SOME PROBLEMS IN THE PHYSIOLOGY OF THE PROTOZOA 299 



chance brings the organism in the sphere of influence of the stimulant. 

 On the other hand, it is equally probable that many of the so-called 

 directive effects may be explained by this assumption, and extreme 

 caution is needed in interpreting all such reactions. Garry ('oo) 

 has applied the distinction, first suggested by Loeb ('93), to be used 

 in interpreting the responses of organisms to stimuli ; if a directive 

 effect is produced (that is, if the organism is distinctly oriented in 

 respect to the centre of the stimulus), the effect is said to be chemo- 

 tropic, but if an effect is produced which is not directive, it is said to 

 be chemokinetic (Unterschiedsempfindlich, Loeb). According to Jen- 

 nings, the reaction of Paramceciitm would be neither chemokinetic nor 

 equivalent to the unilateral stimuli induced by light or heat (photo- 

 taxis and thermotaxis). 



Fig. 151. Motor response in Paramcccium. [JENNINGS.] 

 a-f. Successive positions after meeting with an obstruction A. 



The various instances which have been described of positive and 

 negative chemotaxis must be reinvestigated in the light of Jennings's 

 conclusions, for it is obvious that the mere collection of organisms in 

 a substance does not necessarily indicate the directive effect of that 

 substance. Nevertheless, in some cases such an effect appears to be 

 definitely established. Garry showed that certain organic acids have 

 a positive directive effect upon the flagellate Chilomonas paramcecium, 

 while various other acids, alkalies, and salts have merely a chemo- 

 kinetic effect. He also obtained results which seem to indicate that 

 it is the ions dissociated in certain solutions which are capable of 

 stimulating the organism. These observations open out an unex- 

 plored field for investigation and promise extremely interesting 

 results. 



Irritability on the part of the Protozoa, in response to mechanical 

 stimuli, has long been known. Rosel v. Rosenhof (1755) observed 

 that Amoeba becomes globular upon being shaken, an observation 

 which was followed by numerous experiments by De Bary on Myce- 



