EFFECT OF VOLUME OF WATER 



95 



optimum. He found, in the result, that the size of the shell 

 varied directly in i)roportion to the volume of the water in which 

 it lived, and that this was the case, wliether an individual speci- 

 men was kept alone in a given quantity of water, or shared it 

 with several others. At the close of G5 days the specimens 

 raised in 100 cubic cm. of water were only 

 6 mm. long, those in 250 cubic cm. were 

 9 mm. long, those in 600 cubic cm. were 

 12 mm. long, while those kept in 2000 

 cubic cm. attained a length of 18 mm. 

 (Fig. 37). 



An interesting effect of a sudden fall 

 of temperature was noticed by Semper in 

 connection with the above experiments. 

 Vessels of iniequal size, containing speci- 

 mens of the Limnaca, happened to stand 

 before a window at a time wdien the tem- 

 perature suddenly fell to about 55° F. The sun, which shone 

 through the window, warmed the water in the smaller vessels, 

 Ijut had no effect upon the temperature of the larger. The 

 result was, that the Limnaea in 2000 cubic cm., which ought 

 to have been 10 nnn. long when 25 days old, were scarcely 

 longer, at the end of that period, than those which had lived in 

 the smaller vessels, but whose water had been sutticiently warm. 



Fig. 37.^Four equally old 

 shells of Limnaea stagna- 

 Us, hatched from the same 

 mass of ova, but reared 

 in difl'erent volumes of 

 water : A in 100, B iu 

 250, C ill 600, and D in 

 2000 cubic centimetres. 

 (Alter K. Semper.) 



