from the centre to the circumference, showing

Transverse section of a Sea-Anemone or Actinia.

still more distinctly the typical structure of the division to which it belongs.

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

Hippocrene seen in profile.

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 partitionwalls, 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

Melicertum seen from above, with the tentacles spreading: oo, radiating tubes with ovaries; m, mouth; tttt, tentacles.

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 StarFishes, Sea-Urchins, and Holothurians or Bechesde-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

Common Sea-Urchin, Echinus, seen from above.

the body as in Polyps, or hollowed out of the substance of the body as in Jelly-Fishes, are here enclosed within independent walls of their own,

Echinarachnius, opened by a transverse or horizontal section, and showing the internal arrangement: o, mouth; e e e e e, ambulacra, with their ramifications cm cm cm; wwww, interambulacra.

quite distinct from the wall of the body. But notwithstanding this difference, a transverse sec

tion 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 and the same structural idea.

1

To those already familiar with these animals some technical details showing the absolute identity of structural plan in these three classes of Radiates may not be uninteresting.

Let us therefore return to the Polyps, and look at the Sea-Anemone in a new aspect. Suppose the inner sac to be turned out; it will then present the appearance of a bottle-shaped body, with a row of hollow projections around the base of the neck; the neck itself being the stomach turned outward, so that its inner surface becomes its outer surface. If we now compare this with the Jelly-Fish, placing both in the same attitude, with the mouth either downward or upward, it becomes apparent that the so-called arms surrounding the mouth of the Jelly-Fish correspond exactly to the neck of our bottleshaped animal, with the sole difference that it is split into lobes in the Jelly-Fish, instead of remaining tubular as in the Polyp. There are, however, many Jelly-Fishes in which it is strictly

tubular as in the Polyps. To carry the comparison further, widen the partitions between the chambers of the Polyps, and the chambers are then reduced to narrow tubes, which completes the homology. In Echinoderms the difference consists, as we have seen, in the fact that the various cavities of the body, instead of being simply scooped out of its substance, have walls of their own; these walled cavities being enclosed as intestines by the outer wall of the body. I shall return to this subject again, when I explain the homologies of Radiates more in detail, but have thought it well to allude to it here in connection with this more general sketch of their structure.

I have mentioned only three classes of Radiates. Cuvier had five in his classification; for he had placed among them the Intestinal Worms and the Infusoria or Animalcules. The Intestinal Worms are much better known now than they were in his day. Their anatomy and embryology have been traced, and it has been shown that the essential features of these parasites are the same as those of all Articulates, their whole body being divided into successive movable joints or rings. Cuvier was misled by the circular arrangement of certain parts around the mouth, and by the presence of a wreath of feelers around the head of some of these Worms, resembling the tentacles of many Radiates. This is, however,