[Illustration: Star-Fish from the ab-oral side.]
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, carrying the ovarian plates between them.
We have then a star, just as, dividing, for instance, the peel of an
orange into five compartments, leaving them, of course, united at the
base, then stripping it off and spreading it out flat, we should have a
five-rayed star.
[Illustration: One arm of Star-Fish from the oral side.]
But in thus dividing the broad zones of the Sea-Urchins, we leave the
narrow zones in their original relation to them, except that every
narrow zone, instead of being placed between two broad zones, has now
one-half of each of the zones with which it alternated in the
Sea-Urchin on either side of it and lies between them. The adjoining
wood-cut represents a single ray of a Star-Fish, drawn from what we
call its lower side or the oral side. Along the centre of every such
ray, diverging from the central opening or the mouth, we have a
furrow, corresponding exactly to the narrower zones of the Sea-Urchin.
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