FURTHER INVESTIGATIONS ON HELIOTROPISM 99 



tube thicker than the other, for the thickness of the wall of 

 perfectly straight tubes varies greatly in different places of the 

 same cross-section. The thickness of the wall is therefore no 

 criterion in answering our question. I can therefore formu- 

 late the following theory of the origin of the heliotropic 



curvature in the tube, only 

 by reserving the right to 

 test, and perhaps modify 

 it later. I believe that, 

 when illuminated from one 

 side only, the animal 

 strives at first to bring the 

 axis of its gills as nearly 

 as possible in the direction 

 FIG of the rays of light. In do- 



ing so the animal perhaps 



bends the tube by aid of its muscular force. Since the tube, 

 however, tends to resume its original position because of its 

 elasticity, the body of the Spirographis must rub more 

 strongly against the concave wall of the tube than against the 

 other. This increased friction brings about a great activity 

 of the skin glands, whose secretion forms the material of the 

 tube. That friction indeed leads to secretion, and with it to 

 the formation of a tube, I have been able to prove directly 

 in the case of the Actinian, Cerianthus membranaceus. I 

 have been able to establish the following facts regarding 

 Spirographis which seem to indicate a similar behavior. I 

 cut small pieces from the tube. The animal was in conse- 

 quence obliged to rub against the cut margins during its 

 movements ; and a copious secretion was indeed formed in a 

 short time, which soon closed the opening with a new mem- 

 brane. There is, moreover, always more or less friction on 

 the anterior margin of the tube when the animal stretches 

 out its head. In fact, the tube grows constantly from this 



