486 



SPONTANEOUS AGGREGATION OF FLAGELLATES 



A grain of sand lying in the path of the band of Bodo as it advances 

 from the center towards the edges of the preparation has no effect 

 on the band : the latter approaches and passes the grain of sand with- 

 out being deflected. An air bubble, however, keeps the band at a 

 distance from it. If the bubble lies sufficiently far in from the edge 

 of the cover-glass the advancing band of flagellates becomes bent 

 inwards to form a bay enclosing the bubble (Fig. 6). As the main 

 band continues to move outwards the horns of the bay meet so that 

 an inner ring of flagellates is left behind surrounding the bubble 



Fig. 5. Equilibrium position of flagellate band. 





K*:^?"^-^ 



Fig. 6. Air bubble keeping back an advancing flagellate band, a, air bubble. 

 Figs. 6, 7, and 8 are successive stages of one experiment. 



(Fig. 7). The bubble, then, behaves to the band just as the water- 

 air surface at the edges of the preparation does : it keeps the flagellate 

 band at a certain distance from it. But there is this difference 

 between the two cases, that whereas the main band comes to a halt 

 and remains stationary at a certain distance inside the edges of the 

 preparation, the band encircling the bubble slowly approaches the 

 latter. This continues until the flagellates come into contact with 

 the surface of the bubble itself (Fig. 8), where they remain for a 

 short time and then dissipate, swimming out to join the main band. 



