The Annals and magazine of natural history; zoology, botany, and geology . he left side two binary and onesimple. The bivium, on the contrary, includes symmetricallythe unpaired interradium. Eight and left are determined hereby the position of the anal aperture, and also, except in theCh/peastridce, by that of the madreporic plate; in the regularEchinoidea, hitherto, by the latter alone. Of the five genital plates, the Sjmtangidce are destitute ofthe hinder one, which elsewhere is placed near the end of theimpaired, anal interradium. It has never been developed, anymore than the genital gland,
The Annals and magazine of natural history; zoology, botany, and geology . he left side two binary and onesimple. The bivium, on the contrary, includes symmetricallythe unpaired interradium. Eight and left are determined hereby the position of the anal aperture, and also, except in theCh/peastridce, by that of the madreporic plate; in the regularEchinoidea, hitherto, by the latter alone. Of the five genital plates, the Sjmtangidce are destitute ofthe hinder one, which elsewhere is placed near the end of theimpaired, anal interradium. It has never been developed, anymore than the genital gland, which otherwise, as in the fourothers, should have its efferent duct through it. In all theknown living Spatangidce^ apparently with the exception of asingle genus, its place is occupied by the madreporic plate ;the filtering-apparatus of the aquiferous system, which spreadsitself out in the posterior part of the vertex, often occupies agreater space than any of the four genital plates, is posteriorlyin immediate contact with the last plates of the anal inter- Fio-.
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