190S.] NATURAL SCIENCES OF PHILADELPHIA. 



Table XX — Alternate Starving and Feeding. 



443 



The results — which may be classed as follows : effect of artificial aera- 

 tion, of surface aeration, of volume, of number of individuals — were 

 exactly the same as those found for Lymnma. 



VI. Effect of External Conditions on the Number of Eggs Laid. 



To supplement the experiments on the effect of external conditions on 

 growth, and to observe the effect of external conditions on some 

 physiological process rather different from growth, the following series 

 of experiments was arranged. When adult Lymncea is brought into 

 the warm laboratory in the late winter or early spring it laj^s an im- 

 mense number of fertile eggs. This fact was made the basis of some 

 experiments. Snails gathered at such a time were placed under 

 various conditions and the number of eggs laid during a given time 

 recorded. Conditions that one would not consider to have any effect 

 whatever on fertihty were quite effective in their results. 



1. Sediment. — As we have seen on p. 424, the presence of sediment 

 is beneficial to snail growth, yet the presence of sediment is also of 

 advantage in increasing the fertility of the snail. 



Four adult snails were isolated in four jars with a small amount of 

 sediment, and four jars were similarly treated without the sediment. 

 At the end of some days the eggs in each jar were counted. See Table 

 XXI. 



