THE CLASSES OF ANIMALS. 13! 
Having thus sketched out the monophyletic pedigree of 
the animal kingdom in its most important features, we must 
now turn to a closer examination of the historical course of 
development which the seven tribes of the animal kingdom, 
and the classes distinguished in them, have passed through 
(p. 132). There is a much larger number of classes in 
the animal than in the vegetable kingdom, owing to the 
simple reason that the animal body, in consequence of its 
more varied and perfect vital activity, could differentiate 
and develope in very many more different directions than 
could the vegetable body. Thus, while we were able te 
divide the whole vegetable kingdom into six main classes 
and nineteen classes, we have to distinguish, at least, sixteen 
main classes and thirty-eight classes in the animal kingdom. 
These are distributed among the seven different tribes of the 
animal kingdom in the way shown in the Systematic Survey 
on pages 152 and 133. 
The group of Primeval animals (Protozoa) within the 
compass which we here assign to this tribe, comprises the 
most ancient and the simplest primary forms of the animal 
kingdom; for example, the five oldest phyletic stages of 
development previously mentioned, and besides these the 
Infusoria and Gregarinz, as well as all those imperfect 
animal forms, for which, on account of their simple and in- 
different organization, no place can be found in any of the 
other six animal tribes. Most zoologists, in addition to these, 
include among the Protozoa a larger or smaller portion of 
those lowest organisms, which we mentioned in our neutral 
kinedom of Protista (in Chapter XVI). But these Protista, 
especially the large division of the Rhizopoda, which are so 
rich in forms, cannot be considered as real animals for 
