BIOLOGY OF PHYSA 63 



opinion among different workers. All of these snails have a 

 specific gravity less than water when their lungs are full of air, 

 and all too, secrete a mucous ribbon; therefore it is obvious that 

 reasons must be sought other than the one offered by Tye to 

 account for the fact that some species of these genera spin and 

 others do not. A characteristic difference in the shape of the 

 foot between the spinning and non-spinning gastropod suggests 

 an explanation. Without exception, the snails that spin have \ 

 slender, tapered or pointed feet, while those that do not hava-> 

 bluntly pointed ones. The more slender and pointed the foot, 

 the greater the ability of the snail to spin. Thus Physa stand 

 as the type of spinning mollusk and Planorhis trivolvis as a 

 type of the opposite extreme whieh cannot spin at all. Varia- 

 tion in the shape of the foot may account for the variation in 

 spinning. 



VII. Biological significance of spinning. 



A snail cannot spin when its lung is not sufficiently full of 

 air to buoy it to the surface. When it is submerged with an 

 empty lung it is obliged to crawl upon some solid to the surface 

 film. It cannot therefore be urged that the spinning is resorted 

 to in time of greatest respiratory need. It is perhaps true that 

 in any habitat where spinning may occur, the animal by spinning 

 gets more oxygen than it otherwise would. It probably has 

 also more frequent access to the food that may be on the surface 

 film and hence there may result both respiratory and food 

 advantages which the spinning snail has over the non-spinner. 

 Spinning may also furnish a means, in some habitats, of reach -~^ 

 ing the surface film \\-here no other means are available. The ' 

 greatest advantage perhaps that spinning gives the snail, is 

 that it may leave its submerged food upon a thread, get air, 

 and usually return to the same spot and to its food supply since 

 spinning usually takes place in a food environment. While a 

 spinning snail may get more oxygen and more and perhaps a 

 greater variety of food than one that does not have this habit; 

 yet spinning cannot be thought to be of vital importance to 

 the animal since it \vill be shown later that the animal breathes 

 by means of both lung and integument and that food is abundant 

 in most localities to so great an extent, in fact, that the animal 

 is more in danger of too much than too little food, since abun- 

 dance may lessen the oxygen content of the water. 



