PHYLUM ARTHROPODA 211 



withstanding desiccation. In fact, the eggs of some forms refuse 

 to develop if they are returned to water immediately and 

 develop normally only after being dried out. From the egg is 

 derived a larva in the nauplius or metanauplius stage. Many 

 phyllopods swim upon the back with the ventral surface upper- 

 most. This is especially characteristic (Fig. 97) of Eubranchipus. 

 Eubranchipus and the brine shrimp Artemia are examples of 

 the suborder Anostraca, the members of which have no carapace 

 and bear stalked eyes. The members of suborder Notostraca, 

 of which Apus and Lepidurus are examples, include branchiopods 

 with a shield-shaped carapace covering part of the trunk and 

 with sessile eyes. In the suborder Conchostraca are grouped 



Fig. 97. — Eubranchipus vernalis in normal position for swimming. {After 

 Packard, from Kingsley's Hertwig, with permission of Henry Holt and Co.). 



forms which have a bivalve carapace enclosing the entire animal. 

 Estheria, Limnadia, and Limnetus, with their strikingly mollus- 

 can appearances, are examples of this suborder. 



II. Order Cladocera 



The Cladocera are small phyllopods, rarely more than 3 mm. 

 in length, with a distinct head and usually with a carapace cover- 

 ing the trunk and legs. They are commonly called water fleas. 

 Daphnia is a characteristic genus. In a few instances (Leptodora 

 and Polyphemus), the carapace is greatly reduced and serves only 

 as a brood sac, leaving the trunk and legs entirely free. 



The head holds a median compound eye which has usually 

 resulted from the fusion of two lateral eyes and is capable of 

 rotation within a capsule. The antennules are sensory but the 

 second antennae are modified as swimming organs and constitute 

 the chief organs for locomotion. In addition, the head bears a 

 pair of mandibles and a pair of greatly reduced maxillae. 



A dorsal heart, just back of the head, is the only circulatory 

 organ, for vessels are wanting. An ostium on each side of the 



