SYMMETRY. 



267 



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

 arrangement of the structure exhibits some transitions towards a 

 bilateral symmetry. The Echinodermata are separated from the 

 Cielenterata 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 Echinoderm 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 

 Echinoderms, 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 five meridians constitute the 

 intermediate rays or inter-radii, 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 pentamerous 

 radial arrangement (regular 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 madreporic 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 



Fio. 206. 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 madrepcric 

 plate. 



