Br acker .—Observations on Euglena deses. 107 
mentioned above, as E. deses is active at any temperature between 2*5° C. 
and 25° C. Outside these limits its movements are arrested. By raising 
the temperature the movements of the organism are accelerated within 
certain limits, the optimum temperature being about 15° C. 
Conclusion. 
One would conclude from the foregoing experiments that the life of 
Euglena deses is dependent on a series of movements which the organism 
performs each day. With regard to the physiological reasons for such 
movements very little can be said, but it is worthy of note that the two 
series of movements, namely, those in response to light and those in response 
to the tide, are different in nature. The influence of light is a direct one, 
for Euglenae placed in the dark at any time disappear, to reappear when 
replaced in the light. There is no evidence of any periodic effect, for in 
cultures placed in the dark overnight and left there during the next day 
the Euglenae do not reappear at the accustomed time in the morning. As 
soon, however, as the culture is replaced in the light the organisms come to 
the surface. From this one would think that the periodic tidal movement 
is not dependent on light conditions, but is rather due to the shock of 
the waves, as is thought by Bohn (2) to be the case in Convoluta roscof- 
fensis. 
Finally, it is very interesting to observe how the requirements of such 
an organism as E. deses are so well suited to the conditions under which it 
lives. Being unable to swim freely in water, it cannot live at the bottom of 
ponds or stagnant pools, or it would be unable to obtain sufficient oxygen to 
carry on its life processes, and on the other hand it is unable to withstand 
drought and could not live in mud which is liable to become completely 
dried in the summer. The periodic tidal flow is therefore an ideal condition, 
since the organism is kept moist and has also long periods of exposure 
when it is able to carry on respiration to a greater extent than when covered 
by water. The tide tends to keep the mud at fairly equable temperatures, 
and this is of distinct advantage, since the organism is unable to withstand 
extremes of temperature. 
Being thus so well adapted to its mode of life, the organism is able to 
exist in the vegetative state all the year round, and this may account for the 
fact that no encysted forms, so common in E. viridis during dry seasons, 
have as yet been observed here. 
In the struggle for existence the organism has evolved a structure 
which enables it to live in an habitat where few other organisms occur, and 
so the seasonal dying out of the species, which occurs in ponds when one 
form of organism supplants another, plays no part here, and the organism is 
present in immense numbers all the year round. 
