Wager: Movements of Aquatic Micro-Organisms. 21 1 
We have no definite proof that the red pigment spot is a 
light perceiving organ, but we have some indirect evidence in 
favour of this view in the fact that in all those chlorophyll- 
containing unicellular organisms which are very sensitive to 
light and capable, by means of their flagella or cilia, or res- 
ponding quickly to changes in its direction or intensity, an 
eye-spot is present. Those motile cells which do not possess 
an eye-spot are either not sensitive to light at all, or only to a 
slight extent. Even in the zoospores of Chytridium and 
Polyphagns which are sensitive to light, it is interesting to note 
that at the base of the flagellum there is a conspicuous orange- 
coloured oil globule, which may act in the same way as an 
eye-spot. 
Again, Engelmann has shown for Euglena and Strasburger 
for swarmspores of Algae, that the rays of light which are 
most active in their influence upon the movements of these 
organisms are found in the blue portion of the spectrum, and 
these are just the rays which are absorbed by the eye-spot. 
Further, it has been shown by Engelmann that it is the 
colourless anterior end of Euglena that is sensitive to light. In 
this colourless anterior end of the cell, both the eye-spot and 
the apparatus which directly causes the movement of the cell 
— the flagellum — are placed. 
We may fairly conclude therefore that the eye-spot acts as a 
light absorbing organ, and that the light absorbed is capable 
in some way or other, not at present understood, of setting up 
chemical or physical changes in the cell which react either 
directly upon the flagellum, or through the intermediary 
protoplasm, so as to bring about a modification of its move- 
ments and a consequent change in the direction of locomotion 
of the cell. 
The light responses of numerous micro-organisms are 
frequently modified by the action of gravity. Euglena 
viridis shows this very clearly. If a tube containing water 
with a sufficiently large number of Euglenae to give it a pro- 
nounced green colour is placed upright in the dark, the majority 
of the Euglenae will in a very short time sink to the bottom, 
where they form a dark green layer of .motile cells moving up 
and down. A few will be found to have accumulated at the 
surface of the water where they form a thin green scum. 
These upward and downward movements are due to the 
action of gravity. The organism sinks by its own weight, 
which is slightly heavier than water, and does not swim 
downwards by its own choice. It is therefore a purely 
mechanical response. 
It is frequently stated that aquatic micro-organisms do 
not respond to gravity simply because they are heavier or 
lighter than water but by reason of some kind of sensibility 
1914 July 1. 
