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and they bend and interdigitate so accurately over the
ambulacra that one might easily overlook the grooves
at a first glance. Tbe peripetalous fasciole is somewhat
irregular ; but in those examples in whicli it is
best defined it forms a wide arcli, extending backwards
on each side a little beyond the lateral ambulacra of
the trivium, and then, contracting a little, forms a
rudely rectangular figure round the hivium. The
paired amhulacral grooves in the male are shallow,
not much deeper than the anterior ambulacrum (Fig.
4.5) ; in the female the pore-plates of the paired
F , g . master p la iip p ii. The apical P o tio n
Within. Slightlj enlaigtcl.
ambulacra are greatly expanded and lengthened, and
thinned out and depressed so as to form four deep,
thin-walled, oval cups sinking into and encroaching
upon the cavity of the test and forming very efiicient
protective ‘ marsupia ’ (Fig. 44). The ovarial openings
are, of course, opposite tbe interradial areas;
but the spines are so arranged that a kind of co\ ered
passage leads from the opening into the marsnpium ;