CRUSTACEA. OSTRACODA 313 



Distribution of the Branchiopoda 



The Branchiopoda live mainly in fresh water, but some 

 are found in the sea, in salt lakes, and in brackish water. 

 Only a very few genera are found fossil ; Protocaris, which 

 resembles Apus, occurs in the Lower Cambrian of North 

 America. In the Upper Palaeozoic and later deposits a 

 few genera are present, Estheria being common. Apus 

 ranges from the Trias to the present day. 



SUB-CLASS III. OSTRACODA 



The Ostracods (fig. 135) are indistinctly segmented 

 and generally of minute size. The body is usually com- 

 pressed laterally, and is completely enclosed in a bivalved 

 carapace, which may be horny or calcareous. One valve 



2 — 



7 



Fig. 135. Lateral view of Cypris Candida. (After Zenker.) 1, antennules; 

 2, antenna? ; 3, mandibles ; 4, first maxillae ; 5, second maxillae ; 

 6, 7, first and second pairs of legs ; 8, tail ; 9, eye. Enlarged. 



is placed on each side of the animal, and the two valves 

 are joined together dorsally by an elastic ligament which 

 serves to open the shell ; sometimes a hinge is formed 

 by means of interlocking teeth and ridges ; an adductor 

 muscle passes from the interior of one valve to the other 

 and by its contraction the shell is closed; usually the 

 muscular impression can be seen from the outside. There 



