The smaller Branchiopoda known as "Water-fleas," forming the order Cladocera, are abundant everywhere in fresh water. _Daphnia pulex_ and other species of the genus, and the little Lynceidæ, of which _Chydorus sphæricus_ (Fig. 56) is the commonest species, are to be found in ponds and ditches, and 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 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 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 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 carapace is most highly developed in the family Daphniidæ (Fig. 57), where a saddle-shaped area on the dorsal side, known as the "ephippium," becomes thickened, and on moulting separates from the rest of the carapace to form a compact case enclosing the two resting eggs. The outer wall of the ephippium 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 entangled 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. [Illustration: FIG. 57--A WATER-FLEA, (_Daphnia pulex_), FEMALE, WITH EPHIPPIUM CONTAINING TWO "RESTING EGGS." × 20. (Partly after Lilljeborg.) The Antenna is cut short. Compare Fig. 12, p. 37.]