HOMOLOGIES. 213 



circular oral aperture, open to receive the prey, 

 and -then are brought together to crush it, the 

 points meeting in the centre, thus working con- 

 centrically instead of moving up and down or 

 from right to left, as in other animals. From 

 the oral opening the ten zones diverge, spreading 

 over the whole surface, like the ribs on a melon, 

 and converging in the opposite direction till they 

 meet in a small space which we have called the 

 ab-oral region opposite the starting-pom fc. 



Here the broad zones terminate in five large 

 plates differing somewhat from those that form 

 the zones in other parts of the body, and called 

 ovarian plates, because the eggs pass out through 

 certain openings in them ; while the five narrow 

 zones terminate in five small plates on each of 

 which is an eye, making thus five eyes alternating 

 with five ovarian plates. The centre of this area 

 containing the ovarian plates and the visual 

 plates is filled up with small movable plates clos- 

 ing the space between them. I should add, that 

 one of the five ovarian plates is larger than the 

 other four, and has a peculiar structure, long a 

 puzzle to naturalists. It is perforated with* mi- 

 nute holes, forming an exceedingly delicate sieve, 

 and this is actually the purpose it serves. It is, 

 as it were, a filter, and opens into a canal which 

 conducts water through the interior of the body ; 

 closed by this sieve on the outside, all the water 



