214 HOMOLOGIES. 
that passes into it is purified from all foreign 
substances that might be injurious to the animal, 
and is thus fitted to pass into the water-system, 
from which arise the main branches leading to 
the minute suckers projecting through the holes 
in the narrow zones of plates. 
Now, in order to transform theoretically our 
Sea-Urchin into a Star-Fish, what have we to do? 
Let the reader imagine for a moment that the 
small ab-oral area closing the space between the 
ovarian plates and the eye-plates is elastic, and 
may be stretched out indefinitely ; then split the 
five broad zones along the centre, and draw them 
down to the same level with the mouth, carry- 
ing the ovarian plates between them. We have 
Star-Fish from the ab-oral side. 
then a star; just as, dividing, for instance, the 
peel of an orange into five segments, left, of 
