STIMULI AND THEIR ACTIONS 



443 



their under side with cilia, which the animals, like' woodlice, use 

 as legs, with which to creep about upon objects in the water. 

 These Infusoria are always seen creeping 

 about busily and restlessly upon the slide, 

 the cover-glass, or particles of mud lying 

 in the ■\\'ater, without ever of themselves 

 losing contact with the objects. The 

 following episode from the life of an Oxy- 

 tricha illustrates this positive thigmotaxis 

 particularly well. In a flat dish contain- 

 ing river-water and an Oxytriclia, there 

 lay some spherical eggs of the river-mussel 

 Anodonta. When the contents were 

 poured into the dish, the Oxytricha in 

 some manner came into contact with one 

 of the eggs. It ran about unremittingly 

 for hours upon the spherical surface with- 

 out being able to leave it, since the egg 

 rested with one point only upon the level 

 bottom (Fig. 219, C). The organism must 

 have travelled an enormous distance. 

 After four hours it was able to forsake its 

 enforced retreat by means of a particle of 

 mud which came to the isolated egg. 

 Experiments which artificially imitated 

 with other Oxytrichae essentially the same 

 conditions give wholly analogous results. 



Jennings^ has recently discovered in 

 Parammcium another typical case of posi- 

 tive thigmotaxis. If a piece of filter- 

 paper, or any other substance provided 

 with a rough surface, be placed under 

 a cover-glass under which are numerous 

 Paramcecia distributed uniformly through 

 the water, after some time the piece is 

 beset with a thick coating of the Infusoria, 

 which touch it with their cilia without 

 moving from their place. By employing 

 high powers it is shown that those cilia 

 that are in direct contact with the foreign 

 body stand straight out and perfectly still 

 (Fig. 220, A), and that the activity of 

 the cilia over all the rest of the body is 

 greatly depressed and eventually wholly stopped. There is here 

 a very pronounced thigmotaxis. In connection with this it is 

 noteworthy that the thigmotactic assemblage of Paramcecia con- 



' Loc. cil. 



Fig. 21S. — Positive thigmotaxis of 

 a plant, a, a, Rod ; 6, 6, c^ J. 

 twining shoot. (After Sachs.) 



