PARAMECIUM CAUDATUM 



65 



to all sudden stimuli by a definite reaction termed a "motor 

 response." It backs away by reversal of its ciliary action, then 

 turns on its axis and moves forward again. If the offending 

 object is still encountered it repeats the action until finally its 

 forward movement is unimpeded. It reacts definitely to the 

 action of a galvanic current, and always moves toward the nega- 

 tive pole, thus showing a well-marked galvanotaxis. \Yith 

 strong acids and alkalis it gives the characteristic motor 

 response. 



EXPLODED TfflCHOCYSTS. 



TRICHOCYSTS 



MACXONUCLEUS. 



DIVIDING niCRONUCLEUS. 



DIVIDED MACRONUCLEUS 



FIG. 28. Paramecium in division. Photograph of a section. 



D. SOME GENERAL BIOLOGICAL PROBLEMS ASSOCIATED WITH 



PROTOZOA 



The single-celled animals have been intimately connected 

 with some of the deepest problems in general biology, for since 

 vital manifestations are everywhere the same, physiologists 

 have turned for their solution to these simple organisms w r here 

 processes, like structures, are relatively uncomplicated. Some 

 of these problems, such as the distinction between animals and 

 plants, have only an academic importance; another, spontane- 

 ous generation, has only an historical importance, but others, 



