Lnnibncus Terrestris 223 



REACTIONS TO LIGHT AND PHOTORECEPTORS OF 

 LUMBRICUS TERRESTRIS. 



Walter N. Hess, DePauw University. 



It is generally known that earthworms are sensitive to light, but 

 comparatively little is known concerning the nature and distribution 

 of the photoreceptors. 



It has been shown by numerous workers that worms of this species 

 are negative to all ordinary intensities of light. In fact, the nocturnal 

 habits of these animals indicates that they can distinguish between 

 light and darkness, and since they are nocturnal in habit they must be 

 negative in their normal reactions to light of the intensity of daylight. 



Although lights of ordinary intensities cause these animals to react 

 negatively, lights of very low intensities have an opposite effect. When 

 normal worms were exposed to a light of about .00118 meter candle power, 

 which was covered by a yellow glass, the worms were no longer negative 

 but showed a definite majority of positive reactions. These positive 

 reactions to very weak lights are certainly in keeping with the nocturnal 

 habits of these worms. 



Injury to the brain by removal, by removal of one lobe, or by 

 severing the circumesophageal commissures, resulted in these worms 

 losing their power to react negatively. In fact, under such conditions 

 these worms are fully as strongly positive as normal worms are negative. 



If the ventral nerve cord of an earthworm is severed, as between 

 segments four and five by a slight ventral incision, the animal is now 

 really physiologically double so far as its reactions to light are concerned ; 

 the first four segments being definitely negative and those caudal of 

 segment four are definitely positive. 



These results show that the seat of negative reactions resides in 

 the brain and when this center is injured or destroyed the worms 

 no longer respond negatively but become positive in their reactions 

 to ordinary lights. 



Now let us turn our attention to other experiments in order to 

 discover if possible the regions of the body that are most sensitive to 

 light, for the purpose of determining the distribution and nature of 

 the photoreceptors. 



By the use of ordinary illumination it is possible to demonstrate 

 that worms of this species are most sensitive to light in their anterior 

 regions, somewhat less in the posterior and least of all in the middle 

 portions of their bodies. 



By means of a strong pinhole light, the prostomium and the 

 three anterior segments were found to be most sensitive, of which the 

 prostomium appeared to be slightly more sensitive than the rest. Al- 

 though all segments were found to be photosensitive, each segment, with 

 the possible exception of the first three and the last one or two, was 

 most sensitive in the middle portion of the dorso-lateral region. No 

 reactions were obtained by illuminating the mid-dorsal areas except 

 those of the twelve anterior and the three iiosterior segments. The 



"Proc. 38th Meeting, 1922 (1923)." 



