soderm.

The Coelentera may thus be briefly defined as Metazoa which exhibit two
embryonic cell-layers only,--the ectoderm and endoderm,--their
body-cavities being referable to a single cavity or coelenteron in the
endoderm. Their position in the animal kingdom and their main
subdivisions may be expressed in the following table:--

I. PROTOZOA.
II. PARAZOA or PORIFERA.
III. METAZOA.
|
+----------+--------------+
| |
Coelentera Triploblastica
= Diploblastica. (including Coelomata).
|
+------+-----------+-----------------+
| | |
Hydromedusae. Scyphozoa. Ctenophora.
|
+--------+----------+
| |
Scyphomedusae. Anthozoa.

In the above-given classification, the Scyphomedusae, formerly included
with the Hydromedusae as Hydrozoa, are placed nearer the Anthozoa. The
reasons for this may be stated briefly.

The HYDROMEDUSAE are distinguished from the Scyphozoa chiefly by
negative characters; they have no stomodaeum, that is, no ingrowth of
ectoderm at the mouth to form an oesophagus; they have no mesenteries
(radiating partitions) which incompletely subdivide the coelenteron; and
they have no concentration of digestive cells into special organs. Their
ectodermal muscles are mainly longitudinal, their endodermal muscles are
circularly arranged on the body-wall. Their sexual cells are (probably
in all cases) produced from the ectoderm, and lie in those radii which
are first accentuated in development. They typically present two
structural forms, the non-sexual hydroid and the sexual medusoid; in
such a case there is an alternation of generations (metagenesis), the
hydroid giving rise to the medusoid by a sexual gemmation, the medusoid
bearing sexual cells which develop into a hydroid. In some other cases
medusoid develops directly from medusoid (hypogenesis), whether by
sexual cells or by gemmation. The medusoids have a muscular velum of
ectoderm and mesogloea only.

The SCYPHOZOA have the following features in common:--They typically
exhibit an ectodermal stomodaeum; partitions or mesenteries project into
their coelenteron from the body-wall, and on these are generally
concentrated digestive cells (to f