50 Herbert Eugene Walter 



A Behavior in Dark 



Darkness may be called the zero point in the scale of light inten- 

 sities. That light is not essential to the activity of planarians is 

 shown by their performances in its absence, as is demonstrated 

 by the following facts. 



Rate of Locomotion. The average rate of ten individuals of 

 Planaria gonocephala was found to be 0.50 mm. per second in the 

 dark while the same ten worms, subjected to a light from above of 

 38 cm., 3 with all the other conditions unchanged, averaged 0.82 

 mm. per second. 



Again, ten worms of the same species were allowed to travel in 

 the dark ten minutes in one set of experiments and six minutes in 

 another, when their average rates were found to be 0.42 and 0.57 

 mm. per second, respectively. 



The method devised for obtaining the above records, previously 

 used in experiments upon fresh water snails (Walter, '06), although 

 tedious was comparatively accurate. A clean glass plate was 

 submerged in a dish of water and the latter placed in a light-proof 

 receptacle. A single worm was then allowed to travel on this 

 glass for a unit of time, after which the plate was removed and 

 "developed" by pouring over it powdered carmine shaken up in 

 water. A sufficient number of the insoluble carmine particles 

 adhered to the mucus-track left on the glass by the gliding worm 

 to make it possible to wipe dry the reverse side of the plate and 

 to trace thereon in ink the exact course taken by the worm. This 

 permanent ink line was then measured by means of a map meas- 

 urer such as is in common use for measuring sinuous lines. 



A series of experiments, to be described more in detail later 

 (Table III, p. 57), forms a basis of comparison with the foregoing 

 records in the dark, and further shows that there is an increase in 

 the rate of locomotion in the light. 



Ten worms, subjected to various intensities of light projected 

 from above and ranging from less than one to several hundred 

 candle meters, showed rates which in all cases were greater than 

 the rate traveled in the dark. 



3 The abbreviation cm. is used to denote candle meters. 



