Both the first and second pairs of antennæ are absent in the mature animal; for the three terminal segments of the antennæ of the pupa, which may always be found cemented under the centre of the surface of attachment, are functionless, after maturity. The eyes are rudimentary, and are singular from being seated far from the anterior extremity of the head. In their rudimentary state, and in the absence of antennæ, we have characters common with certain Suctorial Entomostracans; and this similarity apparently arises from the fixed condition of the animals of both groups. The carapace, which covers the dorsal surface of the larva in the first stage, in the last larval or pupal stage is developed so as to enclose, like a bivalve shell, the whole body. In the mature Cirripede, the whole external covering, whether shell and operculum, or capitulum and peduncle, can be conclusively shown to be the carapace of the pupa, modified. In thus enclosing the mouth and whole body, the modified carapace resembles that of several Entomostracans; but in being apparently formed (as I hope hereafter to show) by the development of the third segment of the head, and in consisting generally of distinct sclerodermic plates, arranged in an imbricated order, there is, I think, a closer resemblance to the same part in some of the Podophthalmia. The carapace, or portions of the carapace, being capable of other movements, besides merely opening and shutting, differs, I believe, from that of all other Crustaceans; as it likewise does[10] in the greater part not being periodically moulted. [10] The carapace, however, of the Isaura, a Branchiopod, according to M. Joly ('Annales des Sc. Nat.,' 2 ser. vol. xvii, p. 293), is not moulted. Moreover, in all Cirripedes there is another striking peculiarity connected with these parts, namely, the exclusive attachment of the whole thorax or included body to the internal ventral or sternal surface of the carapace and head. In the pupa, the thorax, as in all Crustaceans, opens into, and is continuously united with, the large anterior part of the head; but from the singular fact that the thorax of the young Cirripede is developed not within the thorax, but within the head of the pupa (Pl. 30, fig. 2), with its longitudinal axis placed at right angles to that which it held in the pupal condition (the mouth and the whole exterior being developed conformably with that of the pupa), it comes to pass after the metamorphosis, that the Cirripede is, as it were, internally cut in twain (compare Pl. 25, fig. 1, and Pl. 30, figs. 2 and 3). Thus it is, as will hereafter be more fully explained, that the sack originates, and thus the body becomes attached to the internal ventral surface of the carapace and front of head.