These folds or partitions do not meet in the centre, but leave
an open space, which is the main cavity of the body. This open space,
however, occupies only the lower part of the body; for in the upper
there is a second sac hanging to a certain distance within the first.
This inner sac has an aperture in the bottom, through which whatever
enters it passes into the main cavity of the body. A central opening
in the top forms a kind of mouth, around which are radiating tentacles
connecting with the open chambers formed by the partitions within.
Cutting such an animal across in a transverse section, we shall see
the radiation of the partitions from the centre to the circumference,
showing still more distinctly the typical structure of the division to
which it belongs.
[Illustration: Vertical section of a Sea-Anemone of Actinia: _o_, mouth;
_t_, tentacles; _s_, inner sac or stomach; _b_, main cavity; _ff_,
reproductive organs; _g_, radiating partition; _eee_, radiating
chambers; _cc_, circular openings in the partitions; _aa_, lower floor.]
[Illustration: Transverse section of a Sea-Anemone or Actinia.]
[Illustration: Staurophera seen in profile.]
[Illustration: Hippocrene seen in profile.]
[Illustration: Melicertum seen from above, with the tentacles spreading:
_oo_, radiating tubes with ovaries; _m_, mouth; _tttt_, tentacles.]
The second class is that of Jelly-Fishes or Acalephs; and here the same
plan is carried out in the form of a hemispherical gelatinous disk, the
digestive cavity being hollowed, or, as it were, scooped, out of the
substance of the body, which is traversed by tubes that radiate from
the centre to the periphery.
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