ANNULOSA: CRUSTACEA. 293 



dages of the mouth. The feet are few in number, and mainly 

 subserve locomotion ; the carapace is in the form either of 

 a shield protecting the cephalothorax, or of a bivalve shell 

 enclosing the entire body. The mouth is not suctorial, but is 

 furnished with organs of mastication. 



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 branchice 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 (Cypridina], 

 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 ulti- 

 mately transformed into the mandibles. They pass through several stages, 

 with complete moults, before arriving at sexual maturity. The young of 

 Cythere, 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. 145, a), which are of 

 almost universal occurrence in fresh water. The common 



Fig. 145. rFresh-water Entomostraca. a Cypris tris-striata : b Daphnia 

 pulex ; c Cyclops quadricornis . 



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. The closure of the carapace is effected 

 by means of an * adductor muscle. Locomotion is mainly 

 effected by means of a pair of caudal appendages. The Cypris 

 is extremely prolific, and a single impregnation appears to last 



