194 



IRRITABILITY 



interference between the contact stimulus, on the one hand, and 

 the thermal or galvanic on the other, the inhibitory effect of the 

 former may overpower the strong excitatipn of the latter. 



Fig. 42. 



Interference of galvanotaxis and thigmotaxis in Paramaecium aurelia. The 

 individuals which are thigmotactically attached to slime particles 

 remain at rest while the freely swimming individuals move toward the 

 cathodic pole. 



Still more complex and striking is finally the following case 

 of interference between thigmotaxis and galvanotaxis. The 

 hypotrichous infusoria as Stylonychia, Urostyla, Oxytricha, etc., 

 have a marked functional and morphological differentiation of 

 their cilia. They possess a bow-like row of perioral cilia, which 

 sweep in the food ; a number of cilia on the ventral surface used 

 for locomotion by which they move about upon objects in the 

 water; a row of border cilia on each side, which, during swim- 

 ming, contribute the propelling force. The perioral cilia also 



