SUMMARY 



Methods of investigation. 



1. The methods used in the study of anaerobic life in 

 invertebrates involve: (a) The determination of the oxy- 

 gen content in the normal surroundings of the animals, 

 that is, in terrestrial, aquatic and parasitic habitats; (b) 

 The establishment of experimental anaerobic conditions, 

 the chief procedures being evacuation of air, absorption of 

 oxygen and replacement of oxygen by inert gases; (c) 

 The determination of the type of metabolism (whether 

 and to what degree it is aerobic or anaerobic). 



Anaerobic habitats. 



1. A severe lack of oxygen is rare in soils. 



2. The bottom material of both fresh-water and salt- 

 water basins is often without molecular oxygen and rep- 

 resents an important anaerobic habitat. 



3. Circulation ceases periodically in many lakes, with 

 the result that the animals living in the stagnant strata 

 frequently cannot get any oxygen for long periods. 



4. In the oceans, there is, at a certain depth, a layer 

 of minimum oxygen concentration, which seems quite im- 

 portant for anaerobic life. 



5. The intestinal tract of vertebrates represents a par- 

 asitic habitat in which only very small amounts of oxy- 

 gen are available; the conditions seem to be more varia- 

 ble in the intestines of invertebrates. 



6. The tissues and secretions of the body of host ani- 

 mals constitute, in some cases, a partly anaerobic habitat 

 for their parasites. 



Protozoa. 



1. Non-parasitic protozoa leading an exclusively or 

 predominantly anaerobic life occur in the sapropelic hab- 

 itat, in sewage tanks and in the stagnating strata of lakes. 



