CHAP. 11 ENTOxMOSTRACA BRANCHIOPODA 1 9 



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 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 

 tlieir 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 ; tlie larger kinds, 

 however, are capable of gnawing objects of considerable size, jlpus 

 being said to nibble the softer insect larvae, and even tadpoles. 

 Many Cladocera (e.g. Daphnia, Simocej)halus) may he 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 lons;-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 a}-pearance, 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 Bhanchipodidae 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 su^jported on mobile stalks, and project freely, one on 

 either side of the head. 



In the Apodidae ^ the head is broad and depressed, the ventral 

 ^ Bernard, " The Apodidae," Auticre Series, 1892. 



