476 EFFECT OF COMPLEX AGENTS OX GROWTH [Cn. XIX 



air, but differing in that one was stoppered and the other not, 

 had attained about the same size (Fig. 135). Since the oxygen 

 supply of the snails in the two vessels was evidently very dis- 

 similar, YUNG'S explanation is shown to be inadequate. Next 

 DE VARIGNY tested the effect of differences in volume. He 

 reared Limn seas in different quantities of water, in vessels of 

 dissimilar proportions, such that in all there was the same free 

 surface area of water. The differences in volume had only a 

 slight effect on the size of the snails (Fig. 136). On the other 

 hand differences in surface, with constant volume of water, have 

 an important influence on size. The greatest growth occurs in 

 the broader vessel (Figs. 137, 138). The number of individuals 

 in the vessel influences the average size, as SEMPER found. 



To test SEMPER'S theory that the small size of the animals in 

 the small vessel is due to its having exhausted the necessary 

 substance in the water, DE VARIGNY partitioned a vessel of 

 water into two unequal compartments and reared a Lymnea in 

 each. One of the compartments was made by partly submerg- 

 ing a test-tube, from which the bottom had been removed, in a 

 beaker of water. A piece of muslin, tied over the bottom of 

 the test-tube, permitted an interchange of water, but not the 

 intermigration of the Limnjeas between the test-tube and the 

 main vessel of water. The latter constitutes the second com- 

 partment in which the snails lived. After several months the 

 individuals kept in the two vessels usually showed a marked 

 difference in size, those reared in the roomier vessel being the 

 larger (Fig. 139). When, however, the two similar snails 

 were kept in similar tubes plunged in beakers of water of 

 unequal volume, they attained about the same size. 



A second test of SEMPER'S theory applied by DE VARIGNY 

 consisted in comparing the growth of Limmeas in fresh water 

 with their growth in water in which snails had for some time 

 been developing. The snails in the latter showed a slight but 

 nearly constant inferiority in size. It may well be that some of 

 the salts necessary to growth had been extracted from the water 

 by the development of snails therein, making it less fit for growth. 

 Nevertheless, even the most mai^ked differences in the size of 

 the snails in the two kinds of water was not sufficient fully to 

 account for SEMPER'S result by means of SEMPER'S theory. 



