2l6 MANUAL OF ZOOLOGY. 



This division comprises the two orders Ostracoda and 

 Copepoda. 



Order I. Ostracoda. — Small Crustaceans having the entire 

 body enclosed in a shell or carapace, which is composed of two 

 valves united along the back by a membrane. The branchiae 

 are attached to the posterior jaws, and there are only two or 

 three pairs of feet, which subserve locomotion, but are not 

 adapted for swimming. A distinct heart is sometimes present 

 {Cypridind), but is more usually wanting (Cypris and Cythere). 



Little is known of the development of the Ostracoda, but the 

 young of Cypris are said to be " shell-bearing Nauplius forms " 

 (Claus), possessing only the three anterior pairs of limbs, but 

 protected by a bivalve shell. As in other Nauplii, the third 

 pair of limbs, though now locomotive, are ultimately transformed 

 into the mandibles. They pass through several stages, with 

 complete moults, before arriving at sexual maturity. The 

 Cytherides, on the other hand, have at birth the two pairs of 

 antennae and two pairs of jaws, with three pairs of rudimentary 

 abdominal limbs. 



The order includes the Cyprides (fig. 66, a), which are of 

 almost universal occurrence in fresh water. The common 



Fig. 66. — Fresh-water Entotnostraca. a Cyfins tn's-siriata; b Daphnia 

 pulexi c Cyclops quadriconiis. 



Cypris is completely protected from its enemies by a bivalve 

 carapace, which it can open and shut at will, and out of which 

 it can protrude its feet. Locomotion is mainly eifected by 

 means of a pair of caudal appendages. The Cypris is ex- 

 tremely prolific, and a single impregnation appears to last the 

 female for its entire lifetime. It appears, also, that the young 

 females, produced in this way, are capable for some generations 

 of producing fresh individuals without the influence of a male 

 (parthenogenesis). 



