In the pupa, the carapace is produced, not only posteriorly, but anteriorly, so as to cover the entire animal, with the exception of a narrow sternal surface (Pl. 30, fig. 4): in front it is notched, where the sternal surface terminates, and from this notch a faint line runs along the dorsal surface, separating its tergal elements. In the young Cirripede, after the metamorphosis, there is no trace of this medial dorsal suture, or of the wider sternal surface. Looking at the several genera of the Lepadidæ, the external covering of the whole peduncle and capitulum is so continuous and of so uniform a nature, that I think we must consider the whole as a carapace, of which the sternal borders have become completely confluent; formerly I was inclined to look at the capitulum alone as formed by the carapace, and at the peduncle as being composed of the two or three anterior cephalic segments, cased only by their own integuments. As far as can be discerned, the carapace in the pupa, and consequently in the Cirripede, consists only of the tergal elements of the segments; and this seems likewise to be the case with the carapace of the Podophthalmia. Until lately,[67] Prof. Milne Edwards doubted whether the carapace in the higher Crustaceans (to which I believe the carapace of Cirripedes must be compared) was formed by the backward production of the third segment, which bears the second pair of antennæ, or of the fourth, _i. e._ the mandibular segment; but from the distribution of the nerves, he now argues that it must mainly belong to the third segment. In Cirripedes, the course of the nerves leads to the same conclusion; for the whole shell, sack, and peduncle are supplied with nerves proceeding from the compounded ganglion, which belongs to the second and third cephalic segments.[68] [67] Compare 'Histoire Naturelle des Crustacés,' tom. i, p. 27, with 'Annales des Sciences Nat.,' 3d series, tom. xvi, 1851, p. 233. [68] This conclusion is supported by the structure of Proteolepas: in this Cirripede there is not a vestige of a carapace, and as the whole of the animal in front of the mouth is almost utterly aborted, being reduced to a mere covering to the two cement-ducts, and as, on the other hand, the mouth with the mandibles, though peculiarly modified, is not at all aborted, there is a strong probability, that the abortion of the carapace is connected with the aborted state of the three anterior cephalic segments; and that the carapace in its origin is not any way related to the fourth or mandibular segment.