s 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. Such is the difference between the
tentacles in a Radiate and the wreath of feelers in a Worm;--the
external effect may be much the same; but in the former every tentacle
opens into one of the chambers as in a Polyp, or connects with one of
the radiating tubes as in Acalephs, or with the locomotive suckers as
in Star-Fishes, and is therefore closely linked with the whole internal
organization; whereas the feelers in the latter are only external
appendages, in no way connected with the essential structural elements.
We have a striking illustration of this superficial resemblance in the
wings of Birds and Insects. In Birds, wings are a typical feature,
corresponding to the front limbs in all Vertebrates, which are
constructed in the same way, whether they are arms as in Man, or
forelegs as in Quadrupeds, or pectoral fins as in Fishes, or wings as in
Birds. The wing in an Insect, on the contrary, is a flattened, dried-up
gill, having no structural relation whatever to the wing of a Bird. They
are analogous only because they resemble each other in function, being
in the same way subservient to flight; but as organs they are entirely
different.

In adding Infusoria to the Radiates, Cuvier was false to his own
principle of founding all classification on plan. He was influenced by
their seeming simplicity of structure, and placed them in the lowest
d