Br acker. — Observations on Euglena deses. 99 
as the water rises over them. About half an hour after the tide has receded 
they reappear, regain their maximum number between the hours of three 
and five, and disappear again completely just before sunset. On Tuesday 
the process was repeated, but owing to a decrease in time (about five hours) 
between the ebb of the tide and the sunset, the Euglenae do not reappear to 
their fullest extent and the mud does not become very green. On Wednesday 
this period of time is reduced to four hours, and the Euglenae make no 
reappearance after high tide during that day. On Thursday and Friday 
the same phenomenon occurs as the time is still further reduced. On 
Saturday and Sunday the appearance of the organisms is retarded, since 
the period of high tide extends over the usual time when they rise to the 
surface. On Monday and Tuesday of the second week the periods of time 
between sunrise and high-water are two and three hours respectively, but 
the organisms do not rise to the surface until after the tide has receded. 
On Wednesday the period is increased to four hours, and on this day the 
Euglenae appear on the surface in small numbers for a short time before 
high tide. On Thursday the period between sunrise and high water is still 
further prolonged, and the Euglenae appear for a longer time and in greater 
numbers, but it is not until the tide once more approaches the region of 
midday that the green colour on the mud attains its maximum degree of 
intensity twice in one day. 
Thus it may be seen : 
1. That, other conditions being equal, the Euglenae rise to the surface 
about two hours after sunrise and go down shortly before sunset, burrowing 
into the mud during the day for the period of high tide. 
2. That if the period between the ebb of the tide and sunset is not 
longer than about four hours, the organisms do not reappear during this 
interval. 
3. That if the period between sunrise and high water is not longer than 
three hours, the organism does not appear on the surface until the tide has 
receded. 
Behaviour of E. deses when removed from the 
Influence of the Tide. 
It may easily be seen that those Euglenae which live close to the outer 
edge of the river bank are only covered by the tide for comparatively short 
periods, while those lower down may remain covered with water for several 
hours. Moreover, owing to the fortnightly variations in the tide, it happens 
that during neap tides this outer zone may be exposed for several days 
without being covered by the tide. Material was taken from this region 
and kept in dishes, and it was noticed that the organisms responded only to 
the stimulus of light. In other material, however, which was taken from 
H 2 
