684 FRESH- WATER BIOLOGY 



are so abundant in the Copepoda. The young are nourished in the 

 brood-cavity, not only by the yolk of the egg, but also by a secretion 

 from the dorsal wall of the sac. The brood case may be closed 

 behind by extensions of the body the abdominal processes - 

 which have some systematic importance. This parthenogenesis 

 goes on regularly through the favorable season for growth, closing 

 when the pools begin to dry or other unfavorable conditions arise. 

 Several successive broods of females are ordinarily produced in this 

 way, although in Moina, which lives in temporary pools, the second 

 generation may be sexual. Sooner or later true females and males 

 are hatched from the eggs. These females produce one or two eggs, 

 large and opaque, with abundant yolk and thick shell, and which 

 must be fertilized by the male before developing. These eggs pass 

 into the brood sac, whose walls have usually acquired a peculiar 

 structure. In the Chydorinae (Fig. 1159) they are merely thick- 

 ened and darkened. In the Daphnidae (Figs. 1073, I0 79> I0 93)> 

 a semi-elliptical portion of the dorsal region of each valve becomes 

 greatly altered to form the ephippium, so called from its resem- 

 blance to a saddle. In the Chydorinae the sexual egg is deposited 

 in the brood sac and the whole shell is then molted ; the egg remain- 

 ing enclosed in it. Where the ephippium is developed, this separates 

 during the molt from the rest of the shell and closes about the one 

 or two eggs deposited in it. In either case the eggs lie over to the 

 next favorable season before they develop. 



This process of sexual reproduction occurs at different times in 

 various species. Like the blossoming of flowers, it cannot always 

 be directly correlated with any definite conditions of food or tem- 

 perature. In those species which live in the open waters of lakes, 

 sexual reproduction is often greatly reduced or wholly absent and 

 the species is carried on from year to year by asexual generations. 

 In many species the males are very rarely seen and in none are they 

 abundant. 



The males are smaller than the females and usually of similar form. 

 They are distinguished by larger antennules; the post-abdomen is 

 usually somewhat modified (Fig. 1144); the first foot is frequently 

 armed with a stout hook which serves to clasp the females. In Moina 

 this function is performed by the very large antennules (Fig. 1092). 



