n: 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. Cutting it across transversely, or looking
through its transparent mass, the same radiation of the internal
structure is seen again; only that in this instance the radiating lines
are not produced by vertical partition-walls, with open spaces between,
as in the Polyps, but by radiating tubes passing through the gelatinous
mass of the body. At the periphery is a circular tube connecting them
all, and the tentacles, which hang down when the animal is in its
natural position, connect at their base with the radiating tubes, while
numerous smaller tentacles may form a kind of fringe all round the
margin.

The third and highest class includes the Star-Fishes, Sea-Urchins, and
Holothurians or Beches-de-Mer. The radiation is equally distinct in each
of these; but here again the mode of execution differs from that of the
two other classes. The internal cavity and the radiating tubes, instead
of being connected with the outer wall of the body as in Polyps, or
hollowed out of the substance of the body as in Jelly-Fishes, are here
inclosed within independent walls of their own, quite distinct from the
wall of the body. But notwithstanding this difference, a transverse
section shows in these animals, as distinctly as in all the rest, the
radiating structure typical of the whole branch. In these three classes
we have no difference of plan, nor even any modification of the same
plan,--for either one of them expresses it as clearly as any other,--but
simply three different ways of executing one structural idea.

[Illustration: Common Sea-Urchin, Echinus, seen from above]

[Illustration: Echinarachnius, opened by a transverse or horizontal
section, and showing the internal arrangement: c, mouth; eeeee,
ambulacra, with their ramifications cmcmcm; wwww, interambulacra.]

I have mentioned only three classes of Radiates. Cuvier had five in his
classification; for he placed among them the Intestinal Worms and the
Infusoria or Animalcules. The Intestinal Worms are much better known
now than they were in hi