PHYLUM ARTHROPODA 



557 



There is considerable diversity of form among the Branchiopoda. 

 Apus has already been described. Branchipus (Fig. 455, 1) and 

 Artemia (the Brine-shrimp) (Anostraca) are small shrimp-like 

 forms, the former living in fresh-water lakes, the latter in brine- 

 pools ; they have no carapace, and the eyes are raised on un jointed 

 stalks. In Limnetis (2), on the other hand, and in Estheria (3) 

 the carapace takes the form of a shell, formed of two parts 

 or valves, united in Estheria by a hinge, and resembling the 

 shell of a cockle or other bivalved mollusc. The limbs have the 

 same general structure as those of Apus, but the antennae are 

 often of considerable size, and are sometimes modified into 

 prehensile organs. 



ant.1 



ard.2 



ant.2 



3. Estheria 



FIG. 455. Three Branchiopoda. In 3, a is the shell ; b the animal with one valve of the 

 shell removed, ant 1 , antennule ; ant 2 , antenna; ht. heart; m. adductor muscle ; md. 

 mandible ; ov. ovary ; a. unpaired process from head ; p. copulatory appendages ; sh.gl. 

 shell-gland ; t. testis. (After Gerstaecker.) 



In the Cladocera, of which the common fresh-water Daphnia 

 (Fig. 456, 1) is a good example, there is a great reduction in size 

 (1-2 mm.), and a corresponding shortening of the body by a 

 reduction in the number of metameres. Segmentation is very 

 imperfect, and the whole body, but not the head, is covered 

 by a large, folded carapace. The abdomen, is turned downwards 

 and is in constant movement, sweeping out any foreign particles 

 which may have made their way among the feet. Between the 

 abdomen of the female and the posterior part of the carapace 

 is a large brood-pouch (br.p.), in which the eggs are stored. The 



