AND OF PRODUCTS OF METABOLISM. 



303 



supplied was always at an optimum. It was evidently 

 in some way the result of the volume of water available 

 for each snail's needs. Other experiments in which the 

 number of snails was constant, but the volumes of 

 water unequal, gave a similar result. The manner in 

 which the volume of water affected the snail's growth, 

 Semper confessed himself unable to determine; but he 

 supposed that the water must contain some substance, 

 as yet unknown, which is essential for stimulating the 

 growth of the snails. The less of this hypothetical 

 body available, therefore, the more retarded their 

 growth. 



Within recent years De Yarigny * has re-studied the 

 unsolved problem, and has extended Semper's methods 

 in several directions. He used both Limn&a stagnalis 

 and what he termed L. auricularis, though this form 

 was probably L. pereger, judging from his figures.f 

 He confirmed Semper's conclusion that the size is in- 

 fluenced by the number of individuals in the vessel, but 

 he did not find the snails nearly so sensitive to differ- 

 ences in the volume of the water as had Semper. Dif- 

 ferences in the superficial area of the water exposed to 



* Journ. de 1'Anat. et de la Physiol., p. 147, 1894. 

 fNat. Sci. v. p. 168. 



