FURTHER INVESTIGATIONS ON HELIOTROPISM 95 



This experiment was made in another aquarium, in which 

 the light rays entered chiefly from above. The animals in 

 the large aquarium of the zoological station at Naples are 

 usually found mainly in this position; the light enters this 

 aquarium chiefly from above. Here, however, where free- 

 swimming forms easily disturb the orientation of Spiro- 

 graphis, it is not always so perfect as when all possible dis- 

 turbing causes are avoided, as in an aquarium used only for 

 such experiments. 



5. It follows from these experiments that gravitation 

 exerts only a slight effect, if any, upon animals which are 

 subjected simultaneously to the effects of light and gravity. 

 It was, however, necessary to discover whether a geotropic 

 erection of the animals would not occur under the influence 

 of gravity alone in a completely darkened room. 



On March 21, 1890, I placed a large number of Spiro- 

 graphis in a horizontal position upon the floor of an aquarium 

 in the dark room. On March 24 most of the animals had 

 attached themselves by their aboral ends to the bottom of 

 the aquarium. The oral ends of the tubes were then elevated 

 until the gills no longer touched the bottom of the aquarium. 

 The axis of the spiral did not stand vertically (as was the 

 case when light fell vertically into the aquarium, or as should 

 have been the case had the animals been geotropically irri- 

 table), but only at a slight angle from the horizontal. The 

 animals remained in this position until the end of the experi- 

 ment, which was interrupted in the middle of April. Gravity 

 therefore has no important influence upon the orientation of 

 Spirographis Spallanzanii. 



6. The contact-irritability of the gills is manifested by 

 the fact that they bend away from solid surfaces. This 

 form of irritability can modify the result of the heliotropic 

 experiments upon the animals. I placed several of the ani- 

 mals upon the floor of an aquarium which was so shallow 



