102 JEAN DAWSON 



of carbonic acid. A preliminary examination of the water 

 from an optimum Hypnum habitat and from an aquarium having 

 an excess growth of Ceratophyllum showed that the amount 

 of oxygen and carbonic acid varied with the day and time 

 of day. Water from an optimum Hypnum habitat had less 

 oxygen than tap water, and water from the weedy aquarium 

 had less oxygen and more carbonic acid on the average during 

 the day than did that from a less weedy one, which had the same 

 conditions with relation to light. Unfortunately the work 

 was cut short before precise and full data could be obtained 

 and these incomplete results would not be mentioned did they 

 not suggest an explanation of why Physa avoids an optimum 

 pond weed habitat. Birge and Juday (1911, p- 115). 



Water plants give off oxygen and consume carbon dioxide 

 during the process of photosynthesis, which takes place in sun- 

 light. In a crowded condition much of the plant is shaded; 

 hence the process of photosynthesis is greatly lessened. In con- 

 sequence a less amount of oxygen is liberated and less carbonic 

 acid gas is consumed. Respiration, however, is in all proba- 

 bility normal during this time; hence oxygen is being consumed 

 and carbon dioxide liberated during the full twenty-four hours. 

 This may account for the lack of oxygen and the excess of car- 

 bonic acid gas in an optimum water weed habitat and for the 

 small number of snails in such a habitat. 



6. Pulmonary respiration in the young snail. In order to 

 study the respiration of the young snail, a number of egg-masses 

 of Physa were allowed to hatch in the laboratory. When they 

 first came from the Qgg mass, the snails were found to be heavier 

 than water; they could, however, crawl on the surface film. 

 The young snail's lung was either full of water or had not been 

 as yet inflated. If placed in the center of the bottom of the 

 aquarium they reach the side and crawl up to the surface film 

 with the directness of older snails whose lungs are empty. The 

 newly hatched snail is not long near the surface film before it 

 opens its siphon to the air. The lung can be seen to inflate 

 as the air enters and its outline can be plainly traced through 

 the transparent shell. The young snail may be said, therefore, 

 to react to the film in respiration as does the adult. It is not 

 as sensitive to air when it is first hatched as the adult, inasmuch 

 as it does not open the siphon immediately to the film when 



