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 transibrm 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 



