Figs. 53 and 54. Nauplii of Copepoda, the former magnified, the latter magnified 2x. Fig. 55. Nauplius of Tetraclita porosa after the first moult, magnified 90 diam. The brain is seen surrounding the eye, and from it the olfactory filaments issue; behind it are some delicate muscles passing to the buccal hood. Fig. 53 and 54. Nauplii of Copepoda, the former magnified, the latter magnified 2x. Fig. 55. Nauplius of _Tetraclita porosa_ after the first moult, magnified. The brain is seen surrounding the eye, and from it the olfactory filaments issue; behind it are some delicate muscles passing to the buccal hood.
There is only one thing more to be indicated in the developmental history of the parasitic Crustacea, namely that some of them, such as _Achtheres percarum,_ certainly quit the egg like the rest in a Nauplius-like form, inasmuch as the plump, oval, astomatous body bears two pairs of simple rowing feet, and behind these, as traces of the third pair, two inflations furnished each with a long seta, but that beneath this Nauplius-skin a very different larva lies ready prepared, which in a few hours bursts its clumsy envelope and then makes its appearance in a form “which agrees in the segmentation of the body and in the development of the extremities with the first _Cyclops-stage_” (Claus). The entire series of Nauplius-stages which are passed through by the free Copepoda, are in this case completely over-leapt.
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.