SYMMETRY. 



as Radiata is inadmissible, and so much the more so since the radial 

 arrangement of the structure exhibits some transitions toward^ a 

 bilateral symmetry. The Echinodermata are separated from the 

 Ccelenterata by the possession of a separate alimentary canal and 

 vascular system, and also by a number of peculiar features both of 

 organization and of development. 



The arrangement of the parts round the axis of the body is usually 

 pentamerous. Nevertheless when the rays are more numerous, irre- 

 gularities in the repetition of the similar organs are met with. If 

 we take as the fundamental form of the Echinoderin type a spheroid 

 with the principal axis somewhat shortened and the poles flattened 

 and dissimilar, the long axis of the radial body will be this chief 

 axis, and the mouth and anus the two poles (oral and anal poles). 



We can imagine five planes pass- 

 ing through the long axis of this 



spheriod, each of which will divide 



the body into two symmetrical 



halves. The perfect correspondence 



of these halves is, in the body of 



Echinoclerms, disturbed by the dif- 

 ferent forms and significance of the 



two poles, so that our representation 



is not an exact one. The ten meri- 

 dians, which are separated from one 



another by equal intervals and fall 



in these five planes, are differently 



related to one another, inasmuch as 



five alternate ones, which are called 



the chief rays, or radii, contain the 



most important organs, the nerves, 



the vascular trunks, the ambu- 



lacral feet, etc., while the other 



intermediate rays or inter-radii, 



FIG. 20G. The shell of a regular Sea- 

 urchin seen from above. R, Radius 

 with double row of perforated plates ; 

 J", inter-radius with the genital organs 

 and their pores. In the right ante- 

 rior inter-radius is the inadrepcric 

 plate. 



five meridians constitute the 

 and also contain certain organs 



(fig. 206). It is only in cases of complete equivalence of the radii 

 and inter-radii that the echinoderm body presents a pentamerou> 

 radial arrangement (r<>yi>hir Echinoderms). It is, however, easy to 

 show that this regular radial symmetry never occurs in its perfect 

 form. Since one organ or another, e.g., the rnadreporic plate, the 

 stone canal, heart, etc., always remains single, and does not fall in 

 the axis of the body, it will be only those planes, in the radius or 

 inter-radius of which the unpaired organs fall, which can fulfil the 



