26 ORGANIZATION AND DEVELOPMENT OP ANIMALS IN GENEEAL. 



can be drawn, passing between the antimeres. A vertical section 

 through a radial line divides the corresponding antimere into two 



FIG. I2a. Sea-urchin (diagrammatic). 

 J, inter-radius with, the double row 

 of interambulacral plates and the 

 genital organs G ; E, radii with the 

 double row of ambulacral-plates 

 perforated by the ambulacral pores. 

 A, anas. 



FIG. 126. Shell of a Sea-urchin seen 

 from above. It, radius with the per- 

 f orated 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. Radiate 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. I2a, 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. 14a). 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 



FIG. 13. Star-fish (diagrammatic). <?, 

 generative organ in inter-radius ; Af, 

 position of the ambulacral feet in the 

 radii. 



