THE OSTRACODA 57 



globular or, in some Halocypridae, greatly elongated. The two 

 valves are probably always more or less unsymmetrical. especially 

 along the ventral margin. On the dorsal side they are connected 

 by a hinge-joint, which may be merely an uncalcified strip of 

 the integument (Halocypridae), or may be strengthened by inter- 

 locking ridges and teeth. The most complex hinges are found 

 among the Cytheridae. The almost globular shell of Gigantocypris 

 is exceptional in that the free edges of the valves occupy only 

 about one-third of the circumference. 



The outer surface of the valves is seldom quite smooth. It 

 may be beset with setae or pitted or sculptured (Fig. 33, C), and 

 is sometimes produced into wing-like processes. When the shell 

 is strongly calcified and opaque more transparent spots may mark 

 the position of the eyes (Podocopa, Fig. 33, e). The attachment 

 of the adductor muscle is usually visible externally as a group of 



e. 



FlO. 33. 



Lateral view of shell of A, PhUomedes brenda (Myodocopa), x 8 ; B, Cypris fuscxtin (Podocopa), 

 x 19 ; C, Cythereis ornata (Podocopa), x 33. a, attachment of adductor muscles ; , median 

 eye ; n, antennal notch. (A after Brady and Norman ; B and C after G. W. Muller.) 



spots, the arrangement of which affords characters of systematic 

 importance (a). When the edges of the valves are brought 

 together they usually fit closely, but in some cases openings are 

 left. The most important of these is the " antennal notch " found 

 in most Myodocopa, and permitting the protrusion of the antennae 

 (Fig. 33, A, n). Unicellular glands opening by pores on the 

 surface of the shell are frequent in Myodocopa, less so in Podocopa. 



The fold which marks off the shell from the body on each side 

 does not extend very far towards the dorsal surface, and certain of 

 the viscera may extend, as in some Cirripedia, into the cavity 

 between the outer and inner integument of each valve. This is 

 the case with the hepatic caeca in some Cyprididae (Pontocyprinae), 

 and more commonly with the reproductive organs, especially the 

 ovaries, which in the Cyprididae are completely and in the 

 Cytheridae partly lodged in the cavity of the shell-fold. In the 

 Cypridinidae a network of blood-channels traverses each valve, 

 radiating outwards from the muscle-impression. 



The posterior part of the body, which is free within the shell, 



