us. go, Gonads.
r, Rotteken's muscle.]

In common with all Coelenterate animals, the walls of the columnar body
and also the tentacles and peristome of Actinia are composed of three
layers of tissue. The external layer, or ectoderm, is made up of cells,
and contains also muscular and nervous elements. The preponderating
elements of the ectodermic layer are elongated columnar cells, each
containing a nucleus, and bearing cilia at their free extremities.
Packed in among these are _gland cells, sense cells_, and _cnidoblasts_.
The last-named are specially numerous on the tentacles and on some other
regions of the body, and produce the well-known "thread cells," or
_nematocysts_, so characteristic of the Coelentera. The inner layer or
endoderm is also a cellular layer, and is chiefly made up of columnar
cells, each bearing a cilium at its free extremity and terminating
internally in a long muscular fibre. Such cells, made up of epithelial
and muscular components, are known as epithelio-muscular or
myo-epithelial cells. In Actinians the epithelio-muscular cells of the
endoderm are crowded with yellow spherical bodies, which are unicellular
plants or Algae, living symbiotically in the tissues of the zooid. The
endoderm contains in addition gland cells and nervous elements. The
middle layer or mesogloea is not originally a cellular layer, but a
gelatinoid structureless substance, secreted by the two cellular layers.
In the course of development, however, cells from the ectoderm and
endoderm may migrate into it. In _Actinia equina_ the mesogloea consists
of fine fibres imbedded in a homogeneous matrix, and between the fibres
are minute branched or spindle-shaped cells. For further details of the
structure of Actinians, the reader should consult the work of O. and R.
Hertwig.

[Illustration: FIG. 2.--1, Portion of epithelium from the tentacle of an
Actinian, showing three supporting cells and one sense cell (sc); 2, a
cnidoblast with enclosed nematocyst from the same specimen; 3 and 4 two
forms of gland cell from the stomodaeum; 5a, 5b, epithelio-muscular
cells from the tentacle in different states of contraction; 5c, an
epithelio-muscular cell from the endoderm, containing a symbiotic
zooxanthella; 6, a ganglion cell from the ectoderm of the peristome.
(After O. and R. Hertwig.)]

The Anthozoa are divisible into two sub-classes, sharply marked off from
one another by definite anatomical characters. These are