92 JEAN DAWSON 



of the snail comes in contact with the film, respiration follows; 

 if the lung is empty or partially so, the response is then as in- 

 variable as is that of the siphon itself. With this ready response 

 to the film it might be expected that the lung would occasionally 

 become greatly distended with air, so as to make descent into 

 the water difficult. The following may be an instance. A 

 snail started to go down the side of an aquarium, after it had 

 been near the air for some time and, without being disturbed 

 in the least, let a bubble of air escape from the lung. The snail 

 had never before been seen to give up air except when disturbed 

 by an unusual occurrence; it is probable that its lung was so 

 distended with air that it found it difficult to crawl down and 

 maintain its hold. This phenomenon has since been observed 

 several times, but only when the snail has taken air frequently 

 at short intervals of time. 



5. Relation between respiration and reaction to gravity. If 

 several Physa are made to give up the air from their lungs 

 and sink to the bottom of the aquarium in which they are ac- 

 customed to live, they begin to crawl about until they reach 

 the perpendicular sides, and then unerringly ascend to the 

 surface film. All efforts to cause them to turn about and go 

 down by giving mechanical stimuli of the usual strength are 

 in vain. Upon the application of unusually strong mechanical 

 stimuli they turn about, but the majority turn immediately 

 back again and ascend to the film. They seem to be insensible 

 to all stimuli but the one that impels them to go upward . Three 

 bottles were stocked with Physa and left with the corks re- 

 moved so that they could have free access to the air for several 

 hours and thus become accustomed to the new habitat. The 

 snails moved in all directions over the sides and bottoms of the 

 bottles and were not seen to collect in any one region. The 

 snails in the first two bottles were then made to give up the 

 air from their lungs and sink to the bottom. The bottles were 

 then filled with water and corked so that there were no visible 

 air bubbles. The snails immediately crawled to the top in both 

 bottles and remained clustered about the cork. The first bottle 

 was then inverted and the snails began almost immediately 

 to turn about and go up toward the bottom, where they re- 

 mained clustered. The third bottle was filled and corked, as 

 were the others, but the snails were not compelled to give up 



