MOVEMENTS OF AQUATIC MICRO-ORGANISMS 301 



of the vessel. Some interesting details are revealed by the 

 study of Euglense enclosed in narrow glass tubes or shallow 

 glass vessels. 



If a tube about 25 cm. long and 7 mm. in diameter filled 

 with water containing Euglenae in sufficient numbers to give 

 a pronounced green colour be placed in the dark in a hori- 

 zontal position, in a little while the Euglenae will be found to 

 be no longer evenly distributed through the tube but aggre- 

 gated, first into a rather irregular network (fig. 2 c) and a little 

 later into a uniform series of vertical bands (fig. 2, a b). Ex- 

 posure to light quickly breaks up the bands by causing the 

 Euglenae to travel towards the light. Under a low power of 

 the microscope each band is seen to consist of a central dark 

 mass of cells sinking with their hinder end downwards, whilst 

 in the lighter-coloured peripheral area are many cells swimming 



Fig. 4. — Diagram showing the direction of movement of Euglena viridis when 



regularly spaced in groups as shown in figs. 2 and 3. 



The direction of movement in each group is indicated by the arrows. 



upwards. The diagram (fig. 4) shows the direction ot motion 

 in these groups. 



A flat-bottomed glass vessel, say 6 mm. deep, may be com- 

 pletely filled with water tinged green with Euglenae, covered 

 with a glass plate, sealed with vaselin to exclude the air and 

 put in a dark place. A coarse network forms quickly (fig. 3 a), 

 which will in a few minutes appear to break up into evenly 

 spaced circular discs, each with a dark-green central spot or 

 streak of irregular outline (fig. 3 b). The circular discs are the 

 ends of cylinders and the central spot consists of a relatively 

 dense mass of Euglenae all sinking ; the peripheral mass of the 

 cylinder consists of Euglenae more widely spaced, all moving 

 upwards (see fig. 4). The grouping may persist during several 

 days if the external conditions remain unchanged. This simple 

 experiment shows that neither the aggregations nor the up- 

 and-down movements of the Euglenae originate in the action 

 of light, by which, however, they may be greatly affected. 



