SUMMARY 133 



2. Most of these animals are facultative, not obligatory 

 anaerobes. 



3. Many free-living protozoa which normally lead an 

 aerobic life are able to withstand experimental anaerobic 

 conditions for quite long periods of time. 



4. Parasitic protozoa living in the alimentary tract of 

 vertebrates, or in related habitats, can usually be cultured 

 in the complete or nearly complete absence of oxygen. 

 These protozoa, however, are not injured by oxygen 

 (except a few). It seems likely, therefore, that in their 

 natural habitat they undergo changes from anaerobic to 

 aerobic life. 



5. The conditions to which protozoa living in the intes- 

 tine of invertebrates are exposed seem to depend largely 

 on the host species. Thus, termite flagellates appear to 

 live primarily anaerobic lives, while the developmental 

 stages of T rypanosomidae seem to require oxygen. 



6. Blood protozoa and tissue protozoa, which have 

 probably always access to some oxygen in their normal 

 habitat, do not seem to be very resistant in completely 

 anaerobic conditions. However, the metabolism of some 

 forms, e.g., trypanosomes and malaria parasites, is partly 

 anaerobic even in aerobic environments. 



Coelenterates. 



1. Coelenterates do not seem to occur in truly anaerobic 

 habitats in nature, but some species survive rather well in 

 water poor in oxygen. 



2. The regeneration of polyps is inhibited in anoxic 

 media, but the luminescence of some forms does not re- 

 quire molecular oxygen. 



AYorms and worui-Iike organisms. 



1. Non-parasitic worms and worm-like organisms are 

 found rather frequently in oxygen-free or oxygen-poor 

 environments, mostly in the sapropelic habitat, at the 

 bottom of lakes and at the bottom of the sea. 



