HOMOLOGIES. 211 



much as possible in these papers, make use of 

 the unfamiliar terms oral and ab-oral regions, to 

 indicate the mouth with the parts diverging 

 from it and the opposite area towards which all 

 these parts converge.* 



Sea-Urchin seen from the oral side, showing the zones with the spines and 

 the suckers ; for the ab-oral side, on the summit of which the zones unite, see 

 the wood-cut on the next page, which shows a portion of that region. 



The whole surface of the animal is divided by 

 zones, ten in number, five broader ones alter- 

 nating with five narrow ones. The five broad 

 zones are composed of large plates on which are 

 the most prominent spines, attached to tubercles 

 that remain on the surface even when the spines 

 drop off after death, and mark the places where 

 the spines have been. The five small- zones are 

 perforated with regular rows of holes, and 

 through these perforations pass the suckers or 



* When reference is made to the whole structure, including the 

 internal organs as well as the solid parts of the surface, the terms 

 actlnal and ab-actinal are preferable to oral and ab-oral. 



