on is frequently present, either in the form of a horny external
investment (_Cornularia_), or an internal axis (_Gorgonia_), or it may
form a matrix in which spicules are imbedded (_Keroeides, Meistodes_).

[Illustration: FIG. 4.--Transverse section of an Alcyonarian zooid mm,
Mesenteries; mb, muscle banners; sc, sulcus; st, stomodaeum.]

Nearly all the Alcyonaria are colonial. Four solitary species have
been described, viz. _Haimea funebris_ and _H. hyalina, Hartea
elegans_, and _Monoxenia Darwinii_; but it is doubtful whether these
are not the young forms of colonies. For the present the solitary
forms may be placed in a grade, _Protal-cyonacea_, and the colonial
forms may be grouped in another grade, _Synalcyonacea_. Every
Alcyonarian colony is developed by budding from a single parent zooid.
The buds are not direct outgrowths of the body-wall, but are formed on
the courses of hollow out growths of the base or body-wall, called
_solenia_. These form a more or less complicated canal system, lined
by endoderm, and communicating with the cavities of the zooids. The
most simple form of budding is found in the genus _Cornularia_, in
which the mother zooid gives off from its base one or more simple
radiciform outgrowths. Each outgrowth contains a single tube or
solenium, and at a longer or shorter distance from the mother zooid a
daughter zooid is formed as a bud. This gives off new outgrowths, and
these, branching and anastomosing with one another, may form a
network, adhering to stones, corals, or other objects, from which
zooids arise at intervals. In _Clavularia_ and its allies each
outgrowth contains several solenia, and the outgrowths may take the
form of flat expansions, composed of a number of solenial tubes felted
together to form a lamellar surface of attachment. Such outgrowths are
called _stolons_, and a stolon may be simple, i.e. contain only one
solenium, as in _Cornularia_, or may be complex and built up of many
solenia, as in _Clavularia_. Further complications arise when the
lower walls of the mother zooid become thickened and interpenetrated
with solenia, from which buds are developed, so that lobose, tufted,
or branched colonies are formed. The chief orders of the Synalcyonacea
are founded upon the different architectural features of colonies
produced by different modes of budding. We recognize six orders--the
STOLONIFERA, ALCYONACEA,