230 



COLLEGE ZOOLOGY 



stimulation, in which condition slight stimuli cause violent 

 responses. 



Regeneration and Grafting. - Earthworms have considerable 

 powers of regeneration and grafting (p. 117). Some of the results 

 of experiments along this line are shown in Figure 160. A 

 posterior piece may regenerate a head of five segments (A) or 

 in certain cases a tail (B). Such a double- tailed worm slowly 

 starves to death. An anterior piece regenerates a tail (C). 



Three pieces from several worms may 

 be united so as to make a long 

 worm (D) ; two pieces may fuse, 



FIG. 1 60. Regeneration and grafting in the earthworm. A, head end of 

 five segments regenerated from the posterior piece of a worm. B, tail re- 

 generated from the posterior piece of a worm. C, tail regenerated from an 

 anterior piece of a worm. D, union of three pieces to make a long worm. 

 E, union of two pieces to make a double-tailed worm. F, anterior and pos- 

 terior pieces united to make a short worm. The dotted portion represents 

 regenerated material. (From Morgan.) 



forming a worm with two tails (E) ; and an anterior piece may 

 be united with a posterior piece to make a short worm (F). 

 In all these experiments the parts were held together by threads 

 until they became united. 



Economic Importance. --Charles Darwin in his book on the 

 Formation of Vegetable Mold through the Action of Worms has 

 shown by careful observations extending over a period of forty 

 years how great is the economic importance of earthworms. 

 One acre of ground may contain over fifty thousand earthworms. 



