184 K. S. LASHLEY 



environment, the persistent effects of former diversities of environ 

 ment, variations in the commensal relations of the polyps and 

 their zoochlorellae, parasitic infection of one clone, and differ- 

 ences in clonal age from the fertihzed egg. The first of these is 

 ehminated by the methods of the experiment. It has been 

 shown that the after effects of injury and 'depression' are only 

 temporary. Experiments attempting to produce permanent 

 modification by extremes of temperature, chemical agents, and 

 artificially induced 'depressions' have been unsuccessful; diver- 

 sities are readily produced but they persist for only a few days. 



Possible variations in the commensal relations of H. viridis 

 are more diflScult to control. Microscopic exammation of the 

 two clones failed to reveal any difference in the green bodies of 

 the two clones. As a test of the influence of commensalism 

 upon variations I partially removed the zoochlorellae from some 

 individuals of Clone L (see below) by the method devised by 

 Whitney ('06-'08). During immersion in glycerin the polyps 

 became smaller but when restored to normal conditions they 

 ate regularly and resumed their original size before beginning to 

 bud. The buds, practically white in color, did not seem other- 

 wise different from the normal members of the clone. I never 

 succeeded in getting out all the zoochlorellae from the polyps 

 so that not enough buds for statistical study could be obtained 

 before the polyps resumed their normal color. 



Five parasites of Hydra have been reported : Amoeba hydrox- 

 ena Entz, Balantidium hydrae Entz, a species of Ophryoglena 

 (Entz '12), Trichodina pediculus Ehr. (Clarke '65), and Kerona 

 polyporum Ehr. The true parasitic nature of only one of these, 

 A. hydroxena, has been demonstrated. Trichodina did not 

 occur in any of the pedigreed cultures recorded here, although 

 it is common in wild populations. In October, 1913, an epidemic 

 of A. hydroxena broke out in my cultures, destroying all of them. 

 The symptoms of infection by this parasite are easily recogniz- 

 able and there is no possibility that it occurred in the earlier 

 cultures. Kerona occurs rarely in wild populations here but 

 did not appear in my cultures; the other two parasites have 



