6S THE EVOLUTION OF MAN. 



that they are to be regarded as occupying the same relative 

 place, or as being homologous, throughout all the various 

 classes of animals, from the Sponges to Man. This homology 

 of the two primary germ-layers, which is of extraordinary 

 significance, extends throughout the animal kingdom, with 

 only a few exceptions in the lowest class, the Primaeval 

 animals {Protozoa). These animals are of an exceedingly 

 low organization, and do not advance to the stage of form- 

 ing germ-layers, and consequently never form real tissues. 

 The whole body merely consists, either of a single cell, as 

 in Amoebae and Infusoria, or of a loose mass of but slightly 

 differentiated cells, or, as in Monera, it does not even attain 

 a form as hio-h as that of a cell. But from the eo^o^-cell of 

 all other animals two primitive germ-layers first proceed, 

 the outer, animal layer (Ectoderm or Exoderm), and the 

 inner, or vegetative layer (Entoderm), and from these the 

 various tissues and organs arise. This is equally true of 

 Sponges, of the other Plant-animals, and of Worms ; it is as 

 true of Soft-bodied animals (MoUusca), Star-animals (Fchin- 

 oclerma), and Articulates (Arthropoda), as of Vertebrates. 

 All these animals may be comprised under the head of 

 Intestinal Animals (Metazoa), in distinction from the 

 Primaeval Animals (Protozoa), which have no intestine. 



It is perhaps more correct not to place the Protozoa 

 among the true animals at all, but to class them in the 

 neutral kingdom of the Protista, those humblest primaeval 

 beings which are neither true animals nor true plants. 

 According to this view the Metazoa can alone be considered 

 as true animals, and the origin from two primary germ- 

 layers may be held to form the primary character of tho 

 animal kingdom. 



