96 JEAN DAWSON 



wire netting was stretched tightly across the aquarium about half 

 way up, so that the snails could not reach the air. The water 

 above the netting was aerated by passing bubbles of air through 

 it. None of the bubbles were allowed to pass beneath the netting, 

 but the movement carried sufficient circulation to keep the water 

 around the snails more or less well aerated. The power of 

 Physa to live without lung respiration was demonstrated by 

 the fact that most of the snails were alive after sixty-two days. 

 Vfhe snails that were breathing entirely through their integument 

 were inactive; compared with a control series, they moved but 

 [Httle and kept near the upper or better aerated water. Shortly 

 after the screen was removed the snails came to the surface, 

 took air as before, and soon became more active. In the field, 

 in habitats having well aerated waters, such as the optimum 

 creek-bed and the optimum Crooked Lake habitats, the snails 

 do not come to the surface and the oxygen which they use is 

 supplied, perhaps, wholly by integumental respiration. 



Physa has been taken by a number of observers from water 

 so deep that there was no possible means of getting air. Forel 

 (1869) who took some of the Lymnaeidae from a depth of over 

 \ f2^o m. in Lake Geneva, vSwitzerland , says that in every case 

 the lungs of the snails taken from this great depth were filled 

 with water, when brought into shallow water they began im- 

 mediately to crawl up to the surface and to inspire air just 

 as if they had never experienced abyssal life. It is impossible 

 to speculate as to the length of time these snails had led a deep 

 water life. They may have been bred there and their ancestors 

 for years before them may have lived in this deep water en- 

 vironment. On the other hand, they may have been ' very 

 recently swept from shallow water. Whether it was years 

 or months that these snails and their ancestors had obtained 

 their air supply from water, the fact remains that they had not 

 ceased to be negatively geotactic when the lung was empty 

 in shallow water, nor did they fail to respond to the stimulus 

 of the surface film immediately, whenever circumstances per- 

 mitted. It would be of interest to know how long it would 

 take to modify or extinguish such reflexes or instincts if totally 

 unused. 



It is a question as to how the snail's lung became full of water 

 in each case where found living as abyssal fauna, since the 



