Chap. 25 FLATWORMS VANGUARD OF THE HIGHER ANIMALS 505 



a planarian is completely frustrated; it can neither fasten its body to the sur- 

 face nor divide. 



Regeneration. The common Dugesia and certain other free-living pla- 

 narians have remarkable powers of regeneration. Parasitic flatworms, like 

 parasites in general, are unable to replace damaged parts. Experiments upon 

 the regeneration of sponges, hydras, and especially planarians have shown 

 important principles governing the organization and growth of the body. The 

 possibility of grafting human tissues was discovered by experimenting on 

 lower animals. The experiments on the regeneration of planarians carried on 

 by T. H. Morgan about 1 890 are among the classics of experimental zoology. 



Pieces of a planarian's body maintain the natural polarity of the whole 

 body. Remove the head and tail leaving only the middle part of the body, and 

 a new head will grow from the front edge and a new tail from the hind edge 

 (Fig. 25.7). 



WINTER 



WINTER 



SPRING 



FALL SUMMER 



ASEXUAL 



FALL 



SUMMER 



SEXUAL 



Fig. 25.6. Diagrams of the life cycles of Dugesia tigrina (Pkmaria maculata) 

 as they vary under different ecological conditions. Left, purely asexual repro- 

 duction. Transverse divisions occur throughout the warmer months. The parts of 

 the animals grow to a certain size; the rear end adheres to the surface and the 

 front part proceeds forward, pulling at the middle of the body which quickly 

 breaks. Right, the more common succession of sexual and asexual reproduction; 

 the sexual organs are highly developed in spring; many egg capsules are laid; by 

 midsummer sexual reproduction ceases and asexual reproduction by fission begins. 

 (Courtesy, Morgan: Animals in Winter. New York, G. P. Putnam's Sons, 1939.) 



