The Reactions of Planarians to Light 95 



nations which serve as a direct stimulus to other planarians, attract- 

 ing them and causing them to come to rest in groups. 



In this connection it is worth mentioning that several times 

 when Dendroccelum lacteum was put in an aquarium with other 

 species of planarians, the individuals of this species would later be 

 found gathered into a separate group by themselves. This manner 

 of isolation was also repeatedly noticed in examining on the under 

 sides of stones taken from the pond at Falmouth, Mass. A similar 

 segregation of species in the case of P. alpina and P. gonocephala, 

 was noted by Collin ('91). He says ('91, p. 180) "Iijima fand 

 diese beiden Arten zusammenlebend, wahrend sie im Harz stets 

 getrennt vorkamen; auch in der Gefangenschaft schien die P. 

 alpina die grossere P. gonocephala in demselben Behalter zu 

 meiden und ihr angstlich auszuweichen. " It would be difficult to 

 explain how these planarians avoid each other so as to fraternize in 

 this fashion, except on the basis of some delicate chemotactic 

 response which caused them to halt when they entered the chem- 

 ical halo of their own kind, but not to do so in the different chem- 

 ical halos of other species. As in the case of goniotaxis, the mani- 

 festations of phototaxis may be entirely superseded by the effect 

 of feeding (Chemotaxis). When once a hungry planarian, driven 

 by directive light into the neighborhood of a crushed snail, becomes 

 subjected to the chemical stimulus arising from the fluids of that 

 object as they are disseminated through the water, it seems to 

 become suddenly indifferent to the light, owing to the greater 

 influence of the chemical stimuli. 



The same inhibition of the influence of light by a chemotropic 

 response to food has been observed by Parker ('03) on the mourn- 

 ing-cloak butterfly, Vanessa antiopa L. He says ('03, p. 457) 

 "when a butterfly alights on a bough, it orients in the sunlight 

 with the usual precision. Should the sap be running from a near 

 stem, the insect is very soon attracted to the spot, begins feeding, 

 and moves about from that time on with no reference to the direc- 

 tion of the sun's rays. Thus, when feeding or near food the but- 

 terflies do not respond phototropically. " Furthermore Darwin 

 ('81, p. 23) observed that earthworms are less disturbed by light 

 while feeding or during copulation than at other times. 



