108 MOLAR AGENTS AND PROTOPLASM [Ch. IV 



they may no longer cling to the glass, but wander, undirected, 

 through the water. Again, while the surface film of water 

 often acts thigmotactically, if the surface tension is reduced 

 by a thin covering of oil, it no longer holds the organisms. 

 It would seem that a certain minimum difference in rigidity, 

 between any surface and the medium, is necessary in order 

 that the surface should act thigmotactically. 



Once in contact with a sufficiently attracting surface, the 

 organism may move to and fro over it, but it can hardly leave 

 it. It is, as Dewitz ('86, p. 366) says, as though the sperma- 

 tozoa were attracted by a magnet. This close adhesion of the 

 organism to the irritating surface is a remarkable phenomenon. 

 Lb Dantbc ('95) suggests that the amoeba adheres to the 

 glass by molecular attraction. On the other hand, it may be 

 doubted whether the close adhesion signifies anything else 

 than the absence of a sufficient stimulus to leave the surface 

 of contact. 



When an organism has been stimulated by contact for some 

 time, it at last becomes changed so that it no longer responds 

 as it did at first. Thus Dr. W. E. Castle has informed me 

 that he has seen a colony of Stentors, in an aquarium, being 

 constantly struck by Tubifex waving back and forth, yet the 

 Stentors did not contract as they usually do when struck. 

 Pfefpee ('88, p. 619) has observed that Urostyla retreats, 

 after a time, from the surface with which it was in contact. 

 These facts indicate that protoplasm can become acclimatized 

 to contact so as to be no longer stimulated by it. 



We now turn to the consideration of Rheotaxis, which may 

 be regarded provisionally as a form of thigmotaxis, although 

 •the possibility of its being rather a case of chemotaxis is not 

 excluded. 



RosANOFP ('68) was the first to notice the rheotaxis of the 

 large plasmodium of ^thalium septicum, but he ascribed it 

 to geotaxis. The correct interpretation was first given by 

 Steasbueger ('78, p. 62), and has been confirmed by Jonin- 

 SON ('83), and Stahl ('84). When ^Ethalium is placed on a 

 strip of saturated filter paper, the upper end of which is 

 dipped in a beaker of water, it is subjected to a current 

 of water in the substratum. At the same time it moves 



