YONACEA the colony consists of bunches of elongate
cylindrical zooids, whose proximal portions are united by solenia and
compacted, by fusion of their own walls and those of the solenia, into
a fleshy mass called the coenenchyma. Thus the coenenchyma forms a
stem, sometimes branched, from the surface of which the free portions
of the zooids project. The skeleton of the Alcyonacea consists of
separate calcareous spicules, which are often, especially in the
Nephthyidae, so abundant and so closely interlocked as to form a
tolerably firm and hard armour. The order comprises the families
_Xeniidae, Alcyonidae_ and _Nephthyidae_. _Alcyonium digitatum_, a
pink digitate form popularly known as "dead men's fingers," is common
in 10-20 fathoms of water off the English coasts.

[Illustration: FIG. 7.--The sea-fan (_Gorgonia cavolinii_).]

In the order PSEUDAXONIA the colonies are upright and branched,
consisting of a number of short zooids whose proximal ends are
imbedded in a coenenchyma containing numerous ramifying solenia and
spicules. The coenenchyma is further differentiated into a medullary
portion and a cortex. The latter contains the proximal moieties of the
zooids and numerous but separate spicules. The medullary portion is
densely crowded with spicules of different shape from those in the
cortex, and in some forms the spicules are cemented together to form a
hard supporting axis. There are four families of Pseudaxonia--the
_Briareidae, Sclerogorgidae, Melitodidae_, and _Corallidae_. In the
first-named the medulla is penetrated by solenia and forms an
indistinct axis; in the remainder the medulla is devoid of solenia,
and in the _Melitodidae_ and _Corallidae_ it forms a dense axis, which
in the _Melitodidae_ consists of alternate calcareous and horny
joints. The precious red coral of commerce, _Corallium rubrum_ (fig.
6), a member of the family _Corallidae_, is found at depths varying
from 15 to 120 fathoms the Mediterranean Sea, chiefly on the African
coast. It owes its commercial value to the beauty of its hard red
calcareous axis which in life is covered by a cortex in which the
proximal moieties of the zooids are imbedded. _Corallium rubrum_ has
been the subject of a beautifully-illustrated memoir by de
Lacaze-Duthiers, which should be consulted for details of anatomy.

The AXIFERA comprise those corals that have a horny or calcified axis,
whic