WATER-FLEA, a name given by the earlier microscopists (Swammerdam, 1669) to minute aquatic Crustacea (q.v.) of the order of Cladocera, but applied also to the smaller members of other groups. The Cladocera are abundant everywhere in fresh water. One of the commonest species, Daphnia pulex, found in ponds and ditches, is less than one-tenth of an inch in length and has the body enclosed in a transparent bivalved shell. The head, projecting in front of the shell, bears a pair of branched feathery antennae which are the chief swimming organs and propel the animal, in a succession of rapid bounds, through the water. There is a single large black eye. In the living animal five pairs of leaf like limbs acting as gills can be observed in constant motion be tween the valves of the shell, and the pulsating heart may be seen near the dorsal surface, a little way behind the head. The body
ends behind in a kind of tail with a double curved claw which can be protruded from the shell. The female carries the eggs in a brood-chamber between the back of the body and the shell until hatching takes place. Throughout the greater part of the year only females occur and the eggs develop "parthenogenetically," without fertilization. When the small males appear, generally in the autumn, fertilized "winter" or "resting eggs" are produced which are cast adrift in a case or "ephippium" formed by a spe cially modified part of the shell. These resting eggs enable the race to survive the winter or drying of the water. (W. T. C.)