3 o8 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS 



whereas in Pelomyxa pahtstris the shock-movement is produced by sudden 

 illumination, and not by the sudden withdrawal of light. Similarly, many 

 bacteria show a shock-movement on passing from strong solutions to weak 

 ones, but not on passing from regions of lower to ones of higher concentra- 

 tion. In addition, a pronounced deformation of the protoplasm is shown on 

 transferring from an almost maximal temperature to a normal one, but not 

 on raising to the higher temperature. The deformation is also absent on 

 cooling to low temperatures, but it appears when the temperature is raised 

 again. In the case of the streaming cells of Char a and Nitella t however, 

 both the sudden application and the rapid removal of pressure, as well 

 as sudden rises or falls of temperature or concentration, may produce a 

 temporary shock-stoppage of streaming. 



SECTION 66. The Forms of Tactic Response to Tropic Stimuli. 



Whenever a tropic stimulus causes a definite orientation of the main 

 axis of a freely motile organism, locomotion in a definite direction is assured, 

 since in most cases the latter takes place along the main axis. Whether 

 the movement is positive or negative in character will depend upon the 

 direction of the anterior end in regard to the orienting stimulus. Local 

 accumulation may also result from the fact that the organisms freely move 

 towards regions of higher illumination or concentration, but experience 

 a backward shock-movement on approaching regions of lower concentration 

 or illumination. Passive accumulation also takes place when organisms 

 which swim freely continually adhere to a mucilaginous region, or are 

 suddenly killed on coming into contact with a poisonous area, or rapidly lose 

 the power of movement in a zone deficient in oxygen. 



Shock-stimulation is responsible for the accumulation of Bacterium 

 photometricum in illuminated areas, as well as for the accumulation of various 

 Bacteria and Infusoria in concentrated solutions. Whereas the phototactic 

 movements of the zoospores of many Algae, and the chemotactic attraction 

 of many antherozoids, and of the zoospores of Saprolegnia and of many 

 Flagellatae, are the result of a tropic orientation of the body axis, as are 

 also the geotactic and galvanotactic movements of various organisms. In 

 both cases we are dealing with stimuli due to dishomogeneity in the 

 surroundings, but the stimuli act upon dissimilar forms of irritability. The 

 shock-stimulation is a temporary action repeated every time the required 

 change of conditions is produced by the movements of the organism, 

 whereas in the typical tropic orientation the inclination the organism 

 assumes is maintained so long as the tropic agency is unaltered, even if the 

 organism adheres to the same spot. Such organisms move with a definite 

 aim, whereas forms like Bacterium photometricum may be said to possess 

 a phobotactic irritability by which they avoid dark areas. Similarly, by 



