BEHAVIOUR OF CELLS 7 
particles which float therein, towards the interior; and the 
particles are then engulphed, no matter what their composition 
may be. Digestible or indigestible, in they go. There is no 
selection of the one or rejection of the other. But, as we 
have seen, the Paramecia collect around a bacterial clot and 
feed upon it. Surely here there is selection of the nutritious ! 
Apparently not. They collect in just the same way towards a 
piece of blotting-paper, cotton-wool, cloth, sponge, or other 
fibrous body, and remain assembled round such an innutritious 
centre just as long as round a bacterial clot. There seems to 
be no choice in the matter ; contact with any substance gives 
rise, aS an organic response, to the lessening or cessation of 
the regular movements in all the cilia except those of the 
mouth-groove and funnel. As the Paramecia swim hither and 
thither, first one, then another, then more, chance to come in 
contact with the bacterial clot, the blotting-paper, or other 
substance, and since the lashing of the cilia is then auto- 
matically lessened, there they stay ; others find their way to 
the same spot in the course of their random movements, and 
they, too, stay ; thus many soon collect. 
But this does not account for the seemingly social assem- 
blages of Paramecia where there is no such substance to arrest 
their progress. Dr. Jennings attributes this to the fact that a 
dilute solution of carbon-dioxide has, what we may call for the 
present, an attractive influence. Ifa bubble of air and a bubble 
of carbon dioxide be introduced into the water in which Para- 
mecia are swimming beneath a cover-glass, the animalcules 
collect around the carbonic dioxide, but not around the air 
bubble. At first they press up close to the bubble of carbon 
dioxide, but gradually form a ring farther and farther from 
its limiting boundary. This is held to be due to the fact that 
it is only the dilute solution of carbonic acid that has the 
peculiar “attraction ’—a stronger solution has a different 
effect. And, as the gas dissolves, the Paramecia collect in a 
ring just where the solution is sufficiently dilute. 
Now carbon dioxide is a product of the organic waste of 
living substance ; it is given off by active Paramecia. Where 
