II BROOD-CHAMBER OF CLADOCERA 47 
beneath the dorsal wall of the thorax and in the middle lne of 
the carapace suggests the possibility that some special nutrient 
substances may pass from the body of the parent into the brood- 
chamber, and in some species the thoracic ectoderm is specially 
modified as a placenta. In Moina (Fig. 16) the dorsal wall 
of the thorax is produced into a dome, covered by a columnar 
ectoderm, which contains a dilatation of the dorsal blood-sinus ; 
and in this form it has been shown that the fluid in the brood- 
pouch contains dissolved proteids. Associated with the apparatus 
for supplying the brood- 
pouch with nutriment 
is a special apparatus 
for closing it, in the 
form of a raised ridge, 
which projects from the 
back and sides of the 
thorax and fits into a 
groove of the carapace. 
A somewhat similar 
nutrient apparatus exists 
in the Polyphemidae, 
Fic. 17.—Moina rectirostris, 9, x 40, showing the 
where the edges of the ephippial thickening of the carapace which pre- 
small carapace are fused cedes the laying of a winter-egg. 
with the thorax, so that 
the brood pouch is completely closed, and the young can only 
escape when the parent casts her cuticle. In some genera of 
this family (e.g. Hvadne) the young remain in the parental brood- 
pouch until they are themselves mature, so that when they are 
set free they may already bear parthenogenetic embryos in their 
own brood-pouches. 
The winter-eggs are fertilised in the same part of the cara- 
pace of, the female in which the parthenogenetic eggs develop, 
but after fertilisation they are thrown off from the body of the 
mother, either with or without a protective envelope formed 
from the cuticle of the carapace. The eggs of Sida are sur- 
rounded by a thin layer of a sticky substance, and when cast 
out of the maternal carapace they adhere to foreign objects, such 
as water-weeds; those of Polyphemus have a thick, gelatinous 
coat; in Leptodora and Bythotrephes the egg secretes a two- 
layered chitinous shell. In these forms the cuticle of the 
