FURTHER INVESTIGATIONS ON HELIOTROPISM 91 



gills fully unfolded takes them at first for plants bearing a 

 palm-like crown (the gills) upon a long naked stem (the 

 tube). A slight jar, however, causes the animals to draw 

 back their gills rapidly into the tubes. 



When the animal is taken from the sea and kept in an 

 aquarium, it is at first indifferent to the light. This con- 

 tinues until the animal has attached itself by its foot to the 

 bottom of the aquarium a period often of several days. As 

 soon as this has taken place, however, the orienting influence 

 of the light begins to be noticeable. If light falls upon the 

 animal from one side only, heliotropic curvatures make their 

 appearance in the tube. The animal turns its oral pole 

 toward the source of light and bends its tube until the axis 

 of its radially expanded gills lies in the direction of the rays 

 of light. The animal maintains this orientation as long as 

 the direction of the rays of light remains unaltered. 



2. To test more accurately to what extent the direction of 

 the rays of light determines the orientation of the animals, I 

 put them into an aquarium which stood at the window, and 

 which could be completely screened from the light by a zinc 

 box. The outlines of the aquarium are indicated in the 

 drawings (Figs. 7 and 8) by black lines, the outlines of the 

 zinc box by dotted lines. The wall abed of the zinc box 

 could be moved vertically upward, so that the amount of 

 light entering the aquarium could be regulated. The zinc 

 box, the walls of which were painted black on the inside, 

 was so placed over the aquarium that the movable wall was 

 on the window side of the aquarium. If this wall was 

 raised only slightly, as shown in Fig. 8, the rays entered 

 the aquarium almost horizontally. When it was drawn 

 farther up, as in Fig. 7, rays entered from above in addition 

 to the horizontal rays. These were more intense than the 

 rays entering horizontally. 



On December 14, 1889, I put nine vigorous specimens of 



