1911.] 



Knowledge of the Protozoa of the Soil. 



171 



"wide, and 2 mm. deep. Examination of the contents of the trough was 

 carried out with a powerful hand lens and with low power objectives. The 

 preliminary trials with this apparatus, using active free-swimming Colpoda 

 and Pleurotricha in a culture, proved that the method was not precise enough. 



A second piece of apparatus was then devised, similar in essentials to the 

 first, but, in place of the glass tubes, three zinc tubes f inch square in section 

 were substituted. These were corked at each end and through the corks 

 glass tubes were passed, and to these, rubber tubes gave connection with the 

 water supply, etc., as before. The zinc tubes were clamped in a horizontal 

 position between two strips of wood, and carefully arranged so as to give 

 good contact with the glass trough when placed on them. Colpoda cucullus 

 and Pleurotricha grandis in a culture liquid were found to respond quite 

 well, especially when the trough was slowly moved more and more on to the 

 tube conveying water at the higher temperature. The tropism was always 

 negative, the organisms moving away from the source of heat towards the 

 region of lower temperature. The movement was fairly precise and general, 

 but there were always a few stragglers, apparently swimming about quite 

 indefinitely. 



A culture containing numerous active ciliated protozoa was mixed with 

 a small quantity of sterilised soil, the mixture put in the glass trough and 

 placed on the tubes. The object of this was to see if it were possible to 

 cause active ciliates to leave the soil particles and collect in the clear liquid 

 at one end of the trough. No satisfactory results were obtained, the 

 presence of soil particles in the liquid appearing to prevent the organisms 

 from responding to the stimulus of heat. They could shelter behind and 

 under the tiny masses of soil, and there can be little doubt that local disturb- 

 ances of temperature were set up also. Moreover, it must be borne in mind 

 that the object of the experiments was to attempt to discover any free-living 

 protozoa in fresh soil. For this reason it was not possible to carry on the 

 experiment for any length of time, owing to the fact that encysted forms 

 might hatch out and so mar the results. Fresh soil was taken on one or two 

 occasions and teased out in sterile 1-per-cent. hay infusion in the glass 

 trough, and treated as described above, but no free-living protozoa were 

 found. The method was therefore discontinued. 



(e) Galvanotaxis. — Verworn* gives an account of the way in which certain 

 protozoa respond to the stimulus of a continuous electric current. It is there 

 shown that when a liquid containing amoebae or certain ciliated protozoa has 

 a continuous current passed through it by means of two non-polarisable 

 electrodes, the organisms move through the liquid and assemble at the 

 * ' General Physiology,' English translation, 1899, p. 455. 

 VOL. LXXX1V. — B. O 



