i66 THE LIFE OF CRUSTACEA 



often swarm in farmyard ponds where the water is 

 foul with decaying matter. In most gatherings from 

 such locahties 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 

 the shell. 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 



Fig. 56 — Chydonis in favourable circumstances may 

 st>hcBrictis, a Common , i- ^ • ^ x, 1 



Species of Water- be exceedmgly rapid. It has 



Lmfeborg.f * ^^^'^' 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 



