of a column fixed by one
extremity, called the _base_, to a rock or other object, and bearing at
the opposite extremity a crown of _tentacles_. The tentacles surround an
area known as the _peristome_, in the middle of which there is an
elongated mouth-opening surrounded by tumid lips. The mouth does not
open directly into the general cavity of the body, as is the case in a
hydrozoan polyp, but into a short tube called the _stomodaeum_, which in
its turn opens below into the general body-cavity or _coelenteron_. In
Actinia and its allies, and most generally, though not invariably, in
Anthozoa, the stomodaeum is not circular, but is compressed from side to
side so as to be oval or slit-like in transverse section. At each end of
the oval there is a groove lined by specially long vibratile cilia.
These grooves are known as the _sulcus_ and _sulculus_, and will be more
particularly described hereafter. The elongation of the mouth and
stomodaeum confer a bilateral symmetry on the body of the zooid, which
is extended to other organs of the body. In Actinia, as in all Anthozoan
zooids, the coelenteron is not a simple cavity, as in a Hydroid, but is
divided by a number of radial folds or curtains of soft tissue into a
corresponding number of radial chambers. These radial folds are known as
_mesenteries_, and their position and relations may be understood by
reference to figs. 1 and 2. Each mesentery is attached by its upper
margin to the peristome, by its outer margin to the body-wall, and by
its lower margin to the basal disk. A certain number of mesenteries,
known as complete mesenteries, are attached by the upper parts of their
internal margins to the stomodaeum, but below this level their edges
hang in the coelenteron. Other mesenteries, called incomplete, are not
attached to the stomodaeum, and their internal margins are free from the
peristome to the basal disk. The lower part of the free edge of every
mesentery, whether complete or incomplete, is thrown into numerous
puckers or folds, and is furnished with a glandular thickening known as
a _mesenterial filament_. The reproductive organs or gonads are borne
on the mesenteries, the germinal cells being derived from the inner
layer or endoderm.

[Illustration: FIG. 1. Diagrammatic longitudinal section of an Anthozoan
zooid,

m, Mesentery. s, Stoma.
t, Tentacles. lm, Longitudinal muscle.
st, Stomodaeum. d, Diagonal Muscle.
sc, Sulc