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 
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 
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