OSTEACODA. 



423 



Order 2. OSTRACODA.* 



Small, usually laterally compressed Entomostraca, with a "bivalve 

 shell and seven pairs of appendages, which function as antennce, jaws, 

 creeping and swimming legs. There is a pediform mandibular palp, 

 and a short abdomen. 



The body of these small Crustacea is nnsegmented and is completely 

 enclosed in a bivalve shell, which gives the animal a resemblance to 

 a mussel. The two valves of the shell join together in the middle 

 line, and are fastened together by an elastic ligament along the middle 

 third of the back. 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. The 

 common tendon of the two heads of this muscle lies nearly in the 



F' MX" SM. MX' Md, Ob 



FIG. 335. Female Cypris before sexual maturity ; the right valve of the sneil has been 

 removed, A', A", first and second pair of antennae ;. Ob, upper lip ; Md, mandible with 

 pediform palp ; G, cerebral ganglion with unpaired eye ; SM, adductor muscle ; MX', 

 MX", first and second pair of maxillae ; F 1 , F", first and second pair of feet ; Fu, caudal 

 fork ; M, stomach ; D, intestine ; L, hepatic tube ; Ge, rudimentary genital organs. 



middle of the body. The edges of the valves are free at both ends 

 and along the ventral side. In the marine Cypridinidce 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. 



* H. E. Strauss-Diirkheim, " Memoire sur les Cypris de la classe des Crus- 

 taces," Mem. du Mug d'hixt. not., Tom VII., 1821. W. Zenker, " Monographic der 

 Ostracoden," Arckiv.fiir NaturgesoK., Tom. XX.. 1854. C. Glaus, "Beitragc 

 zur Kentniss dcr Ostracoden. Entwickelungsgcschichte von Cypris." Marburg. 

 1868. C. Glaus. " Neue Beobachtungen fiber Cypridinen," Zeitschr. fiir miss. 

 Zool. t Tom XXIII. C. Claus, "Die Familic dcr Halocypridcn." Sckriften 

 toologiscTien Inhalt<<t,'Wicu, 1874. G. S.Brady, "A Monograph of the Recent 

 British Oetracoda," Transact, of the Lin. Soc., Vol. XXVI. 



