II CARAPACE OF CLADOCERA 39 
in Ceriodaphnia. In a few forms the whole surface is irregularly 
-covered with spines or scales. The hinder edge of the carapace 
is often produced into a median dorsal spine (Vaphnia, Fig. 19), 
or more rarely there are two spines, one at each ventro-lateral 
corner (Scapholeberis, Fig. 20). 
The cuticle of the carapace is often separated from that of 
the head by a cervical suture, as in Simocephalus (Fig. 10, C.S.), 
and near the line of demarcation many forms exhibit patches of 
Fria. 10.—Simocephalus vetulus, x 30. Side view of female, showing the arrangement 
of the principal organs. A.2, Second antenna; C.S, cervical suture; /, fused 
compound eyes ; H, heart ; ZL, forwardly-directed gastric caeca ; JV, dorsal organ. 
glandular ectoderm which seem to be homologous with the 
dorsal adhesive organs of the Limnadiidae. The commonest 
condition is that of a median dorsal pit (Fig. 10, 1), by means 
of which the animal can fix itself to foreign objects. Certain 
forms may remain for long periods of time attached by the 
dorsal organ to plants, or to the sides of an aquarium, the only 
movement being a slow vibration of the feet, by which a current 
of water, sufficiently rapid for respiratory purposes, is established 
round it.) In Sida erystallina (Fig. 11) the dorsal organ is 
represented by three structures ; in front there is a median raised 
1 Simocephalus vetulus anchors itself to weeds, etc., by a modified seta on the 
exopodite of the second antenna. It does not employ a dorsal organ for purposes 
of fixation. [G. 8.] 
