66 JEAN DAWSON 



especially the Physa, is spent in floating upon the surface of 

 the water, where they feed upon the particles of decaying vege- 

 table matter, this property of thread spinning seems admirably 

 suited to their requirements." 



Tryon (1882) speaking of the spinning of Physa says: " They 

 • come to the surface to respire the free air and sometimes may 

 be observed gliding shell downward, on the surface of the water, 

 anchoring or letting themselves down occasionally by means of 

 a glutinous thread." Adams (1896) speaking of Physa fontinalis 

 says: " The habit may be watched in an aquarium, where the 

 threads may be seen passing perpendicularly downward from 

 the surface of the water and upward from the bottom." 



Perhaps on account of the invisibility of the thread these 

 observers except Tye, were led into the error of supposing 

 the snail could spin a thread downward from the film. Tye 

 went much farther. He clearly saw that an upward thread 

 could not be spun unless the snail could be made lighter than 

 water and that a downwardly directed thread was impossible 

 unless the snails were heavier than water. He did not, how- 

 ever, go far enough, since he attributed the spinning wholly to 

 this ability of the snail to change its specific gravity through 

 pulmonary respiration. According to this, all fresh water pul- 

 monates should have equal power to spin; yet he himself notes 

 that they do not. 



Tye seems to be confused upon the subject of threads. He 

 first speaks of all threads being anchored to the film without 

 telling us how, then he says that " where one of these mollusks 

 descends by the thread it spins in ascending, it gathers it to- 

 gether by a muscular action of the foot, although these threads 

 are sometimes fixed and made to last a considerable time." 

 Tye speaks of several other things which would bear a word of 

 discussion : 



1. He says the great advantage of spinning is that it is a 

 quicker mode of travel. While this is true it is difficult to see, 

 however, where there is a great necessity for hurry when it has 

 been demonstrated that the snail is in no pressing need of either 

 air or food since it can spin only when fed and when its specific 

 gravity is less than water. 



2. He asserts that the greater part of the life of the Lym- 

 naeidae, especially the Physa, is spent in floating upon the 



