98 STUDIES IN GENERAL PHYSIOLOGY 
curvature. The wall on the outer concave side of the tube is 
therefore permanently shortened. It might seem that the 
limit of elasticity of the tube is so low that it retains, like a 
piece of lead, a curvature imparted to it through the muscular 
force of the animal. But this is not thecase. I puta thick rod 
of lead into the straight tube of a Spirographis and bent it till 
the tube was strongly curved. The lead rod was allowed to 
remain in the tube. When a week afterward I withdrew the 
rod from the tube, it retained only a trace of the curve 
impressed upon it. Similar failure followed my attempt to 
straighten by the same method, a heliotropically curved 
tube. Yet, as I have already shown, Spirographis is able to 
straighten its curved tube within a few hours after a change 
in the direction of the rays of light, and, what is more, the 
tube remains straight. The tube retains its curvature even 
after it has been split open. The animal has, however, 
besides pressure and pull, another means at its disposal to 
change permanently the orientation of the tube, namely, the 
production of a secretion and the formation of a new layer 
within the tube. The idea that permanence in the curvature 
is attained in this way is supported by the fact that the 
inner layer of the tube is much more elastic than the outer 
layers, so that the formation of a new inner layer on one 
side of the tube might curve it permanently. The following 
fact supports this view: If a tube is cut open lengthwise, 
the cut margins roll inward. If the individual layers are 
separated, as can be done easily, it is seen that the tendency 
of the inner layers to curl up is greater than that of the 
outer layers, and that of the innermost, newest layer is the 
greatest of all. The formation of a new inner layer on one 
side of the tube would, therefore, be sufficient to maintain 
the curvature of the tube permanently. The formation of a 
new layer cannot be observed directly. One is also disap- 
pointed in the hope of finding one side of the wall of the 
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