The shell may be regarded as, typically, nearly spherical in
shape, with the mouth in the centre of the base, and the excretory
opening or vent at its summit. In both the ancient forms and the
recent ones, the plates of the shell are arranged in ten zones
which generally radiate from the summit to the centre of the base.
In five of these zones--termed the "ambulacral areas"--the plates
are perforated by minute apertures or "pores," through which
the animal can protrude the little water-tubes ("tube-feet") by
which its locomotion is carried on. In the other five zones--the
so-called "inter-ambulacral areas"--the plates are of larger
size, and are not perforated by any apertures. In all the modern
Sea-urchins each of these ten zones, whether perforate or
imperforate, is composed of two rows of plates; and there are
thus twenty rows of plates in all. In the Palaeozoic Sea-urchins,
on the other hand, the "ambulacral areas" are often like those of
recent forms, in consisting of _two_ rows of perforated plates
(fig. 119); but the "inter-ambulacral areas" are always quite
peculiar in consisting each of three, four, five, or more rows
of large imperforate plates, whilst there are sometimes four
or ten rows of plates in the "ambulacral areas" also: so that
there are many more than twenty rows of plates in the entire
shell.
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