D. W. Cutler 443 



substance surrounding eacli particle, which was characterised by an 

 increased percentage of CO2 and nitrogen and the absence of oxygen. 

 If therefore there are anaerobic protozoa in the soil, and experiments in 

 this laboratory indicate that such is the case, this second dissolved atmo- 

 sphere provides a suitable environment. Also the physical conditions of 

 the water around soil particles may differ from those in the free spaces 

 of wet soil : how far, however, these factors will influence the life of the 

 protozoa requires investigation. 



Finally, these experiments have a distinct bearing on the physio- 

 logical condition vaguely termed "positive thigitiotaxis," or the tendency 

 for small living organisms to adhere to hard surfaces. This is a widespread 

 phenomenon occurring both in plants and animals. A case is recorded by 

 Verworn of a small ciliate — Oxytricha — which coming into contact with 

 the egg of a rival mussel (Anodonta) remained on the surface for four 

 hours, unable to leave it until a piece of mud drifted sufficiently near to 

 the egg to allow escape. Jennings has also described how Paramoecia 

 will adhere in countless numbers to a piece of filter paper introduced into 

 the fluid in which they are living. 



Also there is the well known phenomenon of the spermatozoa 

 clustering and adhering to the egg during the process of fertilisation. 

 This is no place to enter into the discussion of this physiological question, 

 but it may be pointed out that the observations can be explained on 

 surface action factors probably of a kind similar to those governing the 

 relation between protozoa and the soil particles. 



Summary. 



1. It has been shown that the direct counting method for soil pro- 

 tozoa devised by Kopeloff and Coleman for use in Hqiiid media gives 

 results entirely comparable with those obtained by a dilution method. 



2. The factors governing the relation between the protozoa and the 

 soil particles are those of surface action, and the capacity of various sub- 

 stances, sand, soil and clay, for retaining these organisms is specific and 

 constant. 



3. Coarse sand is capable of withdrawing per gram approximately 

 145,000 amoebae and flagellates per c.c. from a suspension of any 

 strength. Fine sand withdraws approximately 980,000 per c.c: soil 

 and partially sterilised soil 1,650,000, ignited soil 1,500,000 and clay 

 2,450,000. 



4. These figures are constant for given material and organisms and 



