ARTHROPODA 255 



The Phyllopods are divided into two sub-orders : (a) the 

 Cladocera and (&) the Branchiopoda. 



a. Cladocera. — The, Cladocera or water -fleas are all small. 

 Their hody is laterally compressed, and their carapace takes the 

 form of a hivalved shell, within which the larger part of the 

 hody lies concealed. A pair of large Mramous antennae are used 

 as swimming organs. 



The Cladocera include many species common in freshwater 

 streams and inland lakes. Daphniapulex and Daphnia longispina 

 occur frequently in ditches and ponds in England, and although 

 they are of minute size, 4 or 5 mm. in length, they form a 

 convenient type of this sub-order. The sexes differ both in 

 size and structure, and it will be convenient to describe the 

 female first, and afterwards to mention those points of differ- 

 ence which the male presents. 



As is usual in Crustacea, the body of Baphnia is divisible 

 into three regions : the head, the thorax, and the abdomen. 

 The large bivalve shell which encloses the body like the 

 valves of a Lamellibranch, is an extension of the dorsal surface 

 of the cephalic segments. The head is provided with a pair 

 of antennules,. uniramous, very small, and bearing olfactory 

 hairs ; a pair of antennae which are biramous and very long 

 and are used for swimming; a pair of mandibles; and one 

 pair of maxillae. The larva has two pairs of maxillae, 

 but the second pair disappear before the adult stage is 

 reached. The thorax consists of five segments, which are free 

 from the shell. Each segment bears a pair of lamelliform 

 swimming -feet. The abdomen is three-jointed, and carries no 

 appendages ; it is curved forwards ventrally, and terminates 

 in an unsegmented post -abdomen or telson. The abdomen 

 bears dorsally several processes which assist in enclosing the 

 brood pouch, a space left between the dorsal side of the thorax 

 and abdomen and the shell. The post-abdomen on which the 

 anus opens bears two long dorsal tactile setae, and ends in 

 two hooks or styles. 



Appendages of Daphnia. 



1. Antennules, nniramous and small. 6. 1st thoracic swimming-feet, 



2. Antennae, biramous. 7. 2nd „ ,, 



Mandibles. 4. 1st maxillae. 8. 3rd 



5. 2nd maxillae, disappear during 9. 4tli 



larval life. 10. 5tli 



