But this is but a small part of the mechanism of this interesting
tribe. If you put into a basin of sea-water one of the pretty kind[89]
which we find so abundantly under stones at low water,--whose
green spines are tipped with rosy purple, like the tentacles of an
_Anthea_,--you will presently observe it marching majestically along
by means of the hundreds of sucker-feet, which it possesses in common
with the Star-fish. Now, if you have in your cabinet the empty box of
an Urchin of this same kind, and taking it in your hand, hold it up to
the light, and look into the cavity from the under or mouth side, you
will have a very interesting spectacle. The light streams in through a
multitude of minute holes, as smooth and regular as if drilled with a
fairy’s wimble; and these holes are arranged in a pattern of elegant
symmetry. They run in lines, like meridians, from pole to pole of the
flattened globe; but instead of being set at uniform intervals, they
constitute five principal sets or bands, with blank intervals between,
about twice as wide as the drilled bands. Then each band comprises two
series, each of which contains a double row of orifices. These last,
again, do not constitute a single unbroken line, but an interrupted or
zig-zag line, which is, in fact, made up of a number of short diagonal
rows; three holes in each diagonal, set one after another.
Put the living and the dead together. These tiny orifices, as minute as
the point of the finest cambric-needle could make in a bit of paper,
afford exit to the suckers, which are, of course, equally numerous.
Through these pass the slender pellucid tubes, filled with elastic
fluid, which carry at their tips a flat ring of calcareous shell,
affording to each the form and firmness to make each one an adhesive
sucking disk, in the centre of which a tiny vacuum is created at will
by muscular retraction.