It is a sort of natural cooperative store,
every polype helping the whole, at the same time as it helps itself. The
interior of the stem, like that of the branches, is solidified by the
deposition of carbonate of lime in its tissue, somewhat in the same
fashion as our own bones are formed of animal matter impregnated with
lime salts; and it is this dense skeleton (usually turned red by a
peculiar colouring matter) cleared of the soft animal investment, as
the hard wood of a tree might be stripped of its bark, which is the red
coral.
In the case of the red coral, the hard skeleton belongs to the interior
of the stem and branches only; but in the commoner white corals, each
polype has a complete skeleton of its own. These polypes are sometimes
solitary, in which case the whole skeleton is represented by a single
cup, with partitions radiating from its centre to its circumference.
When the polypes formed by budding or division remain associated, the
polypidom is sometimes made up of nothing but an aggregation of these
cups, while at other times the cups are at once separated and held
together, by an intermediate substance, which represents the branches
of the red coral. The red coral polype again is a comparatively rare
animal, inhabiting a limited area, the skeleton of which has but a very
insignificant mass; while the white corals are very common, occur
in almost all seas, and form skeletons which are sometimes extremely
massive.
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