THE CLASSES OF ANIMALS. 131 



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 to 

 divide the whole vegetable kingdom into six main classes 

 aAd 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 132 and 133. 



The group of Primceval 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 Gregarinse, 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 

 kingdom 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 



