324 THE INVERTEBRATA 



The genital opening is on the i ith of the trunk somites. Each of these 

 bears a pair of limbs until the 13th (second of the abdomen) is reached, 

 after which there are two to five pairs to a somite as far as the 28th somite. 

 Five limbless somites separate this from the telson. The first thoracic 

 limb is a modified phyllopodium, with the endites long and many- 

 jointed in Apus, though shorter in Lepidurus, and has perhaps taken 

 over the functions of the antennae. The second thoracic limb is less 

 modified in the same direction, the endites being shorter and unjointed : 

 it is used in clambering and in manipulating large food particles. The 

 remaining trunk limbs (Fig. 213) are normal phyllopodia: their 

 structure and function have been alluded to above (p. 317). They 

 decrease in size from before backwards. The limbs of the genital 

 somite are in the female modified for carrying eggs, the flabellum 

 fitting as a lid over a cup formed by the distal part of the axis. Males 

 are rare, reproduction being normally by parthenogenesis. 



Order CONCHOSTRACA 



Branchiopoda with a carapace in the form of a bivalve shell provided 

 with a hinge and adductor muscle, and usually en- 

 closing the head as well as the trunk ; the compound 

 eyes sessile and apposed; the antennae large and 

 biramous with numerous joints in the rami; and 

 ten to twenty-seven pairs of trunk limbs, of which ^\^' ^32- Esthena 

 *u r: * ^ j-rr r ^u 1 • i_ • rninuta, from the 



the nrst one or two diner from the rest in being Trias x 3 From 



prehensile. Woods. 



No member of this order is British. 



Estheria (Fig. 232) is a common European genus. A thoracic limb 

 of a related but exotic form is shown in Fig. 211 C. 



Order CLADOCERA 



Branchiopoda with a carapace in the form of a compressed shell which 

 usually covers the trunk but not the head ; the compound eyes sessile 

 and fused; the antennae large and biramous with few joints in the 

 rami; and six, five or four pairs of trunk limbs, some or all of 

 which may be highly differentiated from the normal type of the 

 Branchiopoda. 



The members of this order are the water fleas. They fall into two 

 sharply contrasting divisions. One, the Calyptomera, contains the 

 majority of the species, which have retained the ancestral habit of 

 straining out their food from the water and have perfected for that 

 purpose an apparatus in which the carapace as well as the trunk limbs 

 are involved ; and the other, the Gymnomera, contains a few species 

 of predaceous habit whose trunk limbs are modified for seizing prey 



