106 Brae her . — Observations on Euglena deses. 
regained their activity and were capable of responding to the light stimulus. 
In none of the experiments was found any sign of encysted forms. 
E . deses is therefore able to withstand drying up to a certain point, 
but as soon as the mud becomes hard and crumbles, the organism is 
apparently killed. 
The Need for Oxygen. 
It has been shown that E. deses will not live in stagnant water. This 
is probably owing to the lack of suitable food and partly to the lack of 
oxygen. 
In order to investigate the second factor separately, a dish containing 
mud and Euglenae was placed under a bell-jar with pyrogallic acid and 
potash. The bell-jar was sealed to the glass plate on which it stood by 
means of wax. 
It was noticed that after three days the organisms ceased to come up 
and down in response to the stimulus of light, and on examination were 
seen to be contracted into very irregular shapes. After a few minutes’ 
exposure to the air they regained their normal appearance. 
A second experiment was carried out in which the organisms were left 
without oxygen for a week, when the organisms were contracted but dead, 
and did not revive on exposure to air. 
Thus when oxygen becomes scarce Euglena is able to resist this 
adverse condition for a time by contraction, but if completely deprived of 
oxygen for some days life is impossible. 
Summary. 
The behaviour of E. deses living on the banks of the Avon is dependent 
on certain external factors, namely, light, tidal flow, and temperature 
changes. 
The influence of light . This influence is a direct one and therefore ot 
great importance. The Euglenae are visible on the mud during day-time, 
but burrow under the surface during the night, which phenomenon can be 
brought about by placing the organisms at any time in darkness. 
The influence of tidal flow. The tidal influence shows itself inasmuch 
as the organism burrows into the mud during the period that it is covered 
by the tide. If the time of high water is not more than three hours after 
sunrise the appearance of the organism is delayed, and correspondingly, if 
the time of high water is not more than four hours before sunset, the re- 
appearance of the Eiglena does not take place that day. Further, E. deses 
possesses a tidal periodicity which causes it to respond to the stimulus for 
about three days after being removed from the actual tidal influence. 
The influence of temperature changes is not so important as those 
