438 



PROCEEDINGS OF THE ACADEMY OF 



[July, 



Although the experiments of Walter ('06) and some of the writers 

 seem to indicate that Lymncea is slightly negatively phototactic, yet 

 darkness is prejudicial to growth (Table XV). 



Table XV — Effect of Light and Dark. 



This factor of light is of less importance than the other external 

 conditions affecting growth, and is one that the snail can directly 

 control to some extent through its behavior, and is also one that can 

 be easily regulated in the laboratory. It is almost impossible to 

 devise experiments on the effect of light on animals whose food con- 

 sists of green plants, and experiments so conducted can have little 

 significance. 



6. Area. — According to De Varigny's exercise theory, dwarfing of 

 Lymncea was caused by too little area for the snail to crawl upon. To 

 test the truth of this hypothesis, structures of various shapes were 

 constructed out of microscopic slides and introduced into one of two 

 similar jars containing snails. As some of the structures were cemented 

 with sealing wax, sealing wax was added to the other jar of the experi- 

 ment, so that there was no difference between the jars, except the fact 

 that one had a larger surface exposed on which the snail could crawl 

 than did the other. The results (Table XVI) were contrary to what 

 might have been expected from De Varigny's hypothesis. It can 

 hardly be that the slides hindered the snails from wandering around ; 

 on the contrary the great area exposed would form a surface on which 

 much more algae would grow. 



