The Earthworm 



283 



B 



REGENERATION 



Any part of an earthworm may be cut off at any point between the 

 end of the prostomium and the fifteenth to the eighteenth segment and 

 a new anterior end will grow out from the cut end of the body. This 

 will consist of a single segment if only one segment was removed ; two 

 segments, if two segments were removed ; and of three, four, or five seg- 

 ments, if three, four, or five seg- 

 ments were removed. But never 

 more than segments one to five are 

 regenerated, regardless of the num- 

 ber removed, and no new reproduc- 

 tive organs appear if the original 

 ones were contained in the severed 

 piece. If the cut is made behind 

 segment eighteen, a tail will grow 

 out from the cut surface of the pos- 

 terior piece, thus producing a worm 

 consisting of two tails joined at the 

 center. Such a creature cannot take 

 in food, and must slowly starve to 

 death. When the regenerated part 

 is different from the part removed, 

 as in the case just cited, the term 

 heteromorphosis is given to the phe- 

 nomenon. 



Regeneration of a tail differs 

 from that of a head, since more than 

 five segments can be replaced. The 

 anal segment develops first, and then 

 a number of new segments are intro- 

 duced between it and the old tissue. 

 The rate of regenerative grow T th 

 depends upon the amount of old 

 tissue removed. If only a few seg- 

 ments of the posterior end are cut off, a new tail regenerates very 

 slowly; if more are removed, the new tissue is added more rapidly. In 

 fact, the rate Of growth increases up to a certain point as the amount 

 removed increases. The factors regulating the rate of regeneratior; have 

 not been fully determined, although several possible explanations have 

 been suggested. 



GRAFTING 

 Pieces of earthworms may be grafted upon other worms without 

 much difficulty. Three pieces may be so united as to produce a very 



Fig. 174. 



A. Hirudo medicinalis, about life size. 



1. Mouth. 2. Posterior sucker. 3. Sen- 

 sory papillae on the anterior annulus of each 

 segment. The remaining four annuli which 

 make up each true segment are indicated by 

 the markings on the dorsal surface. 



B. View of the internal organs of Hirudo 

 medicinalis. On the left side the alimentary 

 canal is shown, but the right half of this 

 organ has been removed to show the excretory 

 and reproductive organs. 



1. Head with eye spots. 2. Muscular 

 pharynx. 3. 1st diverticulum of the crop. 4. 

 11th diverticulum of the crop. 5. Stomach. 

 6. Rectum. 7. Anus. 8. Cerebral ganglia. 9. 

 Ventral nerve cord. 10. Nephridium. 11. 

 Lateral blood-vessel. 12. Testis. 13. Vas de- 

 ferens. 14. Prostate gland. IS. Penis. 16. 

 Ovary. 17. Uterus — a dilatation formed by 

 the conjoined oviducts. (After Shipley and 

 MacBride.) 



