102 STUDIES IN GENERAL PHYSIOLOGY 
stood almost mathematically straight and parallel, upon the 
floor of the aquarium so that the longitudinal axes ab 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 Serpulide, 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 
away 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), afler 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 
Serpulide 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 role. I hope later to make a series of experiments 
regarding this point. 
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