218 



PROCEEDINGS OP THE AMERICAN ACADEMY. 



responsive to a very wide range of intensities. They would probably 

 continue to respond negatively to still stronger light, until the light 

 became strong enough to kill the animal. They respond positively to a 

 light (series 16) less than one three millionth part as intense as the 

 strongest intensity experimented with. The response to the weakest 

 intensity used (series 17) is less than the positive migration in the dark. 

 Hence we cannot speak of this as a phototactic response. This attenua- 

 tion of light was so weak that I could not be sure I saw it myself, and 

 had constantly to reassure myself by approaching it. The slug is evi- 

 dently sensitive to a very minute degree of light. 



Where does the slug's sensitiveness reside? The first and most 

 natural answer is, that the eyes are the important organs. The matter 

 was tested on five different individuals. The normal phototactic response 

 was first taken with a .676 candle power. Then the dorsal tentacles, 

 bearing the eyes, were snipped off with scissors and the animal again 

 experimented on. The results are given in Table XIII. 



TABLE XIII. 

 Effect of Amputation of Tentacles. 



As soon as the operation was performed, the stumps were retracted, as 

 the tentacles are when stimulated by touching, or by strong light. After 

 a moment or two, the animal again rolled out the stumps and began 

 moving forward in perfectly normal fashion, as though nothing had 

 happened. The only observable difference was a perhaps slightly in- 

 creased activity. This table (XIII.) shows a striking effect of the 

 amputation on the phototactic response. In some cases, the animal 

 deviated but very little either positively or negatively from its original 

 position, but kept on moving ahead in a straight line. In other cases, 

 the amputation seemed to cause a change from a strongly negative to a 



