AND OF PRODUCTS OF METABOLISM. 307 



The greater size of the tadpoles bred in the more ex- 

 posed water Yung attributed to this water absorbing a 

 larger proportion of oxygen from the air. This is in all 

 probability the correct explanation both of these obser- 

 vations and of the similar ones of De Yarigny on snails. 

 The greater supply of oxygen would not only stimulate 

 the rate of growth of the tadpoles and of the snails, but 

 would also hasten the oxidation of the harmful products 

 of metabolism. It is true that De Varigny found that 

 a snail kept for eight months in a corked vessel contain- 

 ing about 550 cc. of water and 500 cc. of air attained to 

 only slightly less a size than another snail kept in a 

 similar but unstoppered vessel, but this may have been 

 due to the fact that green plants were flourishing 

 healthily in each vessel throughout the whole period, 

 and these may have been sufficient to remove most of 

 the metabolic products excreted by the snails. 



Further evidence as to the influence of volume of 

 water on the growth of molluscs has been obtained by 

 Whitfield.* This observer kept a Limncea megasoma 

 in a small aquarium, and after some months it depos- 

 ited eggs. These hatched out, grew in size, and in due 

 course themselves deposited eggs. This process con- 

 tinued for four generations in all, the shells of each 

 generation being smaller than those of the one before. 



* Bull. Amer. Mus. Nat. Hist., vol. i. p. 29; and Amer. Naturalist, 

 xiv. p. 51. 



