PHYLUM ANNELIDA 229 



comes in contact with the body, no positive reactions being 

 produced by chemical stimulation from a distance. Negative 

 reactions, on the other hand, such as moving to one side or back 

 into the burrow, are produced even when certain unpleasant 

 chemical agents are still some distance from the body. These 

 reactions are quite similar to those caused by contact stimuli. 

 Darwin explained the preference of the earthworm for certain 

 kinds of food by supposing that the discrimination between 

 edible and inedible substance was possible when in contact with 

 the body. This would resemble the sense of taste as present in 

 the higher animals. 



PHOTOTROPISM. No definite visual organs have been dis- 

 covered in earthworms, but nevertheless these animals are very 

 sensitive to light, as is proved by the fact that a sudden illumina- 

 tion at night will often cause them to " dash like a rabbit " into 

 their burrows. One investigator claims to have found cells in 

 the ectoderm, especially in the prostomium and posterior end, 

 which act as visual organs. The entire surface of the body, 

 however, is sensitive to light, although the anterior region is 

 more sensitive than the tail, and the middle less than either of 

 the others. Very slight differences in the intensity of the light 

 are distinguished, since, if a choice of two illuminated regions 

 is given, that more faintly lighted is, in the majority of cases, 

 selected. A positive reaction to faint light has been demon- 

 strated for the manure worm, Allolobophora fcetida. This 

 positive phototropism to faint light may account for the emer- 

 gence of the worms from their burrows at night. 



PHYSIOLOGICAL STATE. From the foregoing account it 

 might be inferred that only external stimuli are factors in the 

 behavior of the earthworm. This, however, is not the case, 

 since the physiological condition, which depends largely upon 

 previous stimulation, determines the character of the response. 

 Different physiological states may be recognized, ranging from 

 a state of rest in which slight stimuli are not effective, to a state 

 of great excitement caused by long-continued and intense 



