FuRTHER INVESTIGATIONS ON HELIOTROPISM 97 
radial plants are orthotropic; 7. ¢., they place their longi- 
tudinal axes in the direction of the rays of light, or of 
gravity. It will have occurred to the reader that Spiro- 
graphis, the body of which, like that of all Annelids, is built 
on the dorsiventral and not on the radial plan, reacts toward 
the light as a radial plant organ. I have, however, already 
emphasized the fact that only the radially arranged gills of 
the animal are exposed to the light, while the remainder of 
the animal is inclosed in the tube. These observations, 
therefore, show that a radial animal organ also obeys the 
law of orientation established by Sachs for plants (even 
though Spirographis possesses a central nervous system, 
which the plant does not). 
It is also of physiological interest that the respiratory 
organ of Spirographis is so highly sensitive to light that 
the orientation of the whole animal in space depends 
essentially upon this sensitiveness. This fact may perhaps 
explain why Branchiomma, a Serpulida quite similar in 
structure to Spirographis, has well-developed eyes upon its 
gills. 
9. If Spirographis is carefully removed from its tube, it 
is not able to raise its body from the floor. In such a con- 
dition it creeps about like an earthworm, only much more 
slowly. I have occasionally seen such animals creep to the 
window side of the aquarium. They appeared, however, to 
suffer from contact stimuli, to which they were constantly 
exposed in this condition ; they all died within a few days. 
10. I am not in a position to make a definite statement 
concerning the mechanics by which the heliotropic curvature 
of the tube is brought about in these animals. The wall of 
the tube of an adult animal is 1.25-1.5 mm. or more thick. 
It is very flexible and elastic. If the animal ts taken out of 
the tube after the latter has been bent through the heliotropic 
reactions of the animal, the tube nevertheless maintains tts 
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