THE LIFE HISTORY OF PLAXARIA VELATA. 14! 



the cessation of activity, loss of pigmentation, and the disintegra- 

 tion of the pharynx. The fate of worms in such a condition can 

 be altered by manipulation of the temperature to which they are 

 subjected. A rise in temperature will hasten the onset of frag- 

 mentation ; lowering of the temperature will retard it. In some 

 cases worms with degenerated pharynges have been prevented 

 from encystment by a change in temperature from that of the 

 room to 8. These worms have regenerated pharynges, acquired 

 more pigment, and become perfectly normal asexual worms at 

 the lower temperature. 



Sexuality has never been observed in specimens of Planaria 

 velata collected from temporary pools or ditches. It seems quite 

 probable that conditions in that kind of habitat are never favorable 

 to the development of sexual structures. 



In a spring at Valparaiso, Indiana, Planaria Z'data has been 

 found to occur at all months of the year. During October, No- 

 vember, and December the worms are increasing very rapidly in 

 numbers and in size. These worms presumably had their origin 

 in cysts formed by the fragmentation of worms as the water be- 

 came warmer the preceding spring. In the latter part of Novem- 

 ber and through December all of the worms of a length of eight 

 to ten millimeters or over show a very distinct genital pore. 

 Microscopical examination shows testes and ovaries are present. 

 The height of sexual development is reached by the latter part of 

 December. The largest worms collected then show not only a 

 complete development of ovaries, testes, and atrial organs, but in 

 some of them the vasa deferentia are well developed and packed 

 with spermatozoa. 



From mid- winter on there is a decrease in the number of sexual 

 individuals as well as in the number of worms to be found. By 

 the middle of March there is no trace of sex organs in any of the 

 worms collected. Through the months of April and May the 

 worms decrease greatly in abundance, the larger ones undergoing 

 fragmentation and encystment and the smaller ones remaining, 

 so that during the summer months only a few small or medium 

 sized asexual worms are found. 



The most evident variable factor that might be effective in in- 

 fluencing the sexual cycle in this location at Valparaiso is seen at 



