The fourth order, the CLADOCERA, comprises the so-called "Water-fleas," which are abundant everywhere in ponds and lakes (Fig. 12). They are all of small size, almost or quite microscopic. The carapace, as in the Conchostraca, forms a bivalved shell, but does not enclose the head. There is a single large eye, which really corresponds to two eyes fused together. A pair of large antennæ, each with two branches, carrying long feathered hairs, project at the sides of the head, and are used in swimming with a peculiar jumping motion, from which the popular name of the animals is derived. There are not more than six pairs of feet. The "Water-fleas," of which _Daphnia pulex_ is one of the commonest species, are very beautiful and interesting objects for microscopic examination, on account of their transparency, which allows many details of their internal structure to be studied in the living animal. [Illustration: FIG. 13--SHELLS OF OSTRACODA. MUCH ENLARGED. (From Lankester's "Treatise on Zoology," after Brady and Norman, and Müller.) A, _Philomedes brenda_ (Myodocopa); B, _Cypris fuscata_ (Podocopa); C, _Cythereis ornata_ (Podocopa). _n_, Notch characteristic of the Myodocopa; _e_, the median eye; _a_, mark of attachment of the muscle connecting the two valves of the shell. A and C are marine species; B is from fresh water] The OSTRACODA (Fig. 13), which form the second subclass in the system of classification here adopted, are nearly all microscopic animals, and are found abundantly in fresh water as well as in the sea. The carapace forms a bivalved shell, which completely encloses the body and limbs, and is often sculptured in an elegant fashion. The Ostracoda are remarkable for the very small number of their appendages. There are not more than two pairs of limbs behind the maxilla. Most of the species are included in two orders, the _Myodocopa_ and the _Podocopa_, of which the former may generally be distinguished by a notch in the anterior part of the margin of the shell (Fig. 13, A, _n_). In the _Podocopa_ the margin is entire. [Illustration: FIG. 14--_Cyclops albidus_, A SPECIES OF COPEPOD FOUND IN FRESH WATER. (After Schmeil.) Female specimen carrying a pair of egg-packets. The actual length is about one tenth of an inch]