390 Causes and Course of Organic Evolution 



The earth worms or Ohgochseta consist of 118 genera or 

 thereby, and include many hundreds of species. Most of 

 them are in the truest sense earth-Hvers, but genera Hke Nais, 

 Tuhifex, and Branchinra are fresh-water and may bear gills. 

 Their soft bodies have prevented fossilization, but this need 

 in no way militate against our regarding them as equal in age 

 to, or even older than, the oldest polychaetes. 



The group of the leeches or Hirudinea is made up of about 

 35 genera, only two or three of which are marine and parasitic 

 in rays and sharks, the remainder being fresh-water parasites. 

 Their wide distribution, like the last group, over practically 

 the whole world, is a strong indication that they are parasitic 

 derivatives from free-swimming fresh-water ancestors, which 

 have undergone slow but extensive modification that has 

 suited them to a parasitic life. 



The purely marine group of the Gephyrea is made up of 

 19 genera, the species of which are world-wide in distribution. 



The Kinorhyncha is a small derivative group of about 20 

 species that are all mcarine, and may be derivatives from primi- 

 tive Turbellaria or Gastrotricha. 



The Nematoda that are mostly parasitic include about 6''2 

 genera. There is no indication that they have originated 

 otherwise than in shallow pools, ponds, lakes, or amid moist 

 shaded earth, even though some are now marine. Their high 

 vitality under dessicating conditions, their frequent infest- 

 nient of the roots of land plants, and their occurrence as para- 

 sites in land invertebrates, all favor a land or fresh-water origin. 

 Fifty -four of the genera are fresh-water or parasitic, and eight 

 genera are marine. The allied group Chaetosomatida includes 

 3 marine genera with a few species. The Archiannelida agrees 

 with the last in liabit and genera (3-4). 



The Acanthocephala is made up of four genera, most of 

 the species of wJiich are parasites in fresh-water or in land 

 animals, while a few only are marine. Even one genus like 

 Echinorhynchm includes forms that are parasitic in fresh- 

 water fish, in a species of whale, in the mouse, the toad, in 

 salt-water fish, or in seals. 



