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 the 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 closing
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 minute 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 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 which project through the holes
in the narrow zones of plates.
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