442 



EFFECT OF LIGHT 



[Ch. XVII 



must conclude, with Rothert, that here, as in animals, stimu- 

 lation is a process distinct from perception. 



2. Animals. — Phototropism among animals will naturally 

 be limited to elongated, sessile forms. It has hitherto been 

 detected only among hydroids and worms of the family Serpu- 

 lidse. 



a. Serpulidm. — A type of response to light intermediate 

 between phototaxis and phototropism is described by Loeb 

 ('90) for Spirographis spallanzanii. This worm (Fig. 127) 



Fig. 127. — Persistent phototropic curvature in Spirographis spallanzanii. The ani- 

 mals were originally placed horizontally on the bottom of the aquarium, the 

 majority with their heads towards the side, efgh, of the aquarium away from 

 the window. In their further growth the animals curve until their heads are 

 turned towards the light side, ahcd, of the aquarium, and the axes of their 

 gills stand in the direction of the rays of daylight. (From Loeb, '90.) 



builds, from a secretion of the body, cylindrical tubes of some- 

 what elastic nature which are attached at their base to a solid 

 substratum. When tliese worms, in their tubes, are illumi- 

 nated from one side by a pencil of rays, the upper part of the 

 tube comes, within a few hours, to lie in the axis of the pencil, 

 and the gills, which surround the head, are stretched out 

 towards the source of light. This result seems to be due to 

 a sort of phototactic response modified by the sessile habit of 



