The fold of skin which forms the carapace contains the coils of the single pair of kidneys, and it forms an important organ of respiration, partly from the great size of the blood-vessels it contains, and partly from the presence of red, blue, or brown respiratory pigments in the tissue of the skin itself. In most Cladocera the cuticle of the carapace is cast at every ecdysis, with that of other parts of the body; but in _Iliocryptus_ and a few others it remains after each moult, giving the carapace an appearance of “lines of growth,” like that seen in many Limnadiidae. The segmentation of the =body= behind the head is obscure, but we can generally recognise (1) a thorax, of as many segments as there are pairs of limbs; (2) an abdomen of three segments; and (3) a telson. The thoracic limbs of the Calyptomera are flattened, and resemble those of the Phyllopoda; as a type we may examine the third thoracic limb of _Simocephalus_ (Fig. 12, C), in which the axis bears a large setose gnathobase (_Gn_) on its inner edge, followed by two small endites; the terminal process, or exopodite (_Ex_) is a large flattened plate, with six long plumose hairs on its edge. The outer margin of the axis bears a bract (_Br_) and an epipodite. In _Simocephalus_, as in the other Daphniidae, there are five pairs of thoracic limbs, of which the third and fourth are alike; in the female each limb of the first pair consists of a jointed axis, with strong biting hairs on the inner border, and a rudimentary epipodite (Fig. 12, A), the second limb being more like the third, but with a more prominent gnathobase and a narrower exopodite (B), while the limbs of the fifth pair have the gnathobase and the exopodite filamentous (D). In the Sididae there are six pairs of thoracic limbs, which are nearly alike in the female; in the Bosminidae there are six pairs, the first two modified for prehension, the last much reduced. [Illustration: FIG. 12.—Thoracic limbs of female _Simocephalus vetulus_. =A=, The first; =B=, the second; =C=, the third; =D=, the fifth. _Br_, Bract; _Ep_, epipodite; _Ex_, exopodite; _Gn_, gnathobase. ] In the male, the first thoracic limb is usually provided with a long sensory process and a prehensible hook (Figs. 11, 19). In the Gymnomera the limbs are cylindrical, jointed rods, with a gnathobase on the inner side in the Polyphemidae, but not in _Leptodora_. The number varies from four to six pairs.