8a THE EVOLUTION OF MAN. 



Anatomy and Ontogeny of the Acoelomi is very fragmen 

 tary, and much too imperfect to enable us to point with 

 certainty to the series of the various stages, we will not 

 attempt a detailed arrangement of them. "We will turn 

 instead to the seventh stage in the human pedigree, which 

 belonged to the multiform group of the Blood-bearing 

 Worms (Coelomati). 



The great organic advance in structure by which the 

 Blood-bearing worms, or Coelomati, developed from the 

 older Bloodless Worms, or Acoelomi, consisted in the for- 

 mation of a body-cavity (coeloma), and of a nutritive juice 

 filling the latter, the first blood. All the lower animals 

 with which we have yet occupied ourselves in our Phy- 

 logeny, all the Primitive Animals and Plant-animals, are, 

 like the Acoelomi, bloodless and without a body-cavity. In 

 the formation of a special vascular system, the earliest 

 Coelomati made a very great advance. Much of the com- 

 plexity in the organic structure in the four higher tribes of 

 animals is based on the differentiation of the vascular 

 system, which they have inherited from the Blood-bearing 

 Worms. 



The first development of a true body-cavity (axlomd) 

 is referable to the separation of the two fibrous layers ; to 

 the formation of a spacious cavity between the outer skin- 

 fibrous layer and the inner intestinal-fibrous layer. In the 

 fissure-like gaps, which formed between the two germ-layers, 

 a juice collected, which penetrated through the intestinal 

 wall. This juice was the first blood, and the gaps between 

 the two germ-layers formed the first rudiment of the body- 

 cavity. The union of these gaps formed the simple coelom, 

 the large cavity, containing blood or lymph, which plays m 



