98 Brae her. — Observations on Euglena deses. 
regulating the tap the duration of high tide may be altered to suit the 
experiment. 
As the greenness of the mud varied from hour to hour, readings were 
taken, and for this purpose a colour scale was made consisting of four colours 
to which the numbers o, 5, 10, and 15 were given respectively. 
Shade 15 = Brightest green colour on mud. 
Shade 10 = Green colour shown when Euglenae are beginning to go 
down. 
Shade 5 = Very pale green when nearly all Euglenae have dis- 
appeared. 
Shade o = Pure grey colour of mud. 
By practice it was possible to read which shade the mud most closely 
resembled at the time when the readings were taken. These readings were 
Fig. 4. Ihe graph represents the intensity of greenness on the mud under normal conditions 
with reference to the periods of high water (depicted black) and of darkness (depicted shaded). The 
intensity of greenness is measured according to the following scale: 15 (bright green), 10 (green), 
5 (pale green), and o (pure grey colour of mud). The observation extends over eleven days, the 
divisions of which are marked at noon (N.) and midnight (Mid.). 
plotted on a graph and a curve obtained, while the periods of high tide, 
and also those of day and night, are shown in order to compare the 
appearance of the mud with these (Fig. 4). 
Starting on Monday it may be seen that the organisms make their 
appearance about two hours after sunrise and attain their maximum number 
on the surface between the hours of 8 a.m. and 1 1.30 a.m. About half an hour 
before the tide comes in they begin to go down and completely disappear 
