3l8 THE INVERTEBRATA 



powerful and mobile endites. The Gymnomera are predaceous 

 animals, and have slender, jointed, mobile trunk limbs with which 

 they seize their prey. 



The mouth parts of the Branchiopoda are small and simple in 

 structure, a condition in which they are not primitive but exhibit 

 reduction. 



That on the whole the group is a primitive one as compared with 

 the other classes of the Crustacea, is seen in the varying and usually 

 large number of somites, the usually small amount of differentiation 

 in the series of limbs on the trunk, the vascular system of the lower 

 members (p. 311), and the nervous system of all (p. 303). 



With the exception of a few marine cladocera and of Artemia 

 salina (p. 321), all branchiopoda are inhabitants of fresh waters. 

 Throughout the group, thick-shelled eggs capable of resisting 

 drought or freezing are produced by sexual reproduction. Often 

 there is also parthenogenesis, the eggs of which are usually thinner 

 shelled than those that are sexually produced (see p. 329). 



The first three orders of the Branchiopoda, as they are arranged 

 below, are sometimes collectively known as the Phyllopoda. 



Order ANOSTRACA 



Branchiopoda without carapace ; with stalked eyes ; with antennae of 

 a fair size but not biramous ; and with the trunk limbs numerous and 

 all alike. 



We may take as one example of this group, Chirocephalus dia- 

 phanus (Fig. 225), one of its two British representatives. This 

 creature turns up from time to time in temporary pools of water in 

 various districts. It is about half an inch in length, transparent, and 

 almost colourless, save for the reddened tips of most of the append- 

 ages and of the abdomen, the black eyes, and often a green mass of 

 algae in the gut. It is incessantly in motion, swimming on its back. 

 Its delicate appearance, and the iridescent gleaming of the bristles on 

 its appendages as they are moved have earned it the name of the 

 fairy shrimp. The body is long, subcylindrical, and enlarged ante- 

 riorly to form the head, upon which the mandibular groove (p. 296) is 

 conspicuous . The head has in front a median eye and a neck organ (p .307) , 

 and bears at the sides : {a) the large, stalked compound eyes ; {b) the anten- 

 «M/e5, slender, unjointed, and ending in a tuft of sense-hairs ; [c) the stout 

 antennae, triangular in the female but in the male (Fig. 226) elongate, 

 two-jointed and carrying on the inside at the base a complicated, 

 lobed "frontal appendage" which comes into play when the limb is 

 used for clasping the female; {d) the mandibles, whose bases are 

 prominent at the sides of the head, while the remaining part of each 



