The Branchiopods are of small or moderate size, with flattened and lobate post-cephalic limbs, and with functional gnathobases. Median and lateral eyes are nearly always present. The labrum is large, and the second maxillae are small or absent in the adult. Branchiopods are found in every part of the world; a few are marine, but the great majority are confined to inland lakes and ponds, or to slowly-moving streams. The fresh waters, from the smallest pools to the largest lakes, often swarm with them, as do those streams which flow so slowly that the creatures can obtain occasional shelter among vegetation along the sides and bottom without being swept away, while even rivers of considerable swiftness contain some Cladocera. Several Branchiopods are found in the brackish waters of estuaries, and some occur in lakes and pools so salt that no other Crustacea, and few other animals of any kind, can live in them. The great majority swim about with the back downwards, collecting food in the ventral groove between their post-oral limbs, and driving it forwards, towards the mouth, by movements of the gnathobases (p. 10). The food collected in this way consists largely of suspended organic mud, together with Diatoms and other Algae, and Infusoria; the larger kinds, however, are capable of gnawing objects of considerable size, _Apus_ being said to nibble the softer insect larvae, and even tadpoles. Many Cladocera (e.g. _Daphnia_, _Simocephalus_) may be seen to sink to the bottom of an aquarium, with the ventral surface downwards, and to collect mud, or even to devour the dead bodies of their fellows, while _Leptodora_ is said to feed upon living Copepods, which it catches by means of its antennae. The Branchiopoda fall naturally into two Sub-orders, the PHYLLOPODA including a series of long-bodied forms, with at least ten pairs of post-cephalic limbs, and the CLADOCERA with shorter bodies and not more than six pairs of post-cephalic limbs. =Sub-Order 1. Phyllopoda.= The Phyllopoda include a series of genera which differ greatly in appearance, owing to differences in the development of the carapace, which are curiously correlated with differences in the position of the eyes. Except in these points, the three families which the sub-order contains are so much alike that they may conveniently be described together. In the BRANCHIPODIDAE the carapace is practically absent, being represented only by the slight backward projection on each side of the head which contains the kidney (Fig. 2); the paired eyes are supported on mobile stalks, and project freely, one on either side of the head.