62 JEAN DAWSON 



is continuously disturbed it moves about with its shell held 

 closer to the glass. Just how long this muscular exertion can 

 last is a question. Probably in a habitat such as that of the 

 creek-bed above cited, the snail may relax behind a sheltering 

 rock. There is no doubt in my mind that this same muscular 

 energy is exerted to keep it from being floated to the surface 

 when its specific gravity is less than water, and when at the 

 same time mucus ceases to flow for brief periods of time. This 

 is also true perhaps in cases whfere snails are fasting and but 

 little slime is secreted. A habitat having debris of moderately 

 coarse stems, straws, or grasses has been observed to be favor- 

 able to spinning. Snails may be seen spinning from the stems 

 or fine leaves of the plants in such a habitat much oftener than 

 in a habitat with a smooth substratum unobstructed by this 

 coarse vegetation. 



VI. "^ Some principles underlying spinning. 



Tye (1878) and Warington (1853) believe that all fresh water 

 pulmonates possess this thread spinning power. Tye bases the 

 ability of the snail to spin wholly upon the fact that it can 

 rise to the surface of the water, when its lung is sufficiently 

 full of air. He says (p. 402), in speaking of the method of res- 

 piration of the pulmonate snails: "It is on this simple fact 

 hangs the capabiHty of the mollusks to spin an upward and 

 downward thread." While we must agree with Tye that the 

 specific gravity of the snail is important, nevertheless the fact 

 'is that all pulmonates do not spin threads. I have kept Plan- 

 orhis trivolvis, Planorhis tricarnata, Lymnaea stagnalis, Lymnaea 

 palustris and Lymnaea reflexa under constant observation for 

 two years and have never seen any of them spin a 

 thread except in the case of a very few individuals of the last 

 two named species. These snails have been placed in a variety 

 of the most favorable conditions to produce spinning, but with- 

 out avail. 



On the other hand some individual mollusks of a well known 

 spinning species never spin a thread. Some large Physae gyrina 

 from one of the creeks of Kavanaugh swamp illustrate this 

 type. There is but Httle doubt that the reverse may be true 

 and that certain few of the snails of a species that do not spin 

 threads may be found to spin. This gives rise to a diversity of 



