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.* 
TN 
Ne 2 
DNS 
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 
actinal and ab-actinal are preferable to oral and ab-oral. 
