HELIOTROPISM 



123 



Fig. 23. 



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, which is based on many observations, the pro- 

 cess of hehotropic curvature is in this case as follows: when the Ught 

 strikes the circle of 

 gills from one side 

 only, in these elements 

 certain chemical re- 

 actions occur more 

 quickly, or to a larger 

 extent, than on the 

 opposite side. This 

 results in correspond- 

 ing alterations 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 (state of contraction) 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 Ught, and this pressure and 

 friction lead to an increased secretion and the formation of a new 

 layer inside the tube. 



Observations on another marine worm which lives in a stony tube, 

 Serpula uncinata (Fig. 24), add an interesting detail.* These worms 

 occur in colonies of thousands whose tubes are in close contact. The 

 tubes of this form differ from those of Spirographis in that they are 

 made of calcium salts (probably carbonates), and are inflexible. Never- 

 theless, these worms are positively hehotropic, Hke Spirographis, and 

 in the ocean all the tubes of a colony are straight and parallel, and 



* Loeb, /of, cii. 



