BIOLOGY OF PHYSA 99 



descended further into the water than did those that touched 

 only their tentacles, i. e., the reaction seemed to be more intense. 

 The lungs of the snails were partly empty and they were there- 

 fore negatively geotactic; nevertheless they reacted negatively 

 to the film on account of the presence of the carbonic acid gas. 

 The snails were left in the bottle for about twelve hours. At 

 the end of this time most of them were to all appearances dead, 

 while those that were still able to crawl were found down in 

 the neck of the bottle as far away from the film as they could 

 crowd. All of the snails revived after they had been kept in 

 well aerated water for some time. Physa reacted very strongly 

 away from the carbonic acid. Indeed, the snail's behavior 

 to the gas was such that an observer might be inclined to say 

 that it was very much surprised at finding something so dis- 

 agreeable in the place of air and that it was loath to believe 

 the impressions of its tentacles and siphon and went back to 

 touch the film many times before it could be convinced that 

 so unexpected a change had occurred. The snail that was 

 crawling up to the film slowly, touched it, stopped, sensed the 

 carbonic acid, and reacted negatively. The one, however, 

 that was moving at a rapid rate reacted to the carbonic acid 

 film as though it were an air film, before either the tentacles 

 or siphon discriminated between them. The snail's reactions to 

 carbonic acid gas and oxygen were modified by its momentum 

 much as was its reaction to food and w^e ma}^ note that the 

 effect of the mechanical again precedes that of the chemical 

 stimulus. 



The following experiments give an idea of the delicacy of the 

 reaction of Physa to oxygen. A large number of Physa w^ere 

 placed in a galvanized iron tank six by four feet and containing 

 water about five inches in depth. Water was allowed to fall into 

 the tank drop by drop, from about one inch above the surface, 

 and caused a slight foam to gather at the point where it struck. 

 The Physa collected in great numbers about the foam in a circle 

 about a foot in diameter. The greatest number of snails was 

 found near the place where the water dropped into the tank 

 and the number grew less as the distance from this point in- 

 creased. vSnails were not found to collect on the bottom of 

 the tank in this region nor about the outlet pipe. The cause 

 of this distribution admits of several possible factors being 



