206 WILLIAM BRODBECK HERMS 



of rather crude methods: the larvae were placed on a board, 

 which was thrust forward out of the shade in such a manner 

 that the oral pole of the larva was subjected to sunlight. This 

 caused the animal to withdraw its head and take up a position 

 parallel with the rays. A similar experiment with reference to the 

 aboral pole of the larva did not result in a like orientation. Sever- 

 ance of the anterior segments also resulted in an inability to orient 

 when subjected to light. Loeb rightly lays little stress on the 

 results of this latter method. 



The experiments cited are characterized by the following 

 statement made by Loeb earlier in the same paper (p. 21). "Die 

 Thatsachen die ich nachzuweisen habe, sind von so einfacher Art, 

 dass fast jedes technische Hiilfsmittel dabei entbehrt werden 

 kann." Later observations by many different investigators have 

 proved that the reactions of the lower organisms to light are not 

 of the simple nature inferred by Loeb. 



It will be observed that the position often occupied by the feed- 

 ing larvae in reference to light must lead one to suspect that the 

 posterior parts are not strongly sensitive, if at all. The head under 

 such conditions (i.e., feeding) is buried in the tissues of the flesh, 

 while all the rest of the body may be protruded in full daylight. 

 Furthermore, when the larva travels away from a source of light, 

 its aboral portions are fully exposed to the light, while the head 

 is obviously kept in shadow as much as possible. The manner in 

 which the larvae wave the head about in response to weak inten- 

 sities leads one also to suspect that the rays falling on this part 

 serve as a directive agency. 



A pencil of light may be employed with which to explore the 

 entire body of the larva in order to ascertain the sensitive regions 

 or region. Even with a fine pencil of light there will be some diffu- 

 sion, but by means of a pinhole aperture (fig. 2, J) diffusion can 

 be reduced to a minimum as compared with the length of the larva. 



All parts of the length of the animal were carefully explored, 

 but only one region was found where the light caused the larva to 

 turn, and that was at the very tip of the oral end. By throwing 

 the pencil on this region continuously, the larva could be forced 

 to crawl in a circle as illustrated by fig. 13. Blackening this region 



