?8 THE EVOLUTION OF MAN. 



Worm group, the Archelminthes. They inherited this 

 ciliated dress directly from the Gastrsea. 



If we now make various vertical sections (longitudinal 

 and transverse) through the simple body of the Gliding- worms 

 (and that of the Archelminthes which are certainly very 

 closely allied to the former), we soon discover that their 

 internal structure is considerably higher than that of the 

 Gastrseads. We first observe that the two primary germ- 

 layers (inherited from the Gastrsea) have differentiated into 

 several cell-strata. The skin-layer and the intestinal layer 

 have each split into two strata. The four secondary germ- 

 layers, which are thus produced, are the same that we found 

 resulted from the first differentiation of the two primary 

 germ-layers in the embryo of the Vertebrate also. (Cf. the 

 transverse sections through the larval Amphioxus and 

 Earth-worm, Figs. 50 and 51, p. 236, and Plate IV. Fig. 2; 

 Plate V. Fig. 10.) 



The highly important histological differentiation of these 

 four secondary germ-layers led directly to further organolo- 

 gical processes of differentiation, by which the organism of 

 the Primitive Worms was soon considerably raised above 

 that of the Gastrseads. In the latter there was really, in 

 a morphological sense, but a single organ, the primitive intes- 

 tine, with its mouth-opening. The whole body was nothing 

 but an intestinal canal ; the intestinal wall was at the 

 same time the wall of the body. Of the two cell-layers, 

 forming this intestinal wall, the inner accomplished the 

 functions of nutrition, the outer those of motion and 

 covering. As some of the cells of the primary germ-layers 

 developed into egg-cells, and others into sperm-cells, these 

 layers also performed the function of reproduction. In the 



