1 88 AN INTRODUCTION TO ZOOLOGY 



seem to show that the worms are stimulated in different degrees 

 by them, since the interval of time between the application of the 

 stimulus and the reaction differs for each substance, being shorter 

 for sodium chloride, and successively longer for the others in the 

 order named (153). In man these substances all taste practi- 

 cally alike, because of the presence of chlorine. 



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 (144). 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. If an animal, which is lying along the ground with 

 its tail clinging to the top of the burrow, is suddenly illuminated, 

 it promptly withdraws into its hole; if not in touch with its bur- 

 row, it will crawl away from the source of light. 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. Thus far we have 

 considered light as causing a negative response; but a positive 

 reaction to faint light has been demonstrated for Allolobophora 

 fatida (135). This positive phototropism to faint light may 

 account for the emergence of the worms from their burrows at 

 night. Experiments with lights of different colors show that red 

 is preferred to any other, green being next, and blue last, and that 

 the intensity of the colored rays determines the effect (156). 



COMBINATIONS AND INTERFERENCE OF STIMULI. It has been 

 shown that contact and chemical stimuli may combine, as in 

 the case of the burrowing reaction of Allolobophora fcetida. In 

 other instances stimuli interfere with one another; for example, 

 light calls forth no reaction if the animals are feeding or 

 mating. 



