HELIOTROPISM IN EUGLENA 397 



straight ahead but that it is only through the turning on its long 

 axis that the resultant of all its various swervings may become a 

 straight line. 



But, while the motions of the rope appear to be identical with 

 those of the flagellum, the mechanism is certainly different. For 

 in the flagellum the loop may stop when it is half way down the 

 flagellum and then go on again. It looks as if the contractile tissue 

 of the flagellum were arranged in a spiral as it is in the stem of 

 Vorticella. In this case, however, the whole of the spiral does not 

 contract at once as in Vorticella, but a wave of contraction pro- 

 gresses from the proximal to the distal end.^ 



The shock-movements are produced by a very simple mechan- 

 ism which acts merely by diminishing in various degrees the cur- 

 vature of the proximal part of the flagellum. In the Irish moss 

 and india ink solutions it could easily be seen that during the 

 usual shock-movement the flagellum extended at right angles to 

 the body (fig. 2, b) ; and that when the organism moved backwards 

 the fiagellum was extending forwards (fig. 2, c). The behavior 

 of the flagellum during slight widenings of the spiral was not 

 directly observed but there can be little doubt that it is caused 

 by the flagellum being directed obliquely backwards. Thus we 

 see that apart from the contraction of the body, the character- 

 istic response of the locomotor mechanism to stimulation consists 

 in a diminution in the curvature of the proximal part of the 

 flagellum.^ 



It is clear then that Euglena turns towards the dorsal side be- 

 cause its locomotor mechanism is such that it cannot turn in any 



"It should be noted that in the action of the flagellum of Euglena we have noth- 

 ing of the nature of the stroke and recovery of an oar. Each loop of contraction 

 as it reaches the tip of the flagellum leaves this organ ready for the next so that 

 there is no need for a recovery. Thus the locomotion, in this species of Euglena 

 at any rate, cannot be likened to that of a boat whose bow is provided with a 

 single oar which is dipped first on one side and then on another as is done by Ver- 

 worn ('03 p. 534). But its locomotion must rather be likened to that of a submarine 

 propelled by a backward driving screw situated at one side of the bow. 



^A similar locomotor mechanism has been described by tjlehla ('11, p. 674) in 

 Euglena doses. But in t.his species the flagellum is extended at right angles to the 

 long axis during the normal forward swimming; and points forward in varying 

 degrees in order to bring about the shock-movements. 



