A somewhat similar nutrient apparatus exists in the Polyphemidae, where the edges of the small carapace are fused with the thorax, so that the brood-pouch is completely closed, and the young can only escape when the parent casts her cuticle. In some genera of this family (e.g. _Evadne_) the young remain in the parental brood-pouch until they are themselves mature, so that when they are set free they may already bear parthenogenetic embryos in their own brood-pouches. [Illustration: FIG. 18.—Newly-cast ephippium of _Daphnia_, containing two winter-eggs. ] The winter-eggs are fertilised in the same part of the carapace of the female in which the parthenogenetic eggs develop, but after fertilisation they are thrown off from the body of the mother, either with or without a protective envelope formed from the cuticle of the carapace. The eggs of _Sida_ are surrounded by a thin layer of a sticky substance, and when cast out of the maternal carapace they adhere to foreign objects, such as water-weeds; those of _Polyphemus_ have a thick, gelatinous coat; in _Leptodora_ and _Bythotrephes_ the egg secretes a two-layered chitinous shell. In these forms the cuticle of the parent is not used as a protection for the winter-eggs, although it is generally, if not invariably, thrown off when the eggs are laid. In the Lynceidae the cuticle is moulted in such a way that the winter-eggs remain within it, at least for a time; the cuticle is occasionally modified before it is thrown off; thus in _Camptocercus macrurus_ the cuticle of the carapace, in the region of the brood-pouch, becomes thickened and darkly coloured, forming a fairly strong case round the eggs. The modification of the cuticle round the brood-pouch is much more pronounced in the Daphniidae, where it leads to the formation of a saddle-shaped cuticular box, the “ephippium,” in which the winter-eggs are enclosed. The ripening of a winter-egg in the ovary of a _Daphnia_ is accompanied by a great thickening of the cuticle of the carapace (_cf._ Fig. 18), so that a strong case is formed in the position of the brood-pouch. The winter-eggs are laid between the two valves of this case, and shortly afterwards the parent moults. The eggs are retained within the ephippium, from which the rest of the cuticle breaks away (Fig. 18). After separation, the ephippium, which contains a single egg (_Moina rectirostris_) or usually two (_Daphnia_, etc.), either sinks to the bottom, as in _Moina_, or floats.