446 PROCEEDINGS OF THE ACADEMY OF [Julv, 



Some snails prefer to lay their eggs on the water plant, others alwaj'S 

 lay their eggs on the glass jar, while still others show no preference at 

 all. 



VII. Summary of the Conclusions. 



1. The effect of Myriophylliim and of Elodea on the growth of Lytn- 

 7icea is quite complex. That it is not a simple factor that is being dealt 

 with is indicated by the inconstancy of the results of the experiments. 

 Notwithstanding the fact that one factor has been isolated, yet it is 

 probable that there are other factors besides. This determined factor 

 is the presence or absence of sand in the so-called "gizzard." In the 

 latter case plant tissue, although ingested cannot be assimilated, so 

 that the snail is smaller because it actually lacks food. On the other 

 hand it is possible that the products of plant metabolism may have a 

 harmful effect on the growing snail. 



2. The accumulations of fsecal matter of Lymncea, instead of having 

 the harmful effect on growth as described by De Varigny ('94), when 

 washed and filtered, have a beneficial effect. These tangled masses 

 of unassimilated food form a great harbor for algse, and so increase the 

 food supply of the snail. 



3. The "original planting" of the aquarium, i.e., algse accidentally 

 introduced with the water, causes great variation in the size of the 

 snails. 



4. Experiments on artificial aeration confirm the conclusions of Yung 

 and Willem; yet experiments on surface aeration do not seem so clear. 



5. Vernon reported that Echinoderm larvae raised in solutions in 

 which other larvae had been raised were dwarfed. De Varigny found 

 the same thing true for Lymncea. Experiments on Lymncea columella 

 confirm the results of the two authors referred to. Weak solutions 

 were found beneficial and concentrated solutions harmful. Experi- 

 ments using urea gave the same result. AVliy dilute solutions of 

 excreted matter and urea are beneficial and concentrated solutions are 

 harmful may be explained in the following way. The presence of the 

 excretions which contain plant food may cause more algse, snail food, 

 to grow; on the other hand the solutions are harmful to snail growth. 

 In dilute solutions, however, the quantity of toxic substance may be 

 so little harmful that an increase of food will overbalance the harmful 

 effect. However, this explanation is not very satisfactory as the 

 Echinoderm larva experimented on by Vernon had no mouth and so 

 did not eat. 



6. As Semper and De A^arigny showed, the number of individuals in 



