218 GENERAL ZOOLOGY 



RELATION TO ENVIRONMENT 



We could find no better animal to show how an organ- 

 ism may be adapted to its environment than the earthworm. 

 In the first place, the slender, rounded body with its pointed 

 anterior end is the best possible form for progression through 

 a dense medium, the substance of which is usually displaced 

 by muscular energy. The appendages, which in none of 

 the earthworm's relatives are hard enough to be used for 

 digging, are here reduced to the minimum, only the tips of 

 bristles remaining. The animal is thus not impeded by 

 useless organs. The smooth skin is thin, and is kept moist 

 for use as a breathing-surface. Branched gills, such as are 

 developed on many relatives of the earthworm that live 

 in the sea, would in the earthworm be superfluous, and 

 much in the way. Reasons that explain the presence of 

 eyes in animals which live in the atmosphere, or in the 

 water, lead to the explanation of the absence of eyes in 

 the earthworm. 



It is a wonderful fact that the earthworm has no eyes and 

 still is able to distinguish light from darkness ; for, except in 

 the time of a rainstorm or in case of disease, it never leaves 

 its burrow in the daytime. By means of the very simple 

 sense-organs in the skin the earthworm can distinguish dif- 

 ferent intensities of light. It was found recently (1902), 

 during experiments carried on at Harvard University, that 

 earthworms not only appear to know the direction from 

 which light comes, but that they crawl away from light of 

 high intensity and crawl toward light of low intensity. Of 

 course, in the experiment, they did this without reference 

 to the matter of food. In nature, earthworms come out at 

 night, in the warm seasons, probably in response to the 

 stimulus of a low intensity of light, but also for the purpose 

 of obtaining food. 



