64 ORGANISMS OF ONE CELL 



vacuoles are formed by the daughter cells which separate and 

 begin an independent existence. The entire process requires 

 from half an hour to two hours according to the temperature. 



Irritability Like ^4 moeba proteus again, Paramecium responds 

 to external stimuli but having definite motile organs its re- 

 sponses are more definite and more easily studied. It answers 

 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 always moving toward the nega- 

 tive pole thus showing a well-marked galvanotaxis. With 

 strong acids and alkalis it gives the characteristic motor response. 



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 where 

 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, 

 like old age, the origin of death, the significance of fertilization 

 are all deep problems which rest upon the very foundations of 

 biological science. 



Animals and Plants. At first glance it would appear to be a 

 simple matter to distinguish between animals and plants and 

 superficial observers do not hesitate to do so with full assurance. 

 But when we come to examine them more closely we find that 

 no definite boundary line can be drawn between them, and no 

 single characteristic belongs absolutely to one or the other 

 group. Formerly it was argued that plants are quiescent while 

 animals move, and the power of spontaneous movement was 



