BIOLOGY OF PHYSA . 59 



The thread, if used by the snails to any great extent, attains 

 considerable size since every time a snail crawls up or down it 

 leaves a coating of mucus. Observations show, however, that 

 the threads are of very short duration since the mucus becomes 

 brittle and easily broken and is eaten by the snail, especially if 

 food particles settle upon it. 



The following are variations from the process above described 

 in spinning: 



1 . Instead of the snail taking air, when it reaches the film and 

 then descending in the usual manner as above described, it 

 may crawl with its body partly upon the film and then turn 

 and descend upon the thread. 



2 . The snail may crawl off upon the film after it has ascended 

 upon a thread and may or may not stop to take air. 



3. Several snails may crawl upon an anchored thread at the 

 same time, one crawling over the other if it chances to be moving 

 faster or if one is going up and the other going down. 



4. When the thread chances to be spun near the side of a 

 dish or other surface, the snail in taking air may touch the head 

 or tentacles against this solid and ma}^ crawl upon it instead 

 of on the thread or surface film. 



5. Snails may crawl up on a thread, take air and descend ; 

 when they get to the bottom, turn, ascend again, take air, and 

 repeat the journey up and down the thread several times with- 

 out stopping. They may crawl oft' finally on the film or on the 

 substratum. 



V. Conditions affecting spinning. 



I. Food. Spinning may be largely controlled by the food 

 supply. This may be proven experimentally at any time that 

 the snails are in good condition, but are not spinning, by feed- 

 ing them upon apple or raw meat. Repeated experiments 

 show that snails brought from the field and placed in an aqua- 

 rium with a good food supply, spin very actively while those 

 in a control aquarium with little or no food do not spin, al- 

 though they move about actively engaged in eating from the 

 sides of the aquarium and are in general in good condition. 

 Now if this experiment is re^^ersed and the well fed snails are 

 given no food and the control snails well fed, the spinning will 

 gradually be reversed. It is rare to see a poorly fed snail spin 

 a thread and equally rare to see a well fed snail refrain from 



