OSTRACODA. 383 



Halocypridae it is richly supplied with glands. The two valves 

 of the shell are joined together by an elastic ligament along the 

 middle third of the back, and the action of this ligament is opposed 

 by a two-headed adductor muscle, which passes from one valve 

 of the shell to the other and causes impressions discernible from 

 without (Fig. 253, 13 ). The tendon common to the two heads 

 of the muscle lies nearly in the middle of the body. The edges 

 of the valves are free at both ends and along the ventral side. 

 In the free-swimming marine Cypridinidae (Fig. 254) and 

 Halocypridae there is a deep indentation in the edges of the 

 valves, to allow the antennae to pass out. When the valves 

 of the shell are open, several pediform appendages can be 

 protruded on the ventral side, which enable the animal to 

 move in the water either by crawling or by swimming. The 

 abdomen can also be protruded ; it either ends in a caudal fork 

 (Cypris and Cythere), or has the form of a plate armed with 

 spines and hooks on its posterior margin (Cypridina). 



Appendages. Anterior antennae, uniramous. 

 Posterior antennae, biramous. 

 Mandibles, with a usually biramous palp. 

 First maxillae, usually jaws (in Polycopidae short bira- 

 mous legs). 



Second maxillae, jaws, maxillipeds, or legs. 

 Sixth pair of appendages, jaws, or legs, or 0. 

 Seventh pair of appendages, legs, or specially modified,. 



or 0. 



Eighth pair of appendages, represented by the brush- 

 shaped organs in the males of Podocopa, by the penis 

 in Cypridinidae (Miiller). 



The two pairs of antennae are placed on the anterior region 

 of the body (Figs. 253, 2 and 3, and 254, A', A"), and are used as 

 creeping or swimming legs. In the Cypridinidae and Halocy- 

 pridae the anterior pair is provided with large olfactory hairs. The 

 second pair of antennae are generally the most important organs of 

 locomotion. In the exclusively marine Cypridinidae and Halocy- 

 pridae they have the form of biramous swimming feet, and 

 consist of a broad triangular basal plate, a many- jointed exopodite 

 beset with long swimming setae, and a reduced endopodite r 

 which, however, is stronger in the male and furnished with hooks 

 of a considerable size. The Polycopidae have the two rami 

 approximately equal in size, while in the other families the 

 endopodite is the principal ramus, the exopodite being reduced 



