FURTHER INVESTIGATIONS ON HELIOTROPISM 97 



radial plants are orthotropic ; i. e., 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 is taken out of 

 the tube after the latter has been bent through the heliotropic 

 reactions of the animal, the tube nevertheless maintains its 



