THE OSTRACODA 



57 



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

 valves are probably always more or less iinsymnietrical, especially 

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

 by a hinge-joint, which may be merely an uncalcificd 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 (luitc smooth. It 

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

 is sometimes produced into -\ving-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 A'isible externally as a group of 



Lateral view of shell of A, /'/(Wo/ncrfes^iTHrta (Myodocopa), x 8; B,C!iprisfiiscatti (Podocopa), 

 X 19'; C, Cythereis ornnta (Podocopa), x 33. a, attachment of adductor muscles ; r, meclian 

 eye ;:?i, antennal notch. (.\ after IJrady and Norman ; B ami C after G. W. Milller.) 



spots, the arrangement of which affords characters of systematic 

 importance (a). When the edges of the valves are l)rought 

 together they usually fit closely, l)ut 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 IMyodocopa, 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, especiall}^ 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 A'alve, 

 radiating outwards from the muscle-im])ression. 



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



