The Reactions of Planarians to Light 43 



gonocephala, P. cornuta and P. alpina, 2 Voigt ('04) incidentally 

 refers to the manner in which these animals come to rest in the 

 darkest part of a dish. He affirms that when an aquarium is sud- 

 denly lighted at night only those that are hungry, i. e., those with 

 comparatively empty digestive tracts, are found in motion, and- 

 he notes that in certain conditions worms may remain quiescent 

 for weeks. The statement made earlier by Duges, that the eyes 

 of planarians play no part in finding food, Voigt confirms. These 

 organs he explains are an aid in distinguishing differences in light 

 intensity as well as the direction from which light comes but are 

 entirely incapable, owing to the simplicity of their structure, of 

 discerning the form of objects. In his opinion worms crawl into 

 hollow stems and similar sheltered places to escape light rather 

 than for warmth, as Wilhelmi ('04) suggests. Neither author, 

 apparently, considers the possible part played by thigmotaxis 

 under such circumstances. Of the delicacy with which worms 

 react to light Voigt says ('04, p. 173) : "Die Empfindlichkeit der 

 Planariden gegen plotzliche Belichtung tritt so scharf hervor, 

 das sie fur den Unterricht eines der anschaulichsten Beispiele zur 

 Demonstration der Lichtflucht bei niederen Tiere darbieten." 

 Notwithstanding this high degree of sensitiveness to light, he finds 

 that the worms when seeking their food leave the shade and come 

 out even into direct sunlight. And, finally, concerning the bearing 

 which light has on the problem of distribution, he concludes ('04, 

 p. 175):- "Auf die Verbreitung im Allgemeinen hat die Belichtung 

 der Bache wenig Einfluss, da sich in der Regel genug dunkele 

 Schlupfwinkel finden, in denen sich die Tiere verbergen konnen. " 



Darwin ('44, p. 242) observed that land planarians, "especially 

 Planaria tasmania, had an immediate apprehension and dislike 

 of light, which they showed by crawling, when the lid of the box 

 was taken off", to the under side of pieces of rotten wood," and in 

 his enumeration of the places where various species of land pla- 

 narians were found, their avoidance of light is plainly shown. 



A note by Leidy ('58) refers to finding Rhynchodemus sylvati- 

 cus crawling about on fences frequently at night, but rarely by day. 



2 P. alpina = P. torva according to Borelli ('93). 



