Corals and coral islands.

42

CORALS AND CORAL ISLANDS.

II. CORAL-MAKING ACTINOID POLYPS.
Of the form, tentacles, mouth, stomach, fleshy septa, lasso.
cells, food, digestion and respiration of the coral-making polyps
here included, nothing need here be said, these characters being
the same as in the Actinie. Their more striking peculiarities
depend on the secretion of coral, making them fixed species,
and involving an absence of the base; and, in the case of the
majority of the species, on the extent to which they multiply
by buds, in imitation of species in the vegetable kingdom.
The coral skeleton which the secretions of polyps form is
called the corallum. These secretions take place among the tisCARYOPHYLLIA CYATITUS.
sues of the sides and lower part of the polyp, but never in the
disk or stomach, as this would interfere with the functions of
these organs. In the above sketches of a simple coral.
from the Mediterranean, the upper extremity is a depression,
or calicle, enclosed by a series of radiating calcareous (coral)