THE EARTHWORM 303 



there is some choice of food. Cabbage leaves and onion, for 

 example, will be selected from among other bits of food. The 

 location of this sense has not been determined experimentally, 

 but it is probable that it is restricted to the mouth region. What 

 could be called smell was evidenced in Darwin's experiments by 

 the fact that the worms found and drew to the surface savory food, 

 such as cabbage and onion, that had been buried under one-fourth 

 of an inch of soil. Since, in vertebrate animals, taste and smell 

 are to be regarded as modifications of a more primitive chemical 

 sense from which both have originated, it is not unUkely that 

 these two senses are closely related, if not identical, in the earth- 

 worm. 



Darwin also studied the responses of the worms to Ught under 

 conditions resembhng those to which they are subjected in nature. 

 In general, he concluded that, while the worms respond indefi- 

 nitely to low and diffuse illumination, light does affect them 

 both by its intensity and by its duration. A strong light flashed 

 suddenly upon the anterior end will cause a retreat into the bur- 

 row, and hght of moderate intensity may cause the worms to 

 withdraw in some instances. What later investigators have 

 called the varying " physiological state " was recognized by Dar- 

 win in his observation that when the worms were employed in 

 dragging leaves into their burrows or in eating them, and even 

 during the short intervals while they ceased from their work, 

 " they either did not perceive the light or were regardless of it; 

 and this occurred even when the light was concentrated through a 

 large lens." The response to stimuli that are effective at other 

 times is also inhibited during sexual union. Experiments con- 

 ducted under the exacting conditions of modern laboratories, 

 show that the worms respond negatively to strong hght and 

 positively to light of low intensity. The results are in har- 

 mony with their daily habits, since ''during the daytime, while 

 the light is of relatively high intensity, these animals retreat into 

 their burrows because of negative phototropism. At night-time, 

 however, they emerge from their burrows, not because of the 

 absence of hght, but by reason of their positive phototropism 

 to light of low intensity, for even in the darkest night there is 

 faint light." As with the tactile sensibihty, the anterior end is 

 the most sensitive, the posterior end is next, and the middle 

 region of the body least of all. 



