34 INTRODUCTION. 
and from this ring the nerves proceed, like rays, to the 
* periphery of the body. The entire body presents a radiant 
form, for the similar parts are not arranged, as in the Articu- 
lates, behind one another in rings, but beside one another in 
a plane. When muscles are present, they are attached to 
the external, sometimes calcareous, covering of the body. 
Let it not be imagined that this modification is insignificant : 
and that the invertebrate animals might very properly be opposed 
to the great division of vertebrate animals, and be afterwards split 
up into three sub-divisions. Such symmetrical separations are 
usually deceptive, and can only be of real service when the two 
groups are of equal rank, and are distinguished by positive charac- 
ters. Beyond doubt, all natural bodies, for instance, are either 
Animals or Non-animals: but who, on that account, would think 
of separating these bodies into an Animal Kingdom, and a Non- 
animal Kingdom? and the Non-animals again into Plants and 
Minerals? Of like value was, in my opinion, the separation of the 
Animal Kingdom into Vertebrate Animals and Invertebrate Ani- 
mals: the latter division meaning only “other than vertebrate 
animals ;”’ it is an indefinite appendage to a defined group, and 
contains no general idea that can be contrasted with another general 
idea. 
But what is especially to be attended to in these four great 
divisions of the animal kingdom is this: that they are not so much 
distinguished by greater or less perfection of organisation, as by 
general form, and by the manner in which the parts respectively 
are related to one another. A great variety of tissues, of organs 
and of subordinate parts, makes the organisation more complicated 
or perfect; but that must be distinguished from the general form, 
from the plan of the organisation. Cuvier did not overlook this 
truth: and even the name of Hundamental Forms (Types), which he 
is careful to use for these four great divisions, indicates the guiding 
idea which led him to adopt them. In each type there is a gradual 
rise and fall of organisation: we descend, says Cuvier, in the type 
of the Molluscs, from the sepia to the oyster, as in that of the verte- 
brates, from man to the fish. But it must not be overlooked, that 
Cuvier did not always sufficiently distinguish the two ideas (the 
Type and the Perfection of organisation), and to this it is to be 
ascribed that his division of adiates comprises many animals which 
