316 



COMPARATIVE ANATOMY 



CHAP. 



Round this principal axis many important parts of the body are 

 grouped in a radiate manner. The typical number of the rays is, 

 with few exceptions, five. In the Echinoderms, as in the radiate 

 Ccelenterates, rays of the first, second, and third order may be distin- 

 guished. The radii or radial regions of the first order, in which the 

 principal organs lie, are called perradii, ambulaeral radii, or simply 

 radii. The five radii of the second order, which regularly alternate 

 with these five principal radii, are the interradii or interambulaeral 



Fir.s. 1268 and 2(i:i.— RepresentatlTes of the principal diyisions of the Eohinodermata. In 

 Fig. 208, in the morphological position; in Fig. 260, in the natural position witli regard to the 

 sea-floor. A, Holotliurian. B, Sea-urcbin. C, Star-fish. D, Crinoid — a, Apical pole ; o, oral 

 pole ; an, anns. 



radii. The far less important ten radii of the third order, each of 

 which lies between a perradius and an interradius, may be called 

 adradii. Between the two poles, at right angles to the principal axis, 

 we have the equator. In those Echinoderms which are provided with 

 large skeletal plates, the body and skeleton is further divided into two 

 zones, separated from one another by the equator ; these are the oral, 

 adaetinal, or ventral zone, and the apical, abaetinal, or dorsal zone. 

 In the centre of the former lies the mouth. 



