Elementary text-book of zoology, tr Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote elementarytextbo01clau Year: 1892-1893 FIG. 12«,—Sea-urchin (diagrammatic). J, inter-radius with, the double row of interambulacral plates and the genital organs G; R, radii -with the double row of ambulacral-plates perforated by the ainbulacral pores. A, anus. Fio. 12i.—Shell of a Sea-urchin seen from above. R, radius with the per- forated plates; J, inter-radius with the corresponding generative organs and their pores. equal parts, while a similar sect
Elementary text-book of zoology, tr Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote elementarytextbo01clau Year: 1892-1893 FIG. 12«,—Sea-urchin (diagrammatic). J, inter-radius with, the double row of interambulacral plates and the genital organs G; R, radii -with the double row of ambulacral-plates perforated by the ainbulacral pores. A, anus. Fio. 12i.—Shell of a Sea-urchin seen from above. R, radius with the per- forated plates; J, inter-radius with the corresponding generative organs and their pores. equal parts, while a similar section through an inter-radial line divides one antimere from its neighbour. Eadiate animals may have two, three, etc., radii; and in animals which possess an uneven number of radii, one radius and one inter-radius always fall in the same vertical plane (fig. 12«, b, and fig. 13). In animals with an even number of radii, on the con- trary, each vertical plane passes through two radii or two inter- radii. A vertical section passing through one radius would, if pro- longed, pass through the radius of the opposite antimere (fig. 14o). For example, an animal with four radii possesses four antimeres, each of which will be divided into two, by two radial vertical sections passing at right angles to each other through the chief axis; while they will all be separated from each other by two similarly directed inter-radial sections. Biradiate forms (the Ctenophora) possess, on the contrary, only two radii, which lie in a common vertical plane. A second vertical plane crossing the first at right angles passes through the inter-radii, and
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