [Alcippe] soon after its metamorphosis, but in the male the development of the extreme anterior end of the body is extraordinarily rapid, and from analogy we may fairly conclude that this is likewise the case with the female. Hence I believe that the horny disc answers to the cemented down, lower end of the peduncle, in other members of the Lepadidæ, and to the basis in the [Balanidæ], and that it is confined to the ventral or rostral surface, owing to the anterior or lower end of the body having been rapidly developed. To make all the parts, internal and external, of [Alcippe], correspond with those of other cirripedes, the main circular part of the horny disc must be turned up at nearly right angles to its present position (the dorsal or carinal integuments, to the right-hand in fig. [5], being shortened), and then we should have a peduncle, certainly very short and broad, but holding its proper relative position.[143]

[143] In the genus Lithotrya, as long as the animal continues to bore into the rock, the calcareous discs by which it is attached in its cavity, stand, as in [Alcippe] (Pl. 8, fig. 2, 2 a′, Darwin’s ‘Monograph on the Lepadidæ’), parallel to the longitudinal axis, but as soon as the animal ceases to bore, and the discs become converted into a cup, they occupy a normal position at right angles to the peduncle. According to Reinhardt, these discs, in Lithotrya, are situated on the carinal or dorsal surface of the peduncle, at which statement I now feel considerable surprise, as undoubtedly the pupa must first permanently attach itself by its prehensile antennæ on its ventral or rostral surface. In Anelasma I failed to discover any cement or cement-ducts; but I am now strongly inclined to believe, considering that the extreme lower or anterior end goes on growing, that the surface of attachment will be found to occur, as in [Alcippe], on the rostral surface, a little way below the orifice.

Sack and its Muscles.—I have already described the curious phalanx of spines, the long fine hairs, and thickened condition of the inner tunic of the sack along the sides of the orifice. This inner tunic is a reflexion from that enveloping the body of the animal, in the usual manner, as may be seen in the section (Pl. [22], fig. [5]). Between the external membrane and the inner tunic of the sack (e), there is of course the usual double fold of corium, these two folds being united by minute, transverse, ligamentous fibres, branched at the two ends, as in other Lepadidæ. Imbedded in the corium there are numerous, longitudinal, striæ-less muscles, which do not run quite up to the orifice, but to an oblique line beneath it. Externally to these muscles there are, as in the other Lepadidæ, fine transverse muscles, confined to the middle part of the animal, and running from the carinal margin more than half way round both sides. Attached to the upper notched or folded end of the orifice (g, fig. [5], above the upper ends of the longitudinal muscles), there is a fan of rather strong, striæ-less muscles, expanding downwards, with their lower extremities attached to the outer membrane of the capitulum; these muscles apparently serve to open the orifice: there is a somewhat analogous muscle in Lithotrya, but in no other member of the Family: in [Cryptophialus], however, there is a closely similar muscle. Owing to the action of these several muscles, the tissues forming the capitulum and peduncle are, according to Mr. Hancock, highly contractile.

I have stated that the under surface of the upper produced end of the horny disc is concave, and serves for the attachment of several muscles. Of these some run to the basal margin of the great labrum, and no doubt, as usual, move the whole mouth; others, as usual, run to the skin between the labrum and the lower end of the orifice,—i. e. in fig. [5], between the lower end of the row of fine hairs (see fig. [11]), which shows where the basal margin of the labrum is situated, and the lower side of the point (a), where the orifice terminates: others run obliquely on both sides towards the point of attachment of the small adductor scutorum muscle (b, fig. [5]): others, of considerable strength, and these are more peculiar, run and are attached to the lower end of the orifice (a), and serve apparently to draw up the orifice from within the fissure-like cavity, in which it lies lodged: others, again, extend transversely on both sides, close beneath the inner tunic of the sack, a little beyond the line whence the ovigerous fræna or branchiæ arise. These transverse muscles lie within the longitudinal muscles, and therefore are quite different from the exterior transverse muscles, which are situated more towards the carinal portion of the peduncle and capitulum, and which are common to most Lepadidæ. The internal transverse muscles, and those running to the lower end of the orifice, are peculiar, but we shall hereafter meet with them even more developed in [Cryptophialus].

Along the medial carinal line there is, between the two layers of corium, the usual circulatory channel. On each side of this line, on the inside of the sack, there are generally some slight irregular swellings, and sometimes a large extent of the inner surface is irregularly carunculated with little knobs. The sack (e in fig. [5]) extends down almost to the basal point of the peduncle (d), more especially when the ovarian cæca are not gorged with ova.

Branchiæ, or ovigerous Fræna.—Within the sack, on each side of the body, rising not far from the ends of the adductor muscle (b), there is a large fillet or fold; the two occupy so exactly the position of the ovigerous fræna that I cannot doubt such is their nature, though, as happens in the case of some species of Pollicipes, they are destitute of their proper glands, and so do not serve for the attachment of the ovigerous lamellæ; this attachment probably is not required, owing to the protected situation which the lamellæ hold in the sack, under the animal’s body, and over the ovarian cæca. From the unusually large size of these so-called fræna, I cannot doubt that they serve as branchiæ, equally well with the plicated folds of membrane, believed to be homologous with the fræna, in the [Balanidæ], which have by every one been considered as branchiæ. The fræna are broad and truncated at their upper ends; their margins are sinuous, and their outer surfaces papillose; they run longitudinally down the sack, narrowing as they extend, almost to the basal point of the peduncle, and hence are of considerable length; they are hidden in the section (fig. [5]) by the medial, somewhat protuberant mass (c) of ovarian cæca, and partly by the (i) prosoma.

Body.—The body is constructed on the usual type, and indeed does not differ greatly from that of Ibla. The labrum is very large, its lower or basal margin is separated by an unusual space (capable of being contracted or folded) from the lower end of the orifice of the sack; hence the labrum and whole mouth is placed quite remarkably near the upper (or carinal) end of the orifice. This upper end of the orifice, I may remind the reader, is homologically the posterior end of the general covering or carapace, and all that portion of the whole animal (as the sectional figure, [5], stands) below the lower margin of the labrum, on the rostral or ventral surface, is formed by the three anterior segments of the head. The main part of the body, carrying the mouth, is formed by the great development of that segment of the thorax which bears the first pair of cirri (h), here closely adpressed, as usual, to the sides of the mouth. The lower portion of this segment forms the prosoma (i), and has the characteristic outline, but is not much developed. On each side of the prosoma an oval space of membrane is yellowish and is thickened, and so gives support to this part of the body. The five succeeding thoracic segments, which ought to carry the five succeeding and posterior pairs of cirri, are together of very small size (as in Ibla), in comparison with either the prosoma, or the whole anterior part of the animal. The segment (k) which should have borne the second pair of cirri, is considerably longer than the following segments, and is at the same time less distinct, owing to an oval convex shield of thickened membrane on the sides, not extending the whole length of the segment, thus causing two transverse creases, which, when the thorax is contracted, appear like two additional segments. Had this segment borne cirri, they would have stood, as in Ibla, at a considerable distance from the first pair. The segments (l, m) which should have borne the third and fourth pairs of cirri are like each other, except that the former is rather the longest. The membrane covering all the thorax is surprisingly thin; and at the articulations, which are straight and transverse, is deeply folded, so that the thorax must be highly extensible, to a degree which I have not seen equalled in any Cirripede except in the males of Scalpellum vulgare and ornatum. The thorax is represented as somewhat extended in fig. [5]. This part of the thorax is amply furnished with striated and striæ-less muscles for its retraction and protrusion, and for lateral movements. The segment (m) which should have borne the fourth pair of cirri, at first sight falsely appears like the terminal segment of the thorax: in one monstrous specimen it bore a single cirrus, showing (if there had been any doubt) that it was a true segment. The three terminal pairs of articulated appendages, form together a brush; they consist of the fifth and sixth pairs of cirri and of the caudal appendages: my reasons for considering the last-named organs as of caudal origin will be given hereafter. A moderately careful inspection, especially of the ventral surface, will show that the fifth pair of cirri are borne on a small segment (n, fig. [5], but plainer in fig. [13]), which is quite distinct from, but partially concealed by, that which ought to bear (and did bear in the monstrous case) the fourth pair of cirri: this segment is oblique, and cannot be traced distinctly all round the dorsal surface. The segment (o, fig. [13]) bearing the sixth pair is much less distinct, and can only be seen by a longitudinal section, or when the cirri are a little separated, but it certainly exists,[144] as is likewise shown by the presence of small apodemes dipping in amongst the muscles, between this and the last segment. The posterior or caudal appendages are closely approximated; they are not separated by any fold from the sixth thoracic segment; but appear as if they were articulated on the dorsal surface of the sixth pair of cirri, in exactly the manner usual in the other Lepadidæ. The segments bearing the fifth and sixth pairs of cirri are highly oblique to the preceding segments, and consequently the cirri, which they support, instead of projecting inwards, lie like a brush in a line with the longitudinal axis of the main part of the thorax. The membrane forming the two small oblique terminal segments of the thorax is strengthened by irregularly shaped plates of thicker and yellowish membrane.

[144] In the middle, the fold is slightly prominent and pointed, and being most finely villose, I for some time looked at this projection as a rudiment of the probosciformed penis.

Mouth.—The mouth is constructed on the strictly normal type of the Family, but is peculiar in every part; it is remarkable from being situated so near the upper (or posterior) end of the capitulum, this being caused by the great length of the labrum, and of the space of body between the latter and the lower end of the orifice. The labrum is a very singular part of the mouth from its vast size and outline: in fig. [8], we have a front view of the mouth, of which the whole upper pointed part consists of the labrum, and h h is the first pair of cirri; in fig. [11], we have a lateral view of the labrum, with the surrounding thin membrane of the body, a—a, still adhering to its edges; h is the first cirrus on the near side; m the mandible, a little distorted in order to show its tooth, marking the position of the transverse crest of the labrum and of the orifice of the œsophagus; b b is the medial longitudinal ridge of the labrum. In the [Balaninæ] the labrum forms a mere rim to the back of the mouth, consisting of an inner fold running down the œsophagus, and of an outer fold, both close together: in the Lepadidæ the folds are separated, the outer one being swollen or bullate; and here this structure is carried to even a greater extreme than in Ibla and its allies. The distance between the transverse crest over the œsophagus and the blunt projecting point on the summit of the medial ridge, b b, equals twice the longitudinal diameter of the rest of the mouth. The lower margin on each side of the labrum is produced into two projections (fig. [11]), the longer one curling round to a point beneath the jaws, with its extremity imbedded as an apodeme. Another very peculiar character in the labrum, prominently noticed by Mr. Hancock, is caused by a longitudinal row, on each side, of closely approximate, long, very finely pointed hairs, which, as already stated, are fronted on the opposed internal surface of the sack by an irregular band of still finer hairs. The surface of the labrum is partially covered by minute toothed scales, and these, seen on the longitudinal medial ridge, b b, give it a finely denticulated structure. At each end of the transverse crest which overhangs the œsophagus, there is a knob, such as occurs on the labrum of every Cirripede. United to these two knobs, which are formed of thick and yellowish membrane, and springing from the adjoining sides of the mandibles, there are two swellings formed of thin membrane (fig. [8]), which occupy the exact position of the palpi, and may be considered as these organs in a rudimentary condition and destitute of bristles.