76 THE EVOLUTION OF MAN. 



sixth ancestral stage of the human race, the stage imme 

 diately following the gastrula. The name "Acoelomi ' 

 signifies " Worms without a body-cavity, or coeloma," and 

 therefore without blood, or vascular system. The extant 

 Acoelomi are generally included in a single class, which, on 

 account of their flattened bodies, are called Flat-worms 

 {Plathelminthes). To this class belong the Gliding- worms 

 (TurbeUaria), which live independently in the water; also 

 the parasitic intestinal Sucking-worms (Trematoda), and 

 the Tape- worms (Cestoda), which have become yet more 

 degraded *by parasitism. The phylogenetic relations of the 

 three forms of Flat- worms are very evident ; the Sucking- 

 worms originated from the free Gliding- worms by adaptation 

 to a parasitic mode of life ; and, by a yet more completely 

 parasitic life, the Tape-worms originated from the Sucking- 

 worms. These are striking examples of the gradually 

 increasing degeneration of the most important organs. 



In addition to these well-known extant Flat- worms, 

 great numbers of other Acoelomi must have lived during 

 the Archilithic ^poch, which in general form were very 

 much like those of the present day, but were, in some 

 respects, yet more simply organized, and were, in their 

 lowest stages of development, immediately connected with 

 the Gastraeads. The whole of these lowest Acoelomi, among 

 which the common parent-form of the whole Worm tribe 

 (the Prothelmis) must have been, may be classed as " Primi- 

 tive Worms " (Archelminthes). 



The two classes of the Acoelomi, the Primitive Wo^m^ 

 and the Flat-worms, represent in their externa) form the 

 simplest bilateral condition of the animal body. The 

 body is a simple oval, usually somewhat flattened, with- 



