Whilst the young Lepas is closely packed within the larva, the capitulum, or shell, about equals the length of the peduncle. Even at this early period the muscles of the peduncle are distinct. The com pound eyes, which we have seen are attached to apodemes springing from the sternal surface of the larval carapace, are consequently cast off with it. Whilst the young Cirripede is packed within the larva, the outer integument of its peduncle necessarily forms a deep trans verse fold passing over the eyes and apodemes. This determines the position and origin of the sac in the young Cirripede.
" The larva," says Mr. Darwin, "fixes itself with its sternal surface parallel and close to the surface of attachment, and the antennae become cemented to it : if the Cirripede after its metamorphosis had remained in this position, the cirri could not have been exserted, or only against the surface of attachment ; but there is a special pro vision that the young Cirripede shall assume its proper position at right angles to the position which it held whilst within the larva, namely, with its posterior end upwards. This is effected in a singular manner by the exuviation of the great compound eyes, which we have seen are fastened to the outer arms of the double • U U• like sternal apodemes. These, together with the eyes, stretch transversely across and internally far up into the body of the larva ; and as the whole has to be rejected or moulted, the membrane of the peduncle of the young Cirripede has necessarily to be formed with a wide and deep inward fold extending transversely across it ; this, when stretched open after the exuviation of the larval carapace and apodemes, necessarily causes the sternal side of the peduncle to be longer than the dorsal, and as a consequence gives to the young Cirripede its normal position, at right angles to that of the larva when first attached." That the homologies of the larva of the Cirripedes are with the Crustacea has before been pointed out. Mr. Darwin says :—" In Crustacea, according to the ordinary view, there are twenty-one seg ments ; of these I can recognise in the Cirripede, on evidence as good as can generally be obtained, all with the exception of the four terminal abdominal segments ; these do not occur in any species known to me in any stage of its development. If that part of the larva, in front of the mouth, bearing the eyes, the prehensile antennae, and in an earlier stage another pair of antennae, be formed, as is admitted in all other Crustacea, of three segments, then beyond a doubt, from the absolute correspondence of every part, and even every coloured mark, the peduncle of the Lepadidce is likewise thus formed. The
peduncle being filled by the ovarian tubes is no objection to this view, for I am informed, on the high authority of Mr. J. D. Dana, that this is the case with the cephalo-thorax in some true Crustaceans; for instance, in Sapplarina. To proceed :—the mouth, formed of mandibles, maxillae, and outer maxillae, corresponds with the fourth, fifth, and sixth segments of the archetype Crustacean. Posteriorly to the mouth we come, in the larva, to a rather wide interspace, without any apparent articulation or organ ; and then to the thorax, formed of six segments, bearing the six pairs of limbs, of which the first pair differs slightly from the others. The thorax is succeeded by three small segments differently shaped, with the posterior one alone bearing appendages. These segments I cannot doubt, from their appearance alone, and from their apparent function of steering the body, are abdominal segments. If this latter view be correct, the thoracic segments are the six posterior ones of the normal seven segments; and there must be two segments missing between the outer maxillae and the first thoracic pair of legs, which latter, on this view, springs from the ninth segment. Now, in a very singular Cirripede named Proteolepaa, the two missing segments are present, the mouth being actually succeeded by eight segments, and these by the three usual abdominal segments, every segment in the body being as distinct as in an Annelids ; hence in Proteolepas, adding the three segments for the mouth and three for the carapace, we have altogether seventeen segments, which, as I have stated, is the full number ever observed in any Cirripede ; the four missing ones being abdominal, and I presume the four terminal segments. That the cavity in which the thorax is lodged in the larva, and therefore in the mature Cirripede, is simply formed by the backward production of the carapace, does not require any discussion. The valves have no homological signification." The preceding wood-cut, copied from Darwin's work, will make these homologies clear. The upper figure is a Stomapod Crus tacean (Leucifer, V. Thompson), and the abdomen, which becomes rudimentary in Cirripedes, is given in faint Hues. The lower figure is a mature Lepas with the antennae and eyes which are actually present in the larva, retained and supposed to have gone on growing. All that is seen of a Cirripede, whether pedunculated or sessile, is the three anterior segments of the head of a Crustacean, with its anterior end permanently cemented to a surface of attachment, and with its posterior end projecting vertically from it.