26o Animal Instincts and Tropisms 



turned and compelled to move to or from the light. 

 This difficulty was overcome by the observation that 



Fig. 43 



sessile animals like the hydroid Eudendrium (Fig. 43) 

 or the tube worm Spirographis (Fig. 44) react to a one- 



FlG. 44 



sided illumination also with heliotropic curvatures like 



sessile plants. ' On the other hand, it had been found 



before by Strassburger that free- swimming plant 



» Loeb, J., Arch. J. d. ^es. Physiol., 1890, xlvii., 391; 1896, Ixiii., 273. 



