CHAPTER II 
CRUSTACEA (CONTINUED) : ENTOMOSTRACA——BRANCHIOPODA— 
PHYLLOPODA——CLADOCERA——WATER-FLEAS 
SUB-CLASS I.—ENTOMOSTRACA. 
THE Entomostraca are mostly small Crustacea in which the 
seginentation of the body behind the head is very variable, both 
in regard to the number of segments and the kind of differentia- 
tion exhibited by those segments and their appendages. An 
unpaired simple eye, known as the Nauplus eye from its 
universal presence in that larval form, often persists in the 
adult, and though lateral compound eyes may be present they 
are rarely borne on movable stalks. In the adult the excretory 
gland (“shell-gland”) opens on the second maxillary segment, 
but in the larval state or early stages of development a second 
antennary gland may also be present, which disappears in the 
adult. The liver usually points forwards, and is simple and 
saccular in structure, and the stomach is not complcated by the 
formation of a gastric mill. With the exception of most Clado- 
cera and Ostracoda the young hatch out in the Nauplius state. 
Order I. Branchiopoda.' 
The Branchiopods are of small or moderate size, with flattened 
and lobate post-cephalic limbs, and with functional gnathobases. 
Median and lateral eyes are nearly always present. The labrum is 
large, and the second maxillae are small or absent in the adult. 
Branchiopods are found in every part of the world; a few are 
marine, but the great majority are confined to inland lakes and 
ponds, or to slowly-moving streams. The fresh waters, from the 
' For this use of the term Branchiopoda, cf. Boas, Morph. Jahrb, vili., 1883, p. 519. 
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