282 



GENERAL BIOLOGY 



under normal conditions causes a negative reaction, may have no effect 

 whatever under such circumstances, the instinctive reaction of the pri- 

 mary instinct being stronger than the artificial secondary stimulus. 



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 consisting 

 of a single segment if only one segment was removed ; two segments, if 

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



three, four, or five segments were 

 removed. But never more than 

 segments one to five are regen- 

 erated, regardless of the number re- 

 moved, and no new reproductive 

 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 posterior 

 piece, thus producing a worm con- 

 sisting 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 growth 

 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 regeneration have 

 not yet been fully determined, although several possible explanations 

 have been suggested. 



Fig. T74. 



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. 

 llth 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. 15. Penis. 16. 

 Ovary. 17. Uterus a dilatation formed by 

 the conjoined oviducts. (After Shipley and 

 MacBride). 



