64 



TKOPISMS 



When such tubes with their inhabitants are put into 

 an aquarium which receives light from one side only, it 

 requires, as a rule, a day or more until the foot end of 

 the tube is again attached to the bottom of the aquarium. 

 As soon as this occurs, the anterior end of the tube is 

 raised by the worm until the axis of symmetry of the gills 

 falls into the direction of the rays of light (Fig. 24) which 

 enter through the window into the aquarium. 288 When 

 the animal has once reached this position it retains it as 



Fio. 25. — The same animals after the position of the aquarium to the window was reversed. 



long as the position of the aquarium and the direction of 

 the rays of light remain the same. When, however, the 

 aquarium is turned 180°, so that the light falls in from 

 the opposite direction, the animal bends its tube during 

 the next tw T enty-four or forty-eight hours in such a way 

 that the axis of symmetry of its circle of gills is again 

 in the direction of the rays of light (Fig. 25). When 

 the light strikes the aquarium from above, the animals 

 assume an erect position, like the positively heliotropic 

 stems of plants when they grow in the open. 



In these phenomena the mechanical properties of the 

 tube play a role. When the animal is taken out of the 

 bent tube, the latter retains its form. This permanent 



