JEAN DAWSON 



IV. RESPIRATION OF PHYSA. 



I. Pulmonary respiration. 



1. Lung. 



2. Process of respiration. 



3. Parts of the snail sensitive to air. 



4. Physiological state of the snail affects respiration. 



5. Relation between respiration and reaction to gravity. 



6. Relative buoyancy of Physa. 



II. Integumental or cutaneous respiration. 



1. Physa may live without atmospheric air. 



2. Physa affected by gases in the water. 



3. Physa reacts positively to oxygen and negatively to carbonic acid. 



4. Physa's reaction to shallow waters may be due to greater oxygen 



content. 



5. Physa's reaction away from an optimum pond-weed habitat may be 



due to a diminution of oxygen and an accumulation of carbonic 

 acid. 



6. Pulmonary respiration in the young snail. 

 III. Summary and discussion. 



I. Pulmonary respiration. 



1. Lung. The lung of Physa is a mere infolding of the mantle 

 with numerous blood vessels, but without internal folds. It is 

 situated on the left side of the aperture of the shell near its 

 caudal end and close to the edge of the mantle. A portion of 

 the pulmonary wall ma}'^ be seen from the outside by looking 

 at the ventral side of the snail while it is crawling on the surface 

 film. This exposed portion of the lung is dome-shaped and 

 shows the aperture, if closed, as a crCvScent-shaped line whose 

 convex border lies close to the edge of the mantle. The region 

 of the lung about the aperture is capable of elongating into 

 a slender, tentacle-like tube, the siphon, the aperture of which 

 is circular when opened upon the film. 



2. Process of respiration. In the laboratory respiration takes 

 place at intervals varying from a few minutes to an hour or 

 more ; in the field in such waters as the optimum Crooked Lake 

 and creek-bed habitats, Physa was not seen to come to the 

 surface for air. Physa may be observed to take air into the 

 lung upon reaching the surface film. The snail, upon nearing 

 the film from beneath stops, swings the left side of the shell 

 about through an arc of about ninety degrees until its axis is 

 parallel to the surface of the water. The foot, however, remains 

 in the position . which it had been when the snail came to rest 

 The siphon is immediately protruded to meet the air; if the 



