HELIOTROPISM IN EUGLENA 395 



darkness or light for varying periods. Subjection to darkness or 

 light for over night was particular y satisfactory in bringing about 

 reactions that were not obtained in other ways. 



In order to prevent evaporation especially in those cases where 

 the same individuals were observed for several days, the Euglenae 

 were usually examined in hanging drop preparations. 

 • In attempting to repeat these observations it should be borne 

 in mind that, since I have not been working with chemically known 

 cu ture media free from bacteria, it may frequently happen that 

 several cultures prepared of the same material in the same way 

 may give different results. Consequently, to obtain the resu'ts 

 described, it may frequently be necessary to modify the procedure 

 here described. But the main facts are so clear, and there are 

 obviously so many different modifiable photochemical react ons 

 involved, that I feel convinced that any one who wil work with 

 Euglenae possessing different kinds of photo-sensitiveness, and 

 systemically subject them to other modification o the environ- 

 ment beside that of light, wi be able to repeat al the results here 

 described and to bring to ight many additional phenomena which 

 have so far eluded observation. 



4. NORMAL LOCOMOTION 



In an unstimulated condition Euglena viridis swims through 

 the water in a narrow spira' ' onstantly swerving towards its 

 dorsal side (larger lip) which thus faces approximately towards the 

 outer side of the spiral. To many changes in its environment it 

 reacts by the shock-movements' pointed out by Engelmann and 

 Jennings which usually consist 'n stopping, swerving in the usual 

 direction through an angle of 90 degrees or more, and then swim- 

 ming ahead again. If the stimulus is very strong the organism 

 may swim backwards a little before turning, and the body may 

 also contract. In the weakest shock-movements merely a slight 

 temporary widening of the spiral is seen 



All of these movements are produced by the contractions of the 

 single flagellum which emerges from near the anterior end of the 



*Used as synonymous with Engelmann's 'Schreckbewegung' and Jenning's 

 'motor reflex.' 



