Vol. LIII 



September, 



No. 3 



BIOLOGICAL BULLETIN 



THE LIFE HISTORY OF PLAN ARIA VELATA. 



W. A. CASTLE. 



Previous investigators working with a common flatworm, 

 Plonaria vclata, have observed that the ordinary type of life his- 

 tory of the species is an entirely asexual one (Child, '13). Fol- 

 lowing the emergence of the young animals from cysts is a period 

 of growth w r hich is followed by processes of histological de- 

 generation and loss of internal structure as soon as the animal has 

 attained a certain size. This is followed by fragmentation of the 

 worm and the encystment of the fragments. The external aspects 

 of this process have been mentioned by Stringer (Stringer, '09) 

 and more fully described by Child (Child, '13). Observations on 

 the histological details of the process of degeneration have been 

 made by Alexander (Alexander, '26). Our observations confirm 

 those of Alexander in regard to the details of this process. 



From the encysted fragments new worms are reconstituted and 

 emerge from the cysts after a period of from two weeks to sev- 

 eral months. The time necessary for complete reconstitution is 

 dependent upon the conditions to which the cysts are subjected. 

 Asexual reproduction may continue in this way for an apparently 

 indefinite number of generations (Child, '14) and until com- 

 paratively recently has been the only observed method of reproduc- 

 tion for the species. 



Under certain conditions in nature Planaria rclata has been 

 observed to undergo a life history and course of development 

 entirely different from the asexual one mentioned above. Ac- 

 companying growth there has been the differentiation of sexual 

 structures. Under the most favorable conditions sexual develop- 

 ment has been completed and the sexually mature worms have 

 copulated and laid eggs. Although none of these eggs laid in the 

 laboratory hatched there is no reason to doubt that they would 

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