h in position corresponds to the axis of the Pscudaxonia, but,
unlike it, is never formed of fused spicules; the most familiar
example is the pink sea-fan, _Gorgonia cavolinii_, which is found in
abundance in 10-25 fathoms of water off the English coasts (fig. 7).
In this order the axis is formed as an ingrowth of the ectoderm of the
base of the mother zooid of the colony, the cavity of the ingrowth
being filled by a horny substance secreted by the ectoderm. In
_Gorgonia_ the axis remains horny throughout life, but in many forms
it is further strengthened by a deposit of calcareous matter In the
family _Isidinae_ the axis consists of alternate segments of horny and
calcareous substance, the latter being amorphous. The order contains
six families--the _Dasygorgidae, Isidae, Primnoidae, Muriceidae,
Plexauridae_, and _Gorgoniaae_.

[Illustration: FIG. 8.

A. Colony of _Pennatula phosphorea_ from the metarachidial aspect. p,
The peduncle.

B. Section of the rachis bearing a single pinna, a, Axis; b,
metarachidial; c, prorachidial; d, pararachidial stem canals.]

In the order STELECHOTOKEA the colony consists of a stem formed by a
greatly-elongated mother zooid, and the daughter zooids are borne as
lateral buds on the stem. In the section _Asiphonacea_ the colonies
are upright and branched, springing from membranous or ramifying
stolons. They resemble and are closely allied to certain families of
the Cornulariidae, differing from them only in mode of budding and in
the dispostion of the daughter zooids round a central, much-elongated
mother zooid. The section contains two families, the _Telestidae_ and
the _Coelogorgidae_. The second section comprises the _Pennatulacea_
or sea-pens, which are remarkable from the fact that the colony is not
fixed by the base to a rock or other object, but is imbedded in sand
or mud by the proximal portion of the stem known as the peduncle. In
the typical genus, Pennatula (fig. 8), the colony looks like a feather
having a stem divisible into an upper moiety or rachis, bearing
lateral central leaflets (pinnae), and a lower peduncle, which is
sterile and imbedded in sand or mud. The stem represents a greatly
enlarged and elongated mother zooid. It is divided longitudinally by a
partition separating a so-called "ventral" or prorachidial canal from
a so-called "dorsal" or metarachidial canal. A rod-like supporting
axis o