ARTHROPODA 255 
The Phyllopods are divided into two sub-orders: (a) the 
Cladocera and (0) the Branchiopoda. 
a. Cladocera.—The Cladocera or water-fleas are all small. 
Their body is laterally compressed, and their carapace takes the 
form of a bivalved shell, within which the larger part of the 
body lies concealed. A pair of large biramous antennae are used 
as swimming organs. 
The Cladocera include many species common in freshwater 
streams and inland lakes. Daphnia pulex 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 Daphnia 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, uniramous and small. 6. 1st thoracic swimming-feet, 
2. Antennae, biramous. 7. 2nd $3 5 
3. Mandibles. 4, Ist maxillae. 8. 3rd 3 a 
5. 2nd maxillae, disappear during 9. 4th 55 Fy 
larval life. 10. 5th By ts 
