CHAPTER VIII 



WORMS AND CCELENTERATA 



Genealogical tree of the animals. Descent of animals. Descent 

 of man. Preservation of intermediate forms. The earth-worm. 

 Regeneration. Leeches. Parasitism. Origin of parasites from 

 free animals. Organic changes in parasites. How parasites are 

 conveyed. Exchange of hosts. Life of the chief parasites, 

 trichinae, maw-worms, dochmius, tape-worms, etc. Danger of 

 taenia. Development of the liver - distoma. Friendships of 

 animals. Symbioses. 



We have already passed in review three stems of the 

 animal world : the vertebrates, articulates, and molluscs. 



We may regard these three groups as sisters, as they 

 proceed side by side from a fourth or parental stem. 

 This consists of the worms or vermalians. 



The worms are primitive forms whose origin goes 

 back long before the geological records commence. 

 It is their simple organisation that makes it possible 

 tor stems to develop from them in three such different 

 directions. 



We have, then, so far considered the three chief 



branches of the tree of organic development, and we 



now come to the trunk in which the three branches 



unite. From this point we shall follow the stem down 



to the lowest point of its roots. That is as far as the 



eye and microscope can reach. The ultimate fibres of 



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