In this order we meet for the first time with an extensive retrograde metamorphosis as a consequence of a parasitic mode of life. Even in some Fish-lice (_Cymothoa_) the young are lively swimmers, and the adults stiff, stupid, heavy fellows, whose short clinging feet are capable of but little movement. In the Bopyridæ (_Bopyrus, Phryxus, Kepone,_ etc., which might have been conveniently left in a single genus), which are parasitic on Crabs, Lobsters, etc., taking up their abode chiefly in the branchial cavity, the adult females are usually quite destitute of eyes; the antennæ are rudimentary; the broad body is frequently unsymmetrically developed in consequence of the confined space; its segments are more or less amalgamated with each other; the feet are stunted, and the appendages of the abdomen transformed from natatory feet with long setæ into foliaceous or tongue-shaped and sometimes ramified branchiæ. In the dwarfish males the eyes, antennæ, and feet, are usually better preserved than in the females; but on the other hand all the appendages of the abdomen have not unfrequently disappeared, and sometimes every trace of segmentation. In the females of _ Entoniscus,_ which are found in the body-cavity of Crabs and _ Porcellanæ,_ the eyes, antennæ, and buccal organs, the segmentation of the vermiform body, and in one species (Fig. 41) the whole of the limbs, disappear almost without leaving a trace; and _Cryptoniscus planarioides_ would almost be regarded as a Flatworm rather than an Isopod, if its eggs and young did not betray its Crustacean nature. Among the males of these various Bopyridæ, that of _Entoniscus Porcellanæ_ occupies the lowest place; it is confined all its life to six pairs of feet, which are reduced to shapeless rounded lumps. The Amphipoda are distinguishable from the Isopoda at an early period in the egg by the different position of the embryo, the hinder extremity of which is bent downwards. In all the animals of this order which have been examined for it,[2] a peculiar structure makes its appearance very early on the anterior part of the back, by which the embryo is attached to the “inner egg-membrane,” and which has been called the “micropylar apparatus,” but improperly as it seems to me.[3] It will remind us of the union of the young Isopoda with the larval membrane and of the unpaired “adherent organ” on the nape of the Cladocera, which is remarkably developed in _Evadne_ and persists throughout life; but in _Daphnia pulex,_ according to Leydig, although present in the young animals, disappears without leaving a trace in the adults.