II THE AUTHOR'S EXPERIMENTS 83 



(in the last instance) the pond-snails care nothing 

 whatever about the aeration of water, since they 

 are not gill-bearers but pulmonated ; they breathe at 

 the surface, and do not breathe the air contained in 

 the water. So aeration has nothing to do with the 

 matter. It has so little to do that I have, in some 

 experiments purposely devised, been able to see that 

 pond-snails live exactly as well in two identical 

 vessels (identical in shape, surface, volume of water 

 and amount of aquatic plants) one of which remains 

 open, in contact with the atmosphere, while the other 

 is stopped by a paraffined cork, the amount of air 

 imprisoned between the cork and surface of the 

 water hardly amounting to 50 cubic centimetres. 

 Even if it is argued that some air may pass in and 

 out, through the cork, the quantity is very small, and 

 we may consider the renewal of the air as very incon- 

 siderable so far as penetration from the atmosphere is 

 concerned. Of course the air does and must remain 

 quite suitable for the animals, since they thrive, and 

 the plants are the agents of this continued purifi- 

 cation. If the animals can and do live under such 

 conditions, and even live as well as they do when 

 the communication with the atmosphere is not inter- 

 rupted, does this not show that aeration must be 

 considered as quite sufficient even when the surface 

 is small ? 



G 2 



