Corals and coral islands.

ALCGYONOID POLYPS.

81

crimson or purple shade. Dun colors also occur, as ash.
gray, and dark brown, and almost black. Some kinds, the
Sponggodiae, are too flexible to stand erect, and they hang
from the coral ledges, or in the coral caves, in gorgeous clusters of scarlet, yellow, and crimson colors.
The species of this order spread from the tropics through
the colder seas of the globe, and occur at various depths, down
to thousands of feet.
The two following are the most striking external peculiarities of the polyps: the number of tentacles is always eight;
and these tentacles are always fringed with papille, though the
papillae are sometimes mere warts. Some of the various forms
of the polyps are shown in the figures on the following pages.
But besides these characteristics, there is also the following: the existence of only eight internal septa, and these septa
not in pairs; consequently, the interior is divided into only
eight compartments (octants), and with each a tentacle is connected. Hence in the Alcyonoids, as Prof. Verrill has observed, the areas externally, and the comn)artments within,
are all ambulacral, or tentacular, which makes a wide distinction between them and the Actinoids (p. 28) in which only
the alternate are tentacular.
The solid secretions of these polyps are of two kinds: Either (1), internal and calcareous; or (2), epidermic, from the
base of the polyp. The latter make an axis to the stem or
branch, which is either homey (like that in Antipathus, p. 62) or
calcareous. A few species have no solid secretions.
All the species are incapable of locomotion on the base;
yet there are some that sometimes occur floating in the open
ocean.
The three following divisions of the Alcyonoids are those
now generally recognized:
f;