A final and very peculiar section of the Crustacea is formed by the two orders of the Cirripedia and Rhizocephala.[[4]]
In these also the brood bursts out in the Nauplius-form, and speedily strips off its earliest larva-skin which is distinguished by no peculiarities worth noticing. Here also we find again the same pyriform shape of the unsegmented body, the same number and structure of the feet, the same position of the median eye (which, however, is wanting in Sacculina purpurea, and according to Darwin in some species of Lepas), and the same position of the “buccal hood,” as in the Nauplii of the Prawns and Copepoda. From the latter the Nauplii of the Cirripedia and Rhizocephala are distinguished by the possession of a dorsal shield or carapace, which sometimes (Sacculina purpurea) projects far beyond the body all round; and they are distinguished not only from other Nauplii, but as far as I know from all other Crustacea, by the circumstance that structures which are elsewhere combined with the two anterior limbs (antennæ), here occur separated from them.
The anterior antennæ of the Copepoda, Cladocera, Phyllopoda (Leydig, Claus), Ostracoda (at least the Cypridinæ), Diastylidæ, Edriophthalma, and Podophthalma, with few exceptions relating to terrestrial animals or parasites, bear peculiar filaments which I have already repeatedly mentioned as “olfactory filaments.” A pair of similar filaments spring, in the larvæ of the Cirripedia and Rhizocephala, directly from the brain.
Fig. 56. Nauplius of Sacculina purpurea, shortly before the second moult, magnified. We may recognise in the first pair of feet the future adherent feet, and in the abdomen six pairs of natatory feet with long setæ.
Fig. 57. Pupa of a Balanide (Chthamalus ?), magnified. The adherent feet are retracted within the rather opaque anterior part of the shell.
Fig. 58. Pupa of Sacculina purpurea, magnified. The filaments on the adherent feet may be the commencements of the future roots.
At the base of the inferior antennæ in the Decapoda the so-called “green-gland” has its opening; in the Macrura at the end of a conical process. A similar conical process with an efferent duct traversing it is very striking in most of the Amphipoda. In the Ostracoda, Zenker describes a gland situated in the base of the inferior antennæ, and opening at the extremity of an extraordinarily long “spine.” In the Nauplii of Cyclops and Cyclopsine, Claus finds pale “shell-glands,” which commence in the intermediate pair of limbs (the posterior antennæ). On the other hand in the Nauplii of the Cirripedia and Rhizocephala the “shell-glands” open at the ends of conical processes, sometimes of most remarkable length, which spring from the angles of the broad frontal margin, and have been interpreted sometimes as antennæ (Burmeister, Darwin) and sometimes as mere “horns of the carapace” (Krohn). The connexion of the “shell-glands” with the frontal horns has been recognised unmistakably in the larvæ of Lepas, and indeed the resemblance of the frontal horns with the conical processes on the inferior antennæ of the Amphipoda, is complete throughout.[[5]]
Notwithstanding their agreement in this important peculiarity, the Nauplii of these two orders present material differences in many other particulars. The abdomen of the young Cirripede is produced beneath the anus into a long tail-like appendage which is furcate at the extremity, and over the anus there is a second long, spine-like process; the abdomen in the Rhizocephala terminates in two short points,—in a “moveable caudal fork, as in the Rotatoria,” (O. Schmidt). The young Cirripedes have a mouth, stomach, intestine, and anus, and their two posterior pairs of limbs are beset with multifarious teeth, setæ, and hooks, which certainly assist in the inception of nourishment. All this is wanting in the young Rhizocephala. The Nauplii of the Cirripedia have to undergo several moults whilst in that form; the Nauplii of the Rhizocephala, being astomatous, cannot of course live long as Nauplii, and in the course of only a few days they become transformed into equally astomatous “pupæ,” as Darwin calls them.
The carapace folds itself together, so that the little animal acquires the aspect of a bivalve shell, the foremost limbs become transformed into very peculiar adherent feet (“prehensile antennæ,” Darwin), and the two following pairs are cast off; like the frontal horns. On the abdomen six pairs of powerful biramose natatory feet with long setæ have been formed beneath the Nauplius-skin, and behind these are two short, setigerous caudal appendages (Fig. 58).
The pupæ of the Cirripedia (Fig. 57), which are likewise astomatous, agree completely in all these parts with those of the Rhizocephala, even to the minutest details of the segmentation and bristling of the natatory feet;[[6]] they are especially distinguished from them by the possession of a pair of composite eyes. Sometimes also traces of the frontal horns seem to persist.[[7]]
As the Cirripedia and Rhizocephala now in general resemble each other far more than in their Nauplius-state, this is also the case with the individual members of each of the two orders.