ANATOMY AND PHYSIOLOGY OF ECIIINODKRMS 239 



those sea-urchins, however, that have assumed the habit of 

 free locomotion on the oral surface (Fig. 230), the mouth has 

 moved from the centre 

 toward one edge and the 

 anus, which is typically at 

 the centre of the aboral 

 pole, has moved toward the 

 opposite edge, so that, in 

 these cases, the body has 

 become bilaterally sym- 

 metrical and the alimentary 

 tract tends to run horizon- 

 tally instead of verticallj'. 

 This result shows the effect 

 of a locomotor habit on the 

 radial form. 



The body -wall of echino- 

 derms contains limy plates 

 (Fig. 230). These are best 

 developed in the sea-ur- 

 chins and sea-lilies, where 

 they encase the entire 

 body. Ten l^ands run 

 along meridians from one 

 pole to the other ; in alter- 

 nating bands the plates are 

 perforated for passage of 

 the tube-feet — such are called the radial l)ands or radii ; the 

 others are called interradii. In starfishes the interradii are 

 small and unimportant, forming the part of the l^ody-wall 

 lying between the arms. The luny plates lie embedded 



Fig. 229. — A sea-eucumber (Cucumaria), 

 showing the radial, tree-Hl<:e, arrange- 

 ment- of parts in an echinoderm. Parts 

 are seen to be arranged on a basis of five. 

 Three rows of tui^ular feet show ; two 

 rows are hidden. There are ten tentacles 

 at the upper end of the body of whieh two 

 (front middle) are small. From Parlier 

 and Haswell. 



