REACTIONS TO LIGHT AND MECHANICAL STIMULI 



IN THE EARTHWORM PERICH/ETA BER- 



MUDENSIS (BEDDARD). 



E. H. HARPER. 



Recent work concerning the behavior of earthworms has related 

 chiefly to their reactions to light. Since the contributions of 

 Hofmeister and Darwin, and that of Hesse ('96) there have been 

 a group of recent papers by Parker and Arkin, Miss Smith, 

 Adams and Holmes, which have been devoted chiefly to the 

 directive influence of light. In the present state of the discussion 

 of this subject the current theory of tropisms has been called in 

 question, according to which the earthworm is oriented directly 

 by light. Holmes has shown that light induces a general state 

 of activity leading to random movements of which those toward 

 the light are checked and those away from it continued, this 

 resulting in final orientation. 



This paper aims to show that random movements are a feature 

 of less strong light, tending to disappear with the increase of 

 intensity, and are replaced by direct orientation in very strong 

 light. It is also shown experimentally that the earthworm is 

 more sensitive in the extended than in the contracted state, and 

 that this has an important bearing upon the production of random 

 movements. The explanation given of this is that when extended 

 the sensitive elements of the skin are expanded over a greater 

 surface. This is shown to have a bearing upon the production of 

 random movements as follows : Locomotion consists of a succes- 

 sion of extensions and contractions and as each extension begins 

 in a state of lower sensibility the anterior end may be projected 

 toward the light, only to be checked when its increase of sensi- 

 bility with extension makes the stimulus appreciated. Movements 

 away from the light are not so checked. In stronger light the 

 sensibility of the worm when contracted is sufficient to suppress 

 movements toward the light at the outset. In such light the 

 worm appears to be orientated without trial movements. It is 

 important that the worms be kept in the dark before all experi- 



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