[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.  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.]