CRUSTACEA OF FRESH WATERS 167 



survive drying or freezing without injury, while the 

 thin-shelled parthenogenetic eggs within the brood- 

 chamber of the mother are easily killed. In addition 

 to having thick shells, the resting eggs are further 

 protected in most, but not in all, cases by the 

 moulted carapace of the parent, which is specially 

 thickened for the purpose. 

 This modification of the cara- 

 pace is most highly developed 

 in the family Daphniidse 

 (Fig. 57), where a saddle- 

 shaped area on the dorsal 

 side, known as the " ephip- 

 pium," becomes thickened, 

 and on moulting separates 

 from the rest of the carapace 

 to form a compact case en- 

 closing the two resting eggs. 

 The outer wall of the ephip- 

 pium is divided up into small 

 hexagonal cells, which become filled with air, causing 

 the ephippium to float at the surface of the water. 

 In this position the ephippia readily become en- 

 tangled in the feathers of birds, and in some cases 

 the shell is provided with spines or hooks, which 

 facilitate transport to other localities by such means. 

 The appearance of males and the production of 

 ephippial eggs — in other words, the ** sexual period" 

 — is generally more or less restricted to one season 



Fig. 57 — A Water-flea, 

 {Daphnia pulex), Female, 

 WITH Ephippium con- 

 taining Two " Resting 

 Eggs." x 20. (Partly 

 after Lilljeborg.) 



The Antenna is cut short. 

 Compare Fig. 12, p. 37. 



