34 



THE INVERTEBRATA 



are "unfavourable", that is from which the organism withdraws; 

 and in doing this it is not repelled in a straight line, but turns away 



Fig. 30. A diagram of the path by which a protozoon is directed by phobo- 

 taxis into the zone of optimum concentration of a substance diffusing from 

 a particle. From Fraenkel, after Kiihn. C, Centre of diffusion. The arrows 

 show the direction of movement of the protozoon, the circles define zones of 

 equal concentration, the circles of dots and dashes enclose the optimal zone. 



Fig. 31. Chemophobotaxis of Bodo sulcatus. From Fraenkel, after Fox. 

 1-5, Positions successively taken up by the members of a culture placed under 

 a coverslip. The position in which the individuals gather in each case is that 

 of the optimum concentration of oxygen, which alters as the supply of the 

 element is lessened by the action of the flagellates. 



at an angle which has no necessary relation to the direction of the 

 stimulus, and may again bring the individual into the unfavourable 



