go INVERTEBRATE ANIMALS. 
CHAPTER IX. 
ACTINOZOA. 
THE second great class of the Cwlenterata is that of the dctin- 
0z0a, comprising the sea-anemones and their allies, the corals, 
the sea-pens, the sea-shrubs, and various other organisms. 
They are all defined as Ccelenterate animals in which there is 
a distinct digestive sac which opens below into the general cavity 
of the body, but is nevertheless separated from the body-walls by 
an intervening space (the “ perivisceral space”), which ts divided 
into a number of vertical compartments by a series of partitions 
or “mesenteries,” to the faces of which the reproductive organs 
are attached. The Actinozoa (fig. 32), therefore, differ funda- 
mentally from the Hydrozoa in this, that whereas in the latter 
the digestive cavity is identical with the body-cavity, in the 
former there is a distinct digestive sac, which opens truly into 
the body-cavity, but is nevertheless separated from it by an in- 
tervening perivisceral space. The result of this is, that whilst 
the body of a Hydrozoon exhibits on transverse section a single 
tube only, formed by the walls of the combined digestive and 
somatic cavity, the body of an Actinozoon exhibits two concen- 
tric tubes, one formed by the digestive sac and the other by the 
general walls of the body. Further, in the Actzzozoa the repro- 
ductive organs are always internal, and are never in the form 
of external processes of the body-wall as in the Hydrozoa. 
In their minute structure the tissuesin the Actzmozoa differ little 
from those of the Hydrozoa. The body is essentially composed 
of two fundamental layers—an ectoderm and endoderm ; but 
there are often well-developed layers of muscular fibres, some- 
what obscuring this simplicity of structure. Thread-cells are 
most commonly present in abundance. Cilia are very generally 
developed, especially in the endoderm lining the body-cavity, 
where they serve to maintain a circulation of the contained 
fluids. The only digestive apparatus consists of a tubular or 
