The =carapace= varies very much. In most genera (the CALYPTOMERA of Sars) it is a large, backwardly-projecting fold of skin, bent downwards at the sides so as to form a bivalve shell, enclosing the whole post-cephalic portion of the body, as in _Simocephalus_ (Fig. 10). The eggs are laid into the space between the carapace and the dorsal part of the thorax, both the carapace and the thorax itself being often modified for their protection and nutrition. In a few forms, the GYMNOMERA of Sars, the carapace serves only as a brood-pouch, which is distended when eggs are laid, but collapses to an inconspicuous appendage at the back of the head when it is empty (e.g. _Leptodora_, Fig. 24, _Bythotrephes_, Fig. 13). In the Calyptomera the surface of the carapace is frequently provided with a series of ridges, which may be parallel, rarely branching, as in _Simocephalus_; or in two sets which cross nearly at right angles, as in _Daphnia_; or so arranged as to form a hexagonal pattern, as in _Ceriodaphnia_. In a few forms the whole surface is irregularly covered with spines or scales. The hinder edge of the carapace is often produced into a median dorsal spine (_Daphnia_, Fig. 19), or more rarely there are two spines, one at each ventro-lateral corner (_Scapholeberis_, Fig. 20). [Illustration: FIG. 10.—_Simocephalus vetulus_, × 30. Side view of female, showing the arrangement of the principal organs. _A.2_, Second antenna; _C.S_, cervical suture; _E_, fused compound eyes; _H_, heart; _L_, forwardly-directed gastric caeca; _N_, dorsal organ. ] The cuticle of the carapace is often separated from that of the head by a cervical suture, as in _Simocephalus_ (Fig. 10, _C.S._) and near the line of demarcation many forms exhibit patches of glandular ectoderm which seem to be homologous with the dorsal adhesive organs of the Limnadiidae. The commonest condition is that of a median dorsal pit (Fig. 10, _N._) by means of which the animal can fix itself to foreign objects. Certain forms may remain for long periods of time attached by the dorsal organ to plants, or to the sides of an aquarium, the only movement being a slow vibration of the feet, by which a current of water, sufficiently rapid for respiratory purposes, is established round it.[31] In _Sida crystallina_ (Fig. 11) the dorsal organ is represented by three structures; in front there is a median raised patch (_N.m_) of columnar ectoderm, containing concretions like those described in the Branchipodidae, and behind this is a pair of cup-shaped organs (_N.e_), with raised margins. [Illustration: FIG. 11.—_Sida crystallina_, male, × 27. Oxford. _A.1_, Elongated first antenna; _N.e_, paired element of dorsal organ; _N.m_, median element of dorsal organ; _Te_, testes; ♂, opening of vas deferens. ]