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.  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 division of the Animal Kingdom on that account.  But even this simplicity was only apparent in many of them.  At certain seasons of the year myriads of these little Animalcules may be seen in every brook and road-side pool.  They are like transparent little globules,