RELATION OF BURSARIA TO FOOD 5 



observed under a high power of a binocular. Granular sub- 

 stances of different chemical or physical properties may be placed 

 in the medium and the path of each individual particle may be 

 easily observed. Mixtures of these substances may also be 

 made and the paths of the different kinds of particles may be 

 determined. 



The different paths of particles which come into varying rela- 

 tions with the organism are shown by the arrows in the outline 

 drawings of figure 1. The paths of the arrows are correct repre- 

 sentations of the paths taken by the different kinds of particles. 

 In general the paths taken by particles may be distinguished 

 according to the following outline: 



1. Paths of rejection 



a. Path of total rejection, arrows A 



b. Path of rejection of larger particles taken into the oral 

 apparatus, arrows B 



c. Paths of rejection of smaller particles taken into the oral 

 apparatus, arrows Ci and Co. 



These paths may also be slightly modified by a combination 

 of the avoiding reaction with the different rejection reactions. 



2. Path of acceptance of large and small particles (large arrows D) 



Path A is taken by those particles which under conditions 

 hereafter to be considered (p. 29) never enter the oral apparatus 

 and are only drawn towards the body by the current; for exam- 

 ple, very toxic particles of yolk. Path B is always taken by 

 those particles which are too large to pass out by way of path 

 Ci and C2 and must be passed back to the exterior by the same 

 way as they were taken in, in order for the organism to get rid 

 of them at all. This may be illustrated by the larger proper I3' 

 treated grains of hard boiled yolk. The path represented by 

 the arrows Ci and C2 has considerable range of variation in part 

 of its course. It may be illustrated by cornstarch grains; these 

 are of convenient size. The variations in the course of these 

 particles may be divided roughtly into two main divisions; some 

 follow the dotted arrows C2 and never directly retrace any 



