70 HUXLEY, ON CLASSIFICATION. 
It is not necessary for my purpose that the groups which are 
named on the preceding table should be absolutely and precisely 
equivalent one to another; it is sufficient that the sum of them is the 
whole of the Animal Kingdom, and that each of them embraces one 
of the principal types, or plans of modification, of animal form; so 
that, if we have a precise knowledge of that which constitutes the 
typical structure of each of these groups, we shall have, so far, an 
exhaustive knowledge of the Animal Kingdom. 
I shall endeavour, then, to define—or, where definition is not yet 
possible, to describe a typical example of—these various groups. 
Subsequently, I shall take up some of those further classificatory 
questions which are open to discussion; inquiring how far we can 
group these classes into larger assemblages, with definite and constant 
characters; and, on the other hand, how far the existing subdivisions 
of the classes are well based or otherwise. But the essential matter, 
in the first place, is to be quite clear about the different classes, and 
to have a distinct knowledge of all the sharply-definable modifica- 
tions of animal structure which are discernible in the animal king- 
dom. 
The first class of which I shall speak is the group of. the Gruea- 
RINIDA. These are among the simplest animal forms of which we 
have any knowledge. They are the inhabitants of the bodies for the 
most part of invertebrate, but also of vertebrate, animals; and they 
are commonly to be found in abundance in the alimentary canal of 
the common cockroach, and in earth-worms, They are all micro- 
scopic, and any one of them, leaving minor modifications aside, may 
be said to consist of a sac, composed of a more or less structureless, 
not very well-defined membrane, containing a soft semi-fluid sub- 
stance, in the midst, or at one end, of which lies a delicate vesicle; 
in the centre of the latter is a more solid particle. (Fig. 1, A.) No 
doubt many persons will be struck with the close resemblance of the 
structure of this body to that which is possessed by an ovum. You 
might take the more solid particle to be the representative of the 
germinal spot, and the vesicle to be that of the germinal vesicle ; 
while the semi-fluid sarcodic contents might be regarded as the yelk, 
and the outer membrane as the vitelline membrane. I do not wish 
to strain the analogy too far, but it is, at any rate, interesting to ob- 
serve this close morphological resemblance between one of the lowest 
of animals and that form in which all the higher animals commence 
their existence. It is a very remarkable characteristic of this group, 
that there is no separation of the body into distinct layers, or into 
cellular elements. The Gregarinida are devoid of mouths and of 
digestive apparatus, living entirely by imbibition of the juices of the 
animal in whose intestine, or body cavity, they are contained. The 
