222 



PHYLUM ECHINODERMATA. 



cells of which are provided with sensory hairs. When these are 

 not present scattered sense-cells are present in the epithelium 

 on the inner sides of the blades. Nerves pass along the stalk 



Fid. 164. Strongylocentrotus droebachiensis. The skeleton of a young individual of 4 mm. 

 spread out and viewed from the oral surface. I to V the radii numbered ; 1 to 5 the inter- 

 radii numbered ; a, b mark the rows in each radius and interradius. The apical system 

 is shown in 2, the interradius of the madreporite (right anterior). 



