HARMFUL EFFECTS OF CROWDING UPON GROWTH iii 



which the latter were dying out. Such results are similar to those 

 reported by Woodruff and others for protozoan infusions. 



Legendre (1907) returned to the problem of the effect of crowding 

 on the growth of snails, using Lymnaea stagnalis and Planorhis 

 corneus, raised in one series of experiments in stagnant water and, in 

 the other, in water changed periodically. As in the case of previous 

 workers, he found the smaller shells in the stagnant water, and at- 

 tributed the cause to the accumulation of excretions. In further work 

 reported the following year, using another species of Lymnaea, 

 Legendre changed the water every 2 hours in order to avoid the 

 accumulation of excreta, and varied the factors of volume of water, 

 surface area, and number of individuals. After 51 days he obtained 

 the same shell size in all such experiments. He recognized that a 

 number of factors might bring about retardation in crowded ani- 

 mals, but laid particular emphasis upon the retarding effect of the 

 excretions. 



Colton (1908) continued work on the effect of crowding on growth 

 in Lymnaea. Food was recognized as an important element, but just 

 how important Colton's work does not reveal. He did find that snails 

 need a certain amount of sediment to aid in grinding their food, and 

 that certain salts, for example calcium sulphate, aid growth. Colton 

 found that washed and filtered snail faeces placed in aquaria has- 

 tened the growth of the snail, probably due to the increase in algae 

 caused thereby. His aeration experiments support the conclusions of 

 Willem that these pulmonate snails have a large proportion of cutic- 

 ular respiration. Concentrated excretory products caused dwarfing; 

 accordingly decreases in volume of water per individual present, 

 whether in isolations or in crowded cultures, caused a decrease in 

 growth rate. Popovici-Baznosanu (1921) minimized the effect of ex- 

 crement, thinking the amount of food more important. 



Crabb (1929) has recently reinvestigated this entire problem with 

 the pond snail Lymnaea stagnalis appressa, taking care that his snails 

 were free from trematode parasites, and supplying them with food 

 known to be favorable for growth in laboratory conditions. He used 

 eggs from the same egg mass for experiments run simultaneously; 

 since this snail reproduces by self-fertilization, individuals obtained 



