EXPEKl.MKNTS IN THE KEEPING OF PLANKTON ANIMALS. 573 



In the second experiment (-002% solution) the cloudiness still con- 

 tinued on the 68th day, and the peptone being then renewed, there was 

 no falling off in the Bacteria on the 90th day afterwards. 



In the third experiment (-01% solution) the cloudiness was un- 

 changed on the 47th day, and the water was then inoculated with 20 c.c. 

 of a culture of a Ciliate, Euplotes sp. On the 9th day subsequently the 

 water was found to be cleared of Bacteria, the Euplotes being 

 present in very large numbers, but owing to my temporary absence 

 in the interval it was not known at what period exactly the 

 water became cleared. This experiment is of interest as exemplifying 

 the destructive action of a Ciliate on a Bacteria culture of Ions 

 standing. 



In all experiments in which Bacteria were grown on peptone, or on 

 the macerating remains of dead plankton, the same action occurred 

 on the part of Infusoria, either naturally present or introduced into the 

 water. Those purposely introduced were from mixed cultures only, in 

 which one or more species were especially prominent. Such a culture 

 was that of Euplotes, which was kept for a long period in a flask in Posi- 

 tion A. Another large species, apparently a Strombidium, was usually 

 obtained in large numbers in " outside " water in which Bacteria were 

 grown, but some difficulty was encountered in retaining this indefinitely 

 as an active culture and often, after a varying period of multiplication, 

 it was lost sight of or died, being then usually superseded by a culture 

 of smaller forms. 



Experiments were made partly in sterilised, partly in unsterilised 

 water. Of those here to be considered, 12 were made in 1-litre flasks 

 in Position A, and 2 were made in ll-Htre bell-jars in Position B. Pep- 

 tone was used in proportions varying from -001% to -0001%, usually 

 in the former percentage, the Bacteria growth being induced by it 

 in some cases once only, in others as many as six times in the same 

 experiment. 



The period occupied by Infusoria in clearing the water varied some- 

 what in the different experiments, apparently also in proportion to the 

 strength of the peptone. Not always exactly observed, the period 

 ranged in 15 exact records, from 3 to 8 days, the average being 6 days, 

 from the time when the peptone was added, the Bacteria usually as- 

 suming a very strong growth within 24 hours later. 



In some experiments in which this cycle of events was repeated a 

 few times, a point was reached when the culture became " sick " and 

 stagnant, the Bacteria growth being feeble, and the Infusoria falling off. 



