CHAP. II ENTOMOSTRACA—BRANCHIOPODA 19 
smallest pools te 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 swift- 
ness 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 down- 
wards, 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 
nay 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. 
In the AropipAE’ the head is broad and depressed, the ventral 
1 Bernard, ‘“‘ The Apodidae,” Nature Series, 1892. 
