SEMPER'S EXPERIMENTS 85 



narrow neck), and it acquired dimensions certainly 

 equal to those of another one living in a large- 

 surfaced vessel. There is some difference certainly 

 between L. stagnalis and auricularia in this respect, 

 and I call attention to this point. 



I have tested Semper's interpretation in another 

 manner. I have caused pond-snails of the same age and 

 brood to live in unequal volumes of the same water in 

 the following manner. In some eases I have used 

 one, or two, or more glass tubes, of same length and 

 diameter exactly (two pieces of the same tube), 

 which were closed at one end with some muslin 

 stretched over the aperture, and made fast by means 

 of a string or thread wound around the tube. The 

 tubes were suspended in a large vessel containing 

 three or four litres, by means of a string, in such a 

 manner as to allow the other end to rise, say two 

 centimetres, above the surface (to prevent the animals 

 from getting out of the tube and going into the 

 vessel). In each tube I put one pond-snail, with a 

 sufficient quantity of aquatic plants (submerged al- 

 ways), and one in the vessel, outside of the tubes. 

 Every day, and many times a day, the tubes were 

 lifted so as to empty them of water, and immediately 

 replunged, so as to ensure the mixture of the water 

 inside them with the water outside; moreover the 

 water in the tubes was in constant communication 



