684 TEXTBOOK OF ZOOLOGY 



undergoes reorganization and growth to regenerate a new indi- 

 vidual. Like the coelenterates, very short pieces may regenerate a 

 head at each end or fail to regenerate a head at all. 



Experiments with x-rays have shown that it is possible to prevent 

 regeneration by this means. X-rays prevent cells in a physiological 

 condition preliminary to cell division from undergoing division, 

 though they do not prevent completion of division in cells under- 

 going mitosis at the time of application. 



Some of the flatworms exhibit a type of agamic reproduction in- 

 distinguishable from regeneration. This is especially true of the 

 tapeworms. The tapeworm forms new segments just distal to the 

 scolex, growing to very considerable length if undisturbed. The 

 more distal segments break off as they become exhausted. 



Vermifuges cause the death and shedding of many segments but 

 are often ineffective in destroying or dislodging the scolex and the 

 growing region next to it, especially if the scolex is barbed. The 

 growing region subsequently produces new segments and the worm 

 may be completely "regenerated." 



Annelida 



Regeneration is a more complicated process in the annelids than 

 in the simpler animals and regenerative capacity is somewhat more 

 restricted. 



If a Lumhriculus be cut into a dozen transverse pieces, a dozen 

 tvorms may be formed. The regenerated head consists of but a few 

 segments but the tail continues to grow by intercalation of new 

 segments just anterior to the anal segment. 



When the anterior end is cut off, the cut surface is closed very 

 quickly by contraction of the body wall. The cut end of the diges- 

 tive tract is contracted and closed. No further extensive changes 

 are visible for a day or two but new ectoderm is soon formed over 

 the cut surface. Old ectoderm gives rise to the new. As the new 

 ectoderm increases in amount, the old is pushed back, leaving a 

 single layer of cells over the cut surface. The new ectoderm pro- 

 liferates, producing a bulge in the new head. New cells arise from 

 the ventral side of the bulge to form the brain, commissures, and 

 ventral nerve cord. Other ectodermal cells form the muscles. 



The blind end of the gut proliferates and grows forward toward 

 the ectoderm. Invagination of the ectoderm takes place so that 



