The internal anatomy of the Hydromedusae shows numerous variations.
The stomach may be altogether lodged in the manubrium, from which the
radial canals then take origin directly as in _Geryonia_
(Trachomedusae); it may be with or without gastric pouches. The radial
canals may be simple or branched, primarily four, rarely six in
number. The ring-canal is drawn out in Narcomedusae into festoons
corresponding with the lobes of the margin, and may be obliterated
altogether (_Solmaris_). In this order the radial canals are
represented only by wide gastric pouches, and in the family Solmaridae
are suppressed altogether, so that the tentacles and the festoons of
the ring-canal arise directly from the stomach. In _Geryonia_,
centripetal canals, ending blindly, arise from the ring-canal and run
in a radial direction towards the centre of the umbrella (fig. 26).
_Histology of the Hydromedusa._--The histology described above for the
polyp may be taken as the primitive type, from which that of the
medusa differs only in greater elaboration and differentiation of the
cell-elements, which are also more concentrated to form distinct
tissues.
[Illustration: FIG. 26.--_Carmarina (Geryonia) hastata_, one of the
_Trachomedusae_. (After Haeckel.)
a, Nerve ring,
a´, Radial nerve.
b, Tentaculocyst.
c, Circular canal.
e, Radiating canal,
g´´. Ovary.
h, Peronia or cartilaginous process ascending from the cartilaginous
margin of the disk centripetally in the outer surface of the
jelly-like disk; six of these are perradial, six interradial,
corresponding to the twelve solid larval tentacles, resembling those
of _Cunina_.
k, Dilatation (stomach) of the manubrium.
l, Jelly of the disk.
p, Manubrium.
t, Tentacle (hollow and tertiary, i.e. preceded by six perradial and
six interradial solid larval tentacles).
u, Cartilaginous margin of the disk covered by thread-cells.
v. Velum.]
The ectoderm furnishes the general epithelial covering of the body,
and the muscular tissue, nervous system and sense-organs. The external
epithelium is flat on the ex-umbral surface, more columnar on the
sub-umbral surface, where it forms the muscular tissue of the
sub-umbrella and the velum. The nematocysts of the ectoderm may be
grouped to form batteries on the tentacles, umbrellar margin and oral
lappets. In places the nematocysts may be crowded so thickly as to
form a tough, supporting, "chondral" tissue, resembling cartilage,
chiefly developed at the margin of the umbrella and forming streaks or
bars supporting the tentacles ("Tentakelspangen," _peronia_) or the
tentaculocysts ("Gehörspangen," _otoporpae_).
[Illustration: After O. Maas in _Results of the "Albatross"
Expedition_, Museum of Comparative Zoology, Cambridge, Mass., U.S.A.