106 STUDIES IN GENERAL PHYSIOLOGY 



isms always place the axes of their radial organs in the 

 direction of the light rays, as do the radial organs of sessile 

 plants. 



2. The fact that sessile animals, such as the Serpulidas, 

 have a central nervous system, while plants have not, does 

 not bring about any difference in the heliotropic effect. 



3. If the light enters from one side, there are produced in 

 the above-mentioned animals heliotropic curvatures which 

 correspond to those obtained in sessile plant organs under 

 similar conditions. 



4. There are sessile animals which attain these helio- 

 tropic curvatures only during the period of growth, as is the 

 case with certain plants. Sertularia and Eudendrium, among 

 others, belong in this group, in which only the growing parts 

 are able to bend heliotropically ; Serpula uncinata, which is 

 able to change the orientation of its otherwise stiff tube only 

 when the latter is growing, also belongs in this group. 



5. Spirographis Spallanzanii, the tube of which is flexible, 

 is capable of heliotropic curvatures without accompanying 

 phenomena of growth, as are also certain jointed plant organs 

 which attain their heliotropic orientation without phenomena 

 of growth. 



Although I do not consider my study of animal heli- 

 otropism ended with this paper, yet I think I have shown 

 that the heliotropism of sessile animals is essentially identi- 

 cal with the heliotropism of sessile plants. 



