BIOLOGY OF PHYSA 97 



pulmonate snail never opens its lung under water unless to take \ \ ' 

 air. They might have been swept into deep water by a strong 

 current or wave while crawling along the bottom; if the snail's 

 lung was full it either gave up its air through the force of the 

 water striking it, or else it was dislodged and came to the top 

 of the water. In the latter case, there would be absolutely 

 no chance for the animal to go under water, unless it did so 

 by expelling the air and falling to the bottom as did the snails 

 which were kept from crawling down the sides of the aquarium. 

 If the lung was comparatively empty it would be but a question 

 of depth of water. The pressure of the water on the outside of the 

 lung would be greater at a certain depth than that of the air within, 

 and the latter would in consequence be displaced by the former. 

 This reasoning is based upon the results of observations made^ 

 in the field which show that Physa does not crawl into deeper-A 

 and deeper water. Hence it would not in this way gradually reach 

 say a depth of one hundred and thirty fathoms in which Forel . 

 found it. Physa reacts so delicately to depth that an increase of 

 a few inches often serves as a barrier. Experimental evidence 

 showing the part that depth of water plays in the local distri- 

 bution of Physa is found in the history of the Cemetery Pool, 

 Ann Arbor, Mich. Formerly, the snail lived only on the side of 

 the pool where an inwash made the waters shallow. The bank 

 was artificially filled and so graded that it was gently sloping 

 on all sides. The limited and discontinuous distribution of 

 Physa was thereby changed to a continuous one and the animal 

 was found inhabiting the shallow marginal waters. 



2. Physa affected by gases in water. If Physa are kept in 

 a deep bottle for some days they gradually collect about the 

 mouth, and they will remain close to the surface film if the water 

 becomes foul. If snails are placed when their lungs are com- 

 paratively empty in water well saturated with carbonic acid, 

 they will die in a short time; but if less of the gas is dissolved 

 in the water, the snails crawl to the surface and rest against 

 the film, or crawl half way into the air. If Physa are kept 

 in boiled water, they crawl to the surface and rest just under 

 the film. If, however, they are put in a bottle of boiled water 

 which is so closed as to shut them off: from the air, they become 

 much less active and die several days sooner than snails kept 

 in tap water under similar circumstances. If a Physa whose 



