82 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 midtiform 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 

 Cielomati 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 (coeloma) 

 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 

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

 a juice collected, which penetrated through the intestinal 

 waU. 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 ccelom, 

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



