PRIMITIVE INTESTINAIi CANAL. 



313 



inner, nutritive, or vegetative layer, and the outer, covering, 

 or animal layer. 



If we try to construct for ourselves an animal body of 

 the simplest conceivable form, possessing such a primitive 

 intestinal canal, and the two primary germ-layers forming 

 its wall, the result is necessarily the very remarkable 

 germ-form of the gastrula, which we have shown to exist 

 in wonderful uniformity throughout the whole animal 



Fig. 274. — Gastrnla of a Chalk-sponge (Olynthcis) : A, from outside; 

 B, in longitudinal section through the axis ; g, primitiTe intestine ; 0, primi- 

 tive mouih ; i, intestinal la^'er, or entoderm ; e, skin-layer, or exoderm. 



series : in the Sponges, Sea-nettles (Acalephce), Worms, 

 Soft-bodied Animals (MoUusca), Articulated Animals (A rthro- 

 poda), and Vertebrates (Figs. 174-179, p. 65). In all these 

 various animal tribes the gastrula reappears in the same 

 entirely simple form (Fig. 274). Its whole body is really 

 merely the intestinal canal ; the simple cavity of the body, 

 the digestive intestinal cavity, is the primitive intestine 



