[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 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, no indication of radiate structure, but a superficial feature
in no way related to the internal organization.
We must carefully distinguish between affinity and analogy among
animals. The former is founded on identity of plan; the latter only upon
external resemblance, produced by similar features, which, when they are
intimately connected with the whole internal organization, as in some
groups, may be considered as typical characters, but when only grafted,
as it were, in a superficial manner on animals of another type, have
no relation to the essential elements of structure, and become at
once subordinate and unimportant.
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