II BROOD-CHAMBER OF CLADOCERA 47 



beneath the dorsal wall of the thorax and in the middle line 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, 



, ,, -. f i FIG. 17. Moina rectirostris, ?, x 40, showing the 



Wfiere tne edges OI tne ephippial thickening of the carapace which pre- 



small carapace are fused cedes the la y in g 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. Evadne) 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 Bytliotreplies the egg secretes a two- 

 layered chitinous shell. In these forms the cuticle of the 



