178 Wager: Movements of Aquatic Micro-Organisms. 
containing Euglena viridis to a lateral illumination from a 
window as shown in Fig V., the vessel being carefully shaded 
above and on three sides so as to present the entrance of light 
rays from other directions. The Euglenae will at once move 
towards the window and will accumulate at d. If the vessel 
is now completely turned round, so that the Euglenae are in 
the position d l , and the vessel is then covered with a piece of 
white paper, they will again move towards the source of light 
and will gather once more at d. The amount of light which 
passes through the white paper, although considerably 
weakened, is still sufficient to attract all the Eugleme to the side 
d. If, however, the experiment is repeated, with a sheet of 
white paper covering only that half of the vessel nearest the 
window', the Euglenae will be seen to collect in large numbers 
Fig-. Y. — Shallow vessel containing Euglena?, to show effect of oblique 
illumination ; the arrow at b indicates the direction of the raj's of light ; 
a, dead black cover to ward off extraneous rays of light ; c, sheet of note 
paper ; rf 1 , e and d, positions taken up by the Euglena? under varying con- 
ditions. 
at the edge of the paper e. and only a small proportion of them 
will continue their movement in the weaker light to the point 
d. We have seen in the previous experiment that the light 
which passes through the paper is sufficiently strong to attract 
all the Euglenae, but the last experiment shows that a large 
proportion of them prefer a stronger light and thus appear to^ 
have some power of choice as to whether they will or will not 
pass into a region of less intensity. If they responded merely 
in a mechanical fashion to the directive action of the rays of 
light, they ought all to pass into the region of lower intensity. 
( To be continued.) 
Naturalist,. 
