" WATER-FLEAS " 



341 



Order 2. Ostracoda. — Small Crustaceans, usually laterally compressed, 

 with an indistinctly segmented or unsegmented body, rudimentary 

 abdomen, and bivalve shell. There are only seven pairs of 

 appendages : — antennules, antennae, mandibles, first maxillae, 

 second maxillae, and two pairs of thoracic hmbs. Parthenogenesis 

 is often prolonged. 



Examples. — Cypris (fresh water), Cypndina (marine). 



Fig. 185. — Daphnia. 



Eye ; A^., second antenna 

 shell gland ; go., gonad ; 

 brood-pouch ; sp., spine ; 

 men ; t., caudal fork ; g. 



A^., first antenna ; dg., digestive caeca ; s.g., 

 h., heart in pericardium ; 0., ovum ; B.p., 

 /., furca ; s., setae ; Ab., rudimentary abdo- 

 gut ; /-J, thoracic limbs. 



Order 3. Copepoda. — Elongated Crustaceans, usually with distinct seg- 

 ments. There is no dorsal shell. There are five pairs of biramose 

 thoracic appendages, but the last may be rudimentary or absent. 

 The abdomen is without limbs, and of its five segments the first 

 two are sometimes united. The females carry the eggs in external 

 ovisacs. Most Copepods move very actively in the water, jerking 



