ENTOMOSTRACA. 30r 
(6) 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 antennie 
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 maxillz. 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, Moina, 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 Daghnia the appendages are:—antennules, antennz, 
mandibles, first maxille, second maxille (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 
~qules, antennz, mandibles, first maxillze, 
é two pairs of thoracic limbs. Parthenogenesis- 
is often prolonged. 
Examples.—Cygris (fresh water), Cyprédina (marine). 
Fic. 155.—Cypris. 
M., Marks of adductor muscle; £., eye seen through the shell (S/.); 
A.J, first antennz ; A.2, second antenne ; F., thoraciclegs. 
