The compound lateral eyes fuse at an early period of embryonic life, so that they form a single median mass in the adult, over which a fold of ectoderm grows, to make a chamber over the eye, like that found in the Limnadiidae, except that it is completely closed. The fused eyes are generally large and conspicuous; in some deep-water forms the retinular elements of the dorsal portion are larger than those of the ventral (e.g. _Bythotrephes_, Fig. 13). In one or two species which live at very great depths, or in caves, the eyes are altogether absent. The appendages of the head are fairly uniform, the most variable being the first antennae. In the females of many genera the first antennae are short and immovable, consisting of a single joint, with a terminal bunch of sensory hairs, and often a long lateral hair, as in _Simocephalus_ (Figs. 9, 10), _Daphnia_, etc. In the female _Moina_ (Fig. 16) they are movable, as they are in _Ceriodaphnia_ and some others; in _Bosmina_ (Fig. 22) and many Lyncodaphniidae they are elongated and imperfectly divided into joints by rings of spines, while in _Macrothrix_ they are flattened plates. In the males the first antennae are elongated and mobile (_cf._ Figs. 11, 19). [Illustration: FIG. 9.—_Simocephalus vetulus_, female. Ventral view, without the carapace; _A_{1}_, _A_{2}_, first and second antennae; _For_, head; _Md_, mandible; _Te_, telson; I-IV, first to fourth thoracic appendages. ] The second antennae, the chief organs of locomotion, are biramous in all genera except _Holopedium_; the number of joints in each ramus, and the number of the long plumose hairs with which they are provided, are remarkably constant in whole series of genera, and are therefore useful for purposes of classification. The creatures row themselves by quick strokes of these appendages, the movement being slow and irregular in the rounder forms, such as _Simocephalus_ or _Daphnia_, rapid and well directed in such elongated lacustrine forms as _Bythotrephes_ or _Leptodora_. The mandibles have no palp; the first maxillae are very small, and the second maxillae are absent (Fig. 9).