HELIOTROPISM 65 



change of form of the tube is apparently caused through 

 the secretion of new layers on the mside of the tube. The 

 youngest layers of the secretion are more elastic than the 

 old layers, and, moreover, have at first a powerful tend- 

 ency to shorten. If such a secretion occurs on one side of' 

 the tube only, or more so than on the opposite side, the 

 former must become shorter than the latter, and the result 

 must be a curvature of the tube, that side becoming con- 

 cave where the new secretion has occurred. 



On this assumption the process of heliotropic curva- 

 ture is in this case as follows : when the light strikes the 

 circle of gills from one side only, in these elements certain 

 photochemical reactions occur to a larger extent, than on 

 the opposite side. This results in corresponding altera- 

 tions of the sensory nerve endings, the sensory nerves and 

 the corresponding motor nerves, and their muscles. The 

 sense of these changes is such as to throw the muscles 

 connected with the nerves of the gills on the light side 

 into a more powerful tonic or static contraction than the 

 muscles on the opposite side of the body. The consequence 

 is a bending of the circle of tentacles, or the head, toward 

 the source of light, which will continue until the axis of 

 symmetry of the circle of tentacles falls into the direction 

 of the rays of light. "When this happens, symmetrical 

 tentacles are struck at the same angle (or in other words, 

 with equal intensity) by the rays of light, and therefore 

 the tone of the antagonistic muscles is the same. The 

 result is that the circle of tentacles becomes fixed in this 

 position. The bending of the head produces an increased 

 pressure and friction of the animal against that side of 

 the tube which is directed toward the light, and this pres- 

 sure and friction lead to an increased secretion and the 

 formation of a new layer inside the tube. 



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