2] 



EFFECT OF EXTENT OF MEDIUM ON SIZE 



475 



explains on the hypothesis that water contains some substance 

 necessary to the growth of the snails, which becomes used up 

 before full size is acquired : the more quickly, of course, the 

 smaller the amount of water. 



Instigated by SEMPER'S experiments YUNG ('85) now under- 

 took similar ones upon tadpoles. Twenty-five just-hatched 

 tadpoles were put into each of three vessels, A, B, and C, contain- 

 ing about 1200 cc. of water, but of different forms, so that A, 

 which measured 7 cm. diameter, had a depth of 30 cm. of water ; 



200 400 600 800 1UOO 1200 liOO 1000 1800 2000 

 CUBIC CENTIMETRES OF WATER 



FIG. 134. Curve of relation between length of shell of Limnrea stagnalis and 

 quantity of water in which it is reared. (From SEMPER, '74.) 



B, whose diameter was 11 cm., had 13 cm. of water; and C, 

 whose diameter was 14.5 cm., had 6.5 cm. of water. At the 

 end of one and a half months measurements were made of the 

 length and the breadth of each tadpole with the following 

 average results : - 



YUNG concluded that the larger size of the animals in the 

 shallower water (C) was due to its absorbing oxygen more 

 thoroughly. 



Finally, DE VARIGNY ('94) has attacked even more system- 

 atically this problem of effect of extent of medium on size. He 

 has found that Limnaeas kept for eight months in similar 

 vessels, each containing 550 cc. of water and about 500 cc. of 



