102 STUDIES IN GENERAL PHYSIOLOGY 



stood almost mathematically straight and parallel, upon the 

 floor of the aquarium so that the longitudinal axes a b of the 

 tubes, which had previously been vertical, now had a hori- 

 zontal position (Fig. 12). The light fell into the aquarium 

 from above. I noticed that in the course of the next day 

 the Serpulidge, which like Spirographis presented only their 

 radially arranged gills to the light, bent them strongly 

 upward. Individual tubes then began to grow, and in such 

 a way that the newly formed portions of the tubes all bent 

 upward until the free tip of the tube lay in the direction of 

 the rays of light (which in this case was identical with the 

 direction of gravity), after which the tubes continued to grow 

 in the direction of the rays of light (and of gravity). Within 

 six weeks the entire block was covered with tubes which 

 curved upward; not a single individual had continued to 

 grow in the original direction ab. The figure shows the 

 Serpulida3 curving upward at the free edge of the block. The 

 final effect in this case therefore again corresponds to the 

 theory of geotropism and heliotropism as presented by Sachs : 

 the axis of the gills which react as a radial organ lies finally 

 in the direction of the rays of light (and of gravity). While 

 in the case of Spirographis, however (the tube of which is 

 flexible), this effect was brought about through a change in 

 the orientation of the old tube, the same effect was attained 

 in the case of Serpula (the tube of which is inflexible) only 

 through the heliotropic curvature of that portion of the tube 

 which was in the process of growth. 



In the above-mentioned experiment the direction of the 

 light rays was identical with the direction of gravity. I 

 have not yet been able to decide whether light alone deter- 

 mines the orientation of the tube, or whether gravity also 

 plays a r6le. I hope later to make a series of experiments 

 regarding this point. 



