412 PROCEEDINGS OF THE ACADEMY OF [J^^ly, 



Both Semper and De Varigny, on a priori grounds, assumed that the 

 manner of respiration in the fresh water puhnonates was entirely per- 

 formed by the so-called lung. This caused them to overlook the 

 factor of the aeration of the water. 



Willem ('96) called attention to this fact and conducted a series of 

 experiments with this particular end in view. Various authors had 

 already noticed that Lymncea in deep lakes never came to the surface, 

 and that under certain conditions they could be made to visit the 

 surface very seldom. Acting on these suggestions, he performed his 

 experiments by running a stream of air bubbles through the water, 

 using De Varigny's experiments as a basis. By this means he was able 

 to explain all of De Varigny's results as due to the simple factor of 

 aeration of the water. 



Parallel to the experiments of De Varigny ('94) and Willem ('96), 

 Vernon ('95) experimented at Naples on the growth of Echinoderm 

 larvffi. His conclusions were that dwarfing in confined spaces was due 

 to the concentration of excretory secretions in the medium. Aeration 

 seemed not to be a factor in the growth of Echinoderm larvae, except 

 for the fact that aeration would tend to oxidize the waste products of 

 metabolism. 



Warren ('00), as a result of experiments with Daphnia in confined 

 spaces, reported that the individuals were dwarfed by the accumulation 

 of their own excretory secretions. This was specific and did not 

 affect the growth and abundance of other crustaceans. 



It will be seen that five factors have been advanced to explain 

 dwarfing in confined spaces. These are lack of oxygen, presence of 

 secretions, lack of exercise, presence of unknown chemical, and the 

 adaption to the necessities of existence. 



In commenting on these explanations Davenport ('99) writes, 

 "There is, however, much reason for believing that Hogg's conclusion 

 is the one which with our fuller knowledge we can hardly improve 

 upon." In the mind of the author Hogg's explanation is not an expla- 

 nation but a statement of the fact that confined spaces do affect growth. 

 It does not help us to understand how and why animals adapt them- 

 selves to their surroundings. 



III. Materials. 



After a few preliminary experiments with Lymncea, Physa and 

 Planorhis, it was soon found that the former was by far the best form 

 for experiments in the laboratory. There are several reasons for 

 this. Lymncea is abundant in the ponds and streams about Philadel- 



