AND OF PRODUCTS OF METABOLISM. 305 



be mentioned that in all these experiments De Varigny 

 lifted each tube out of its vessel of water and replaced 

 it two or three times a day, in order to mix the water in 

 it with that in the external vessel. He concluded, 

 therefore, that Semper's hypothesis is not tenable, and 

 that the size of the snails actually depends in some way 

 on the volume of water containing them, and on the 

 superficial area of this water. His explanation of the 

 phenomenon is that in small vessels the snail would 

 need to move about less in order to obtain food, for this 

 would always be near at hand. With less exercise, the 

 growth rate might accordingly be diminished. This 

 explanation does not account for some of the principal 

 results obtained by Semper and by De Yarigny himself, 

 however. Thus in vessels of equal volume, but con- 

 taining various numbers of snails, the amount of move- 

 ment and exercise necessary would be just the same in 

 each case, and yet, as we have seen, the growth rate 

 varies enormously. 



In all probability, the results obtained both by Sem- 

 per and by De Yarigny can be most simply accounted 

 for in the manner already suggested. Thus De Ya- 

 rigny actually found that snails grown in water in which 

 other snails had already been growing several months 

 were distinctly smaller than those grown in fresh water, 

 and if the excreta of snails had been added as well, they 

 were smaller still. If, then, the observed differences 

 in growth are due to the accumulation of various quan- 

 tities of products of metabolism, how can we account 

 for the results obtained by De Yarigny in his muslin- 

 bottomed tube experiments? We must imagine that 

 the mixing of the internal and external waters two or 



