MOVEMENTS OF IRRITATION. 425 



opposite edge. Under these conditions, therefore, they recede 

 from the light. 



If a shallow plate is filled with water well stocked with Euglenee, 

 and placed near a window, the swarmers collect, if the light is 

 not too intense, on the window margin of the plate. If we rotate 

 the plate through 180 most of the swarmers again rapidly group 

 themselves on the window side. In such experiments with 

 Euglena I have frequently observed such a lively movement of 

 the swarmers towards the source of light, that the directive 

 influence of the light rays could be demonstrated with the same 

 material several times in the course of an hour. 



Under certain conditions, especially when their free forward 

 movement is arrested (e.g. in our moist chamber, by their collec- 

 tion in the illuminated margin of the drop), the shape of the 

 Euglena swarmers changes in a striking manner (metaboly). 

 Euglena viridis as a rule swells in the middle and draws itself 

 out thin at the ends ; the swarmers of other species of Euglena 

 curve into the form of a crescent. 



Suitable material for the observation of swarming movements 

 is also found in Hsematococcus lacustris, an alga which at Jena 

 for example is found in the Leutra, colouring the stones on which 

 it occurs a beautiful red. We place a few of the stones bearing 

 the Hsematococcus in a large shallow dish the bottom of which 

 is only just covered with water, put a sheet of glass over the dish, 

 and let it stand for several days. Then a number of the stones 

 are placed in another vessel and flooded with water. We leave 

 them in the water till the following day, and it will then usually 

 be found that the water contains many red Hsernatococcus 

 swarmers. Like those of Euglena, they are phototactic. If the 

 light is not too intense, and the temperature is favourable (about 

 20 C.), they move towards the source of light. I have satisfied 

 myself that to obtain large numbers of the swarmers, it is impor- 

 tant to keep the stones for a few days, as above described, in a 

 space saturated with moisture, before completely flooding them 

 with water. 1 



Experiments similar to those which we have mentioned may be 

 made with the small green swarmers of Clamydomonas. It is 

 further to be observed that the swarmers in different stages of 

 development may be tuned to light of quite different intensity, 

 a fact which especially in the case of Hsematococcus is not seldom 

 very clearly perceptible. For example, when we study the 



