248 THE EVOLUTION OF MAN. 



mouth), and of the two primary germ-layers which sur« 

 round it in their simplest original form (Entoderm and 

 Exoderm). But in none of the Protozoa are there germ- 

 layers, and therefore no primitive- intestine. The entire 

 protozoan body is formed either of a very simple cytod, a 

 little shapeless mass of protoplasm, as in the Monera, or a 

 very simple cell, as in Amoebae and Gregarinse, or a colony 

 of simple cytods or cells (as in most Protozoa). But in the 

 last case the cells of this cell-community are either entirely 

 homogeneous, or but slightly differentiated, and never 

 separated into true germ-layers. A real intestine never 

 appears in the Protozoa. The Infusoria, which reach the 

 highest degree of physiological perfection among Protozoa, 

 do indeed appear to have an intestine with a mouth and 

 vent. But as the entire body, notwithstanding the con- 

 siderable differentiation of its individual parts, retains only 

 the form-value of a simple cell, we cannot compare this 

 physiological food-canal with its openings, with the true 

 many-celled intestine, which in other animals are morpho- 

 logically characterized by their covering of germ-layers.^^ 



We must therefore primarily divide the whole animal 

 kino^dom into two main divisions : on the one side the 

 Protozoa, without a primitive intestine or germ-layers, 

 without yelk-cleavage or differentiated many-celled tissues ; 

 on the other side, the Intestinal animals (Metazoa) with 

 intestines, with two primary germ-layers, with yelk-cleav- 

 age, with differentiated many-celled tissues. The Intestinal 

 animals, or Metazoa, in which we include the six higher 

 groups of animals, have all descended from the Gastrsea, 

 the previous existence of which may be, even at this day, 

 proved with certainty by means of the Gastrula. Thia 



