The Effect of Gravity wpon Euglena viridis, ete. 95 
which tends to inhibit this, and to cause sluggish movements, such as a very 
low temperature, may entirely prevent the aggregation ; but so long as the 
slightest movement takes place in the organisms, an aggregation may be 
visible, although it may be very indistinct, and only recognisable as a delicate 
ripple-mark like effect. 
The aggregation does not appear to be due to any marked extent, and 
probably not at all, to currents set up in the liquid either by heat or evapo- 
ration ; and it appears not to be dependent upon the presence or absence of 
oxygen or carbon dioxide, except in so far as these may be necessary to 
maintain the motility of the organism. 
The downward movement appears to bea purely mechanical one, dependent 
upon the specific gravity of the organism, and is not due to a stimulus which 
evokes a physiological response as in geotropism or geotaxis. ‘The upward 
movement is, on the other hand, due partly to the activity of the organisms 
themselves, partly no doubt to the upward currents set up in the liquid by 
the friction of the downward moving stream. The upward movement of 
Euglena appears to be controlled, so far as the orientation of its elongate 
body is concerned, by the action of gravity. If not crowded together, the 
motile ceils of Huglena may move in any direction in space, but if anything 
interferes with their movements the pull of gravity immediately causes them 
to take a more or less vertical position with the posterior end pointing 
downwards. ‘This is due to the fact that the organism is heavier than water 
and that the posterior end is heavier than the anterior. | 
The network-hke aggregations and groupings resemble very closely in many 
respects the cohesion figures which are formed, under certain conditions, when 
fine sediments of various kinds, such as precipitates of manganese dioxide, 
osmium dioxide, etc., are allowed to settle slowly in a liquid; and the con- 
clusion has been arrived at that such aggregations are probably of the 
nature of cohesion figures, due to the action of gravity upon organisms 
massed together, combined with the vortical movements set up when the 
streaming movements begin to take place. 
In addition to Huglena viridis, experiments were made with colourless 
forms of the same species, with #. deses, Chlamydomonas, Volvox, Bacteria 
(Spirulum), and one of the fresh-water Peridinez, in all.of which the 
phenomena were visible, but with considerable variation in the extent to 
which it was produced. 
It is a remarkable fact that the movements of micro-organisms should be 
controlled in this purely mechanical fashion, but the advantage to those 
species which, like Huglena, are often found in a confined space in very laree 
numbers must be very great, as by its meaus a constant circulation of them 
it 
