Corals and coral islands.

N ULLIPORES.

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distinctly. The animals, unlike true polyps and the Hydroids,
have two extremities to the alimentary canal, and in this, and
other points, they are Molluscan in type.
The cells of a group never have connection with a common
tube, as in the Hydroids; on the contrary, each little Bryozoum,
in the compound group or zo6thome, is wholly independent of
the rest in its alimentary canaL
Bryozoans occur in all seas and at all depths; and in early
Paleozoic time they contributed largely to the making of lime.
stone strata.
IV. NULLIPORES.
The more important species of the Vegetable Kingdom that
afford stony material for coral reefs are called Nullipores.
They are true Algse or sea-weeds, although so completely stony
and solid that nothing in their aspect is plant-like. They form
thick, or thin, stony incrustations over surfaces of dead corals,
or coral rock, occasionally knobby or branching, and often
spreading lichen-like.
They have the aspect of ordinary coral, especially the Millepores, but may be distinguished from these species by their
having no cells, not even any of the pin-punctures of those
species.
Besides the more stony kinds, there are delicate species, often jointed, called Corallines, which secrete only a little lime in
their tissues, and have a more plant-like 'look. Even these
grow so abundantly on some coasts, that, when broken up and
accumulated along the shore by the sea, they may make thick
calcareous deposits. Agassiz has described such beds as having considerable extent in the Florida seas.