MOVEMENTS OF IRRITATION. 427 



to indicate. 2 We suck up some water containing Euglenae into 

 a capillary tube 0'5 mm. in diameter, open above and below. If 

 we place the capillary in a vertical position in the dark, the 

 swarmers collect in its upper region. 



It is of interest that the swarmers of many algae are distinctly 

 aerotropic. This has been specially demonstrated in the case of 

 Euglena by Aderhold, and we make the following experiment to 

 acquaint ourselves with the phenomenon. A small dish is filled 

 with water containing large numbers of Euglena swarmers. We 

 also fill a test-tube to the brim with the water, close it with the 

 thumb, and invert it with its mouth under the water in the dish. 

 Light being excluded, it will be found after some time that almost 

 all the swarmers have left the water in the test-tube, and collected 

 in the water in the dish. This is by no means due to positive 

 geotropism or other causes, but is explained by the aerotropism of 

 the swarmers. They strive to reach places in which larger 

 quantities of Oxygen are available. This is very clearly brought 

 out by the following observations. When the swarmers have 

 left the test-tube, we replace some of its water by air, spread 

 a layer of oil on the water in the dish, and place the whole in the 

 dark. The swarmers now pass in considerable numbers from the 

 dish into the test-tube, because here larger quantities of Oxygen 

 are presented to them. 



The organisms known as Oscillarise have a wide distribution 

 in stagnant waters and on muddy soil. They are for the most 

 part blue-green in colour, and occur in the form of filaments. 

 These organisms exhibit various movements which can be closely 

 followed under the microscope. Especially striking are the 

 irregular curvatures which the filaments undergo as they move 

 forwards or backwards. 



In the cases, hitherto mentioned the movements have been due 

 to the activity of the organisms themselves, but there are still 

 to be mentioned a number of curious phenomena which are due 

 to purely passive movements on the part of swarm spores. 



If green-coloured water, containing large numbers of swarming 

 Clamydomonads or Euglenae, is poured on to a plate, which is 

 then covered with a sheet of glass and placed under a cardboard 

 box in the middle of a large room, we shall find after a time that 

 the algae have grouped themselves in the form of concentrically 

 arranged clouds or so as to form other regular figures. If we 

 remove the sheet of glass covering the plate, the figures rapidly 



