ENTOMOSTRACA. 301 



(b) Cladocera. Small laterally compressed "water-fleas," with few 

 and somewhat indistinct segments. The shell is usually bivalved, 

 and the head often projects freely from it. The second antennae 

 are large, two-branched, swimming appendages, and there are 

 4-6 pairs of other swimming organs. The heart is a little sac 

 with one pair of openings. An excretory organ (the shell or 

 maxillary gland) opens in the region of the second maxillae. It 

 is the Entomostracan equivalent of the antennary green gland 

 of Malacostraca. The males are usually smaller and much rarer 

 than the females. The latter have a brood-chamber between 

 the shell and the back. Within this many broods are hatched 

 throughout the summer. Periodic parthenogenesis (of the 

 "summer ova") is very common. "Winter eggs," which 

 require fertilisation, are set adrift in a part of the shell modified 

 to form a protective cradle or ephippium. 



Daphnia, Mbina, Sida, Polyphemus, Leptodora, and many 

 other "water-fleas," are extraordinarily abundant in fresh 

 water, and form part of the food of many fishes. A few 

 occur in brackish and salt water. 



In Daphnia the appendages are : antennules, antennae, 

 mandibles, first maxillae, second maxillae (disappearing in the 

 larva), and five thoracic limbs. The abdomen is turned down- 

 wards and forwards, and shows three segments and a telson. 



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 limbs. Parthenogenesis 

 is often prolonged. 



Examples. Cypris (fresh water), Cypridina (marine). 



FIG. 155. Cypris. 



**/., Marks of adductor muscle ; ., eye seen through the shel 

 A.f, first antennae ; A. 2, second antennae ; P., thoracic le 



