28 . E. H. HARPER. 



of the epithelial layer must of course tend to cut off light from 

 these cells. The inrolling of the most sensitive region is another 

 important factor. 



THE BEARING OF EXPERIMENTAL RESULTS UPON THE HABITS 



OF THE EARTHWORM. 



It is a truism that in all experiments upon animals the relation 

 of the experimental results to the normal life of the animal should 

 be kept in mind. The behavior of the earthworm has not been 

 systematically studied as a whole except by Darwin. It is 

 obvious that all the experimenters mentioned have studied the 

 reactions of the earthworm in only one phase of its activity, and 

 that phase is not what we should call the normal life of the worm. 

 It is as if the experimenters had chosen the situation of the earth- 

 worm as we find it crawling on the sidewalks after a heavy rain 

 as being its typical mode of life. None would probably admit 

 this sooner than themselves, and doubtless they have regarded 

 certain facts as too obvious to require mention. Does the fact 

 that the normal life of the earthworm is carried on in a burrow 

 affect our view of the experimental results obtained ? Now the 

 earthworm does spend a portion of its life, during the night time, 

 crawling on the surface of the ground in search of leaves, and also 

 during sexual activity it is less mindful of the light, as is stated. 

 The earthworm leaves its burrow rather reluctantly. Darwin 

 describes the earthworm as retaining the posterior end in the 

 burrow while making searching movements in all directions in 

 search of leaves. In drier weather we know that worms burrow 

 deeply and seldom are found near the surface, depositing their 

 castings in old burrows instead of on the surface. 



If the worm is at home almost exclusively in the burrow we 

 should expect those responses which are typical of the burrow 

 life to be better organized and more definite than its activities 

 when crawling in the open. The movements which are typical 

 of the life in a burrow are mainly in the line of the axis of its 

 body. Are these movements and the responses which control 

 them of a more definite nature than its lateral movements ? We 

 may first consider the typical burrow movements in response to 

 light. These may be imitated easily by using a screen to shade 



