210 Wager: Movements of Aquatic Micro-Organisms. 
grating spectra with the same results, but they do not say 
whether they used an open slit or a closed one, a pure spectrum 
or an impure one. There seems to be no doubt that with 
some organisms, Daphnia for example, there is a tendency 
to accumulate in the yellow and green, but my experiments 
show both with Chlamydomonas and Barnacle larvae, and 
also with Euglena viridis, that in a pure spectrum produced 
with a narrow slit, there organisms accumulate in the blue or 
blue-green, but where the slit is opened and the spectrum 
become less pure they migrate towards the yellow end and 
gather in the green or yellow-green portions of the spectrum. 
If we cut off the blue end of the spectrum, Euglenae tend 
to gather in the green ; if both green and blue are cut off they 
accumulate in greater density in the yellow' and orange ; if 
yellow' and orange are also eliminated, they behave in the red 
light as in the dark. Similar results are obtained with coloured 
filters. When we expose Euglenae to the rays transmitted by 
red and blue filters respectively, they go to the blue ; red and 
yellow', they go to the yellow'; yellow and green, they go to 
both in about equal proportions ; green and blue, the}' are 
attracted more by the blue than the green ; blue and violet, 
they accumulate mainly in the blue. 
We have now to inquire how' it is that Euglena is enabled 
to respond to the stimulus of light. As we have seen, the 
locomotion of Euglena is brought about by means of a specially 
developed vibratile organ, the flagellum. The movements of 
this organ are in all probability set up by chemical or physical 
changes taking place in the protoplasm of the cell and are 
ultimately traceable to the release of the stored up energy 
contained in the protoplasm. They always originate spon- 
taneously w’ithin the organism and are only indirectly traceable 
to external conditions. It is evident therefore that those rays 
of light which are capable of exerting a material influence 
upon the direction of locomotion of the cell can only do so by 
controlling or modifying in some w'ay the mechanism by means 
of w'hich the movements of the flagellum are brought about. 
We have already seen that the flagellum arises from the 
protoplasm at the anterior end of the cell in close relation 
to the red eye-spot. Immediately in front of, and in close 
contact with, the inner concave surface of the eye-spot is a 
bright refractive granule which is borne on the flagellum. We 
have thus a combination of two structures, a red pigment spot 
and the flagellum enlargement w'hich may possibly function 
as a rudimentary light perceiving mechanism, by means of 
which the light responses of Euglena are brought about.* 
* Wager. On the Eye-spot and Flagellum in Euglena viridis. Jour. 
Linn. Soc., Vol. 27, 1899. 
Naturalist, 
