Ill 



EFFECT OF VOLUME OF WATER 



95 



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

 shell varied directly in proportion to the volume of the water in 

 which it lived, and that this was the case, whether an individual 

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

 it with several others. At the close of 65 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. 87). 



An interesting effect of a sudden fall 

 of temperature was noticed by Semper in 

 connection with the above experiments. 

 Vessels of unequal size, containing speci- 

 mens of the Limnaea^ happened to stand 

 before a window at a time when the tem- 

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

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

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

 result was, that the Limnaea in 2000 cubic cm., w^hich ought 

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

 longer, at the end of that period, tlian those w^iich had lived in 

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



Fig. 37. — Four equally old 

 shells of Limnaea stag- 

 nails, hatched from the 

 same mass of ova, but 

 reared in different vol- 

 umes of water: A in 100, 

 B in 250, C in GOO, and D 

 in 2000 cubic centime- 

 tres. (After K. Semper.) 



