MOVEMENTS OF AQUATIC MICRO-ORGANISMS 303 



in water. This is what one would expect from the structure of 

 the organism. Though the anterior end is the larger and would 

 therefore tend to turn downwards in a medium of less density 

 than the cell itself, the middle and posterior regions are pro- 

 bably denser, owing to the inclusion in them of the chlorophyll- 

 bodies, the paramylum-grains and the nucleus, whilst the density 

 of the anterior end is very likely lowered by the pharyngeal 

 cavity and the pulsating vacuole. 



The downward movement appears therefore to be the result 

 of the mechanical action of gravity upon an organism heavier 

 than water and does not necessarily involve anything of the 

 nature of a geotactic response. 



The mechanical action of gravity operates only when the 

 organisms are crowded and by crowding I mean that many of 

 them are only separated by about their own breadth. 1 



Euglena can be seen by the microscope to swim in the 

 direction of its axis along a more or less spiral path. Gravity 

 tends to bring it into a nearly vertical position but it is able by 

 its own exertion to resist a mere gravitational pull. If, for 

 example, Euglenae which are swimming in a horizontal direction 

 towards the light are suddenly shaded, the forward movement is 

 slowed and they swerve into a more vertical position. 



A similar effect is observed when Euglenae become crowded, 

 especially in the dark or in a diffuse light. If they are much 

 crowded, their active movement is inhibited ; the action of 



1 " That the downward pull of gravity is not effective until the organisms have 

 become congested into a heap is beautifully demonstrated by the following experi- 

 ment : A small number of motile Euglenae are placed in a shallow cell, which is 

 completely filled with liquid and sealed up with a cover-glass and vaselin. This 

 is now placed under the low power of a microscope placed in a horizontal position 

 [the stage being vertical]. On examination the organisms will be seen moving 

 about freely, if the light is not too strong or if red light is used ; no special 

 tendency to move either upwards or downwards will be noticeable. If a bright spot 

 of light is concentrated upon the middle of the cell from the source of illumination, 

 which can be most easily done by manipulating the substage condenser, the 

 Euglenae will be attracted by it and will in a very short time become congested 

 in the light space. If now the condenser be moved so as to take away the bright 

 spot, leaving the field in a diffuse light only, the congested mass will be at once 

 acted upon by gravity and begin to fall. . . . When they have begun to fall, their 

 downward movement can be stopped almost at once by bringing back the bright 

 spot of light into a position just below the falling stream ; a few will pass beyond 

 it at first but in a very short time the streaming will be stopped and they will 

 again begin to congregate in the light space" (Wager, Phil. Trans. Series B, 

 vol. 20 1, pp. 369-70, 191 1). 



