. The Cambridge natural history. Zoology. BROOD-CHAMBER OF CLADOCERA 47 beneath the dorsal wall of the thorax and m 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 tbe brood- pouch contains dissolve
. The Cambridge natural history. Zoology. BROOD-CHAMBER OF CLADOCERA 47 beneath the dorsal wall of the thorax and m 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 tbe 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, where the edges of the small carapace are fused with the thorax, so that. Fig. 17.—Moina reciirostris, 9 , x 40, showing the ephippial thickening of the carapace whidi pre- cedes the laying of a winter-egg. the brood pouch is completely closed, and the young can only escape when the parent casts her cuticle. In some genera of this family ( 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 layer of a sticky substance, and when cast out of the maternal carapace they adhere to foreign objects, such as water-weeds; those of Folyphemus have a thick, gelatinous coat; in Zeptodora and Bythotrephes the egg se
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1895
