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248 THE EVOLUTION OF MAN. .; 



t 

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



round it in their simplest original form (Entoderm and '] 

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

 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 .1 

 very simple cell, as in Amoebse and Gregarinse, or a colony f 

 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 |j 

 highest degree of physiological perfection among Protozoa, || 

 do indeed appear to have an intestine with a mouth and Ji 

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

 siderable differentiation of its individual parts, retains only *| 

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

 physiological food-canal with its openings, with the true y 

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

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



We must therefore primarily divide the whole animal 

 kingdom 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. This 



