i66 



THE LIFE OF CRUSTACEA 



the shell. 



often swarm in farmyard ponds where the water is 

 foul with decaying matter. In most gatherings from 

 such localities only female specimens will be found, 

 and nearly all of these will be seen to carry a cluster 

 of eggs or of developing embryos in the " brood- 

 chamber " between the back part of the body and 

 In Daphnia pulex (see Fig. 12, p. 37) a 

 single brood may consist of 

 thirty young, and occasionally 

 of more than twice that number. 

 As the broods may succeed each 

 other at intervals of two or 

 three days, it will be seen that 

 the multiplication of the species 

 in favourable circumstances may 

 be exceedingly rapid. It has 

 been calculated that in sixty 

 days the progeny of a single 

 female might amount to about 13,000,000,000. In 

 addition to these parthenogenetic eggs, which hatch 

 at once while still within the brood-chamber, the 

 Cladocera produce, at certain seasons, another kind of 

 egg which requires to be fertilized by the male before 

 it will develop. These eggs are dark in colour and 

 are enclosed in a thick shell, and they do not hatch 

 at once, but are cast off when the shell of the female 

 is moulted. Very commonly these " resting eggs," 

 as they are called, are produced in the autumn and 

 lie dormant until the following spring, and they can 



FIG. 56 Chydorus 

 spharicus, A COMMON 

 SPECIES OF WATER- 

 FLEA, x 50. (After 

 Lilljeborg.) 



