STIMULI AND THEIR ACTIONS 



441 



actively to stream along the bottom without rising again freely 

 into the water. In other words, upon slight contact with the bottom 

 the living substance of the rhizopods behaves positively thigmotac- 

 tically and turns toward the object. Except in the free-swimming 

 Radiolaria, Heliozoa, etc., extension and wide expansion of the 

 pseudopodia take place always in contact with some body, whether 

 it be the bottom, the cover-glass, the surface of the water, or objects 

 in the water. On the other hand, by strong mechanical stimulation 

 of the tip of an extended pseudopodium of Orbitolites, best by press- 



I 



FIG. 216. Pseudopodium of Orbitolites, in a cut across at * ; b, c, 



reaction. 



<?, f, successive stages of the 



ing upon it with a needle or cutting it across with a scalpel, it is 

 possible to call out negative thigmotaxis of its living substance, 

 the protoplasm at the place of stimulation drawing together into 

 small globules and spindles, and streaming away (Fig. 216). 1 The 

 same phenomenon may be observed still more distinctly in a 

 rapidly reacting fresh-water rhizopod, the shell-bearing Cypho- 

 deria ; here the protoplasm of the pseudopodia withdraws from 

 the place of stimulation with great rapidity (Fig. 217). 



Thigmotactic phenomena are wide-spread. Among plants they 

 1 Cf. Verworn ('92, 1). 



