1908.] 



NATURAL SCIENCES OF PHILADELPHIA. 



425 



was the opposite from that found by De Varigny. However, the 

 latter did not wash the faecal material, and so introduced into the 

 water a large amount of soluble excreted material that he himself found 

 so harmful to the growing snail. 



Table IV — Effect of F.eces. 



^- Rathay's ('98) observations on Helix hortensis and Young's ('88) 

 observations on Helix po?natia and on Arion show that these pulmonates 

 eat a great mass of food, very little of which appears to be assimilated 

 or even digested. 



Observations of the fsecal matter of Lymncea shows the same thing 

 true for these pond snails. Although the writer did not perform 

 any special tests on the cells found in fseces, as did Rathay ('98), yet 

 the appearance of the Pleurococcus and desmids in those masses was so 

 nearly normal that there is very little doubt that there was any 

 change. 



Other Effects of Water Plants. — Warren ('00) discovered that Daphnia 

 in a vessel filled with Vallisneria became less and less productive. If, 

 however, the bulk of the water plant was removed, the crustaceans soon 

 regained their normal number. As green light was found to be unfavor- 

 able to the fertility of Daphnia, Warren concluded that the mass of 

 green plant caused the light to be green and the Daphnia infertile in 

 consequence. 



It can be imagined from what has been said that the effect of water 

 plant on the physiological processes of organisms is not simple and it is 

 not easy at once to discover just how it acts. 



2. Aeration. — This section should be treated under the head of the 

 composition of the water, but as Semper, De Varigny, Willem and 

 Walter have each considered it separately, it was thought best to 

 follow them and make it an independent topic. 



On a priori grounds Semper ('79) and De Varigny ('94) both decided 

 that the only means of respiration in Lymncea was by the specially 



