48 



CRUSTACEA BRANCHIOPODA 



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 Dcqjhnia 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 be- 

 tween the two valves of 

 this case, and shortly 

 afterwards the parent 

 The eggs are 



FlQ. 18. — Newly-cast epliippiuni of Daphnia, 

 contaiuing two winter-eggs. 



moults, 

 retained within the 

 ephippium, from which the rest of the cuticle breaks away (Fig. 

 18). After separation, the ephippium, which contains a single 

 egg (Ifoina rectirostris) or usually two (Daphnia, etc.), either 

 sinks to the bottom, as in Moina, or floats. 



The winter - eggs usually go through the early stages of 

 segmentation within a short time after they are laid, but after 

 this a longer or shorter period of quiescence occurs, during 

 which the eggs may be dried or frozen without injury. The 

 sides and floor of a dried -up pond are often crowded with- 

 ephippia, containing winter -eggs which develop quickly when 

 replaced in water ; and the resting-stage of winter-eggs pro- 

 duced in aquaria can often be materially shortened by drying 

 the ephippia which contain them, though such desiccation 

 does not appear to be necessary for development. Under 

 normal conditions large numbers of winter - eggs remain 

 quiescent through the winter and hatch in the foUowino- 

 spring. 



The individual developed from a sexually fertilised winter- 



