The members of this order are small Crustaceans, which have a distinct head, and have the whole of the remainder of the body enclosed within a bivalve carapace, similar to that of the Ostracoda. The feet are few in number (usually four, five, or six pairs), and are mostly respiratory, carrying the branchiae. Two pairs of antennae are present, the larger pair being of large size, branched, and acting as natatory organs. The Cladocera quit the egg with the full number of limbs proper to the adult.

In the Daphnia pulex (fig. 145, b), or "branched-horned Water-flea," which occurs commonly in our ponds, the body is enclosed in a bivalve shell, which is not furnished with a hinge posteriorly, and which opens anteriorly for the protrusion of the feet. The head is distinct, not enclosed in the carapace, and carrying a single eye. The mouth is situated on the under surface of the head, and is provided with two mandibles and a pair of maxillae. The gills are in the form of plates, attached to the five pairs of thoracic legs. The males are very few in number, compared with the females, and a single congress is all that is required to fertilise the female for life. Not only is this the case, but the young females produced from the original fecundated female are able to bring forth young without having access to a male. Two kinds of eggs occur in Daphnia. In the first of these, or "summer eggs," the ova (from ten to fifty in number) are deposited in an open space between the valves, and are retained there until the young are ready to be hatched. In the second of these, or "winter eggs," which alone are fecundated, the ova (generally two in number) are placed in a peculiar receptacle, which is formed on the back of the carapace, and is called the "ephip-pium" or saddle. After a time the ephippium is cast off, and floats about till spring, when its contained eggs are hatched by the warmer temperature of the water.