182 



COLLEGE ZOOLOGY 



d 



c 



B 



Figure 98. Earthworm. Diagrams illustrating copulation and regeneration. A, a pair in copula- 

 tion. Because this usually occurs at night, it is not often observed. B, a new anterior end 

 (dotted) that has regenerated in place of an anterior end removed. C, a new posterior end 

 (dotted) that has regenerated in place of an anterior end removed. D, a long worm produced 

 by grafting together parts of three worms. (A, courtesy of General Biological Supply House, Inc., 

 Chicago; B, C, D after Morgan.) 



food by supposing that the discrimination 

 between edible and inedible substances was 

 possible when they were in contact with the 

 body. This would resemble the sense of taste 

 as present in the higher animals. 



ReacfioTts to light 



No definite visual organs such as eyes 

 have been discovered in earthworms; never- 

 theless, these animals are very sensitive to 

 light, as is proved by the fact that a sudden 

 illumination at night will often cause them 

 to quickly snap back into their burrows. 

 This sensitiveness to light is due to the 



photoreceptor cells (Fig. 96) that are con- 

 centrated especially in the anterior and pos- 

 terior ends, and are found in every segment 

 of the body. Each of these light-sensitive 

 cells contains a transparent "lens" that 

 focuses light on the neurofibrils which 

 ramify through the cell. By means of these 

 photoreceptor cells, very slight differences 

 in the intensity of the light are distinguished. 

 If a choice of two illuminated regions is 

 given, the one more faintly lighted is se- 

 lected in the majority of cases. A positive 

 reaction to faint light has been demon- 

 strated for the manure worm, Eisenia foe- 



