38 CRUSTACEA—BRANCHIOPODA CHAP. 
into joints by rings of spines, while in Macrothria they are 
flattened plates. In the males the first antennae are elongated 
and mobile (ef. Figs. 11, 19). 
The second antennae, the chief organs of locomotion, are 
biramous in all genera except Holopediwm; the number of 
joints in each ramus, and the number of the long plumose hairs 
with which they are provided, are remarkably constant in whole 
series of genera, and are therefore useful for purposes of classi- 
fication. The creatures row themselves by quick strokes of 
these appendages, the movement being slow and irregular in the 
rounder forms, such as 
Simocephalus or Daphnia, 
rapid and well. directed in 
such elongated lacustrine 
forms as Bythotrephes or 
Leptodora. 
The mandibles have no 
palp; the first maxillae are 
very small, and the second 
maxillae are absent (Fig. 9). 
The carapace varies very 
much. In most genera (the 
CALYPTOMERA of Sars) it is 
Fic. 9.—Simocephalus vetulus, female. Ventral 2 eee Bec Re eae 
view, without the carapace; Aj, As, first jecting fold of skin, bent 
ete teat ait, downwards at the sides 50 
thoracic appendages. as to form a bivalve shell, 
enclosing the whole post- 
cephalic portion of the body, as in Simocephalus (Fig. 10). The 
eggs are laid into the space between the carapace and the 
dorsal part of the thorax, both the carapace and the thorax itself 
being often modified for their protection and nutrition. In a few 
forms, the GYMNOMERA of Sars, the carapace serves only as a 
brood-pouch, which is distended when eggs are laid, but collapses 
to an inconspicuous appendage at the back of the head when it is 
empty (e.g. Leptodora, Fig. 24, Bythotrephes, Fig. 13). In the 
Calyptomera the surface of the carapace is frequently provided 
with a series of ridges, which may be parallel, rarely branching, as 
in Stmocephalus ; or in two sets which cross nearly at right angles, 
as in Daphnia; or so arranged as to form a hexagonal pattern, as 
