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-point. 
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 
