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 Serpulide, 
have a central nervous system, while plants have not, does 
not bring about any difference in the heliotropic effect. 
3. Ifthe 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. 
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