I02 Journal of Comparative Neurology and Psychology. 



see that in strong illumination extensions towards the light are 

 ■checked while those that bring the worm away are followed up, 

 but this is not so obvious in light of weaker intensity. Wheth- 

 er the negative reaction takes place entirely by the selection of 

 random movements is hard to determine with certainty, but 

 there is no doubt that this factor plays a very large part in the 

 process. 



Experiments performed by Miss Rhodes and myself three 

 years ago upon the phototaxis of leeches showed that the meth- 

 od of orientation in these forms is, in principle, the same as that 

 of the earthworm. When specimens of Glossiphonia are placed 

 in strong light their locomotor reflexes are set in action. The 

 mode of progression in Glossiphonia differs from that of the 

 earthworm, although in certain fundamental respects the same. 

 The anterior part of the body is extended, the mouth which 

 acts as a sucker is attached, and then the posterior part of the 

 body is brought forward and fastened by the sucker at its caud- 

 al end. Then the anterior part of the body extends again and 

 the other movements follow as before In its progress the 

 leech frequently raises the extended anterior part of the body 

 and waves it from side to side as if feeling its way. If the ani- 

 mal turns it in the direction of a strong light it is quickly with- 

 drawn and extended again, usually in anotlier direction. If the 

 light is less strong it waves its head back and forth several times 

 and sets it down away from the light; then the caudal end is 

 brought forward, the anterior end extended and swayed about 

 and set down still farther from the light than before. When 

 the leech becomes negatively oriented it may crawl away from 

 the light, like the earthworm, in a nearly straight line. The 

 extension, withdrawal and swaying about of the anterior part of 

 the body enable the animal to locate the direction of least stim- 

 ulation, and when that is found it begins its regular movements 

 of locomotion. Of a number of random movements in all di- 

 rections only those are followed up which bring the animal out 

 of the the undesirable situation. 



The phototactic reactions of the larva; of the J/uscidc? ha.\'e 



