Sub-Order 2. Cladocera.
The Cladocera are short-bodied Branchiopods, with not more than six pairs of thoracic limbs. The second antennae are important organs of locomotion, and are nearly always biramous; the first antennae are small, at least in the female; the second maxillae are absent in the adult. The carapace may extend backwards so as to enclose the whole post-cephalic portion of the body, or may be reduced to a small dorsal brood-pouch, leaving the body uncovered.
The Cladocera or “Water-fleas” are never of great size; Leptodora hyalina, the largest, is only about 15 mm. long, while many Lynceidae are not more than 0·1 or 0·2 mm. in length.
The head is bent downwards in all the Cladocera, so that parts which are morphologically anterior, such as the median eye and the first antennae, lie ventral to or even behind the compound eyes and the second antennae (cf. Fig. [10]).
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).
Fig. [9].—Simocephalus vetulus, female. Ventral view, without the carapace; A1, A2, 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]).