64 JEAN DAWSON 



VIII. Brief discussion of the literature on spinning. 



The spinning of mollusks has been known since Montagu 

 (1803) recorded the follo\nng observations: '' Physa fontinalis 

 spins a filament by which it lets itself down from the surface 

 after floating." Warington (1852) gives an interesting account 

 of the thread spinning of Lymnaea glutinosa, L. stagnalis, various 

 species of Planorhis and Physa fontinalis. He ends by saying 

 that, "All fresh -water snails are possessed of this power." 

 Reeves (1863) does not believe that Montagu and Warington 

 are right in their observations of the spinning mollusks. He 

 gives the following account of his observations : ' ' The Physa 

 especially P. hypnorum are active in habit, whether swimming 

 foot uppermost, on the surface of water, holding themselves 

 stationary at different depths in the water, or gliding through 

 it in sudden jerks by a hydraulic action of the foot. By bringing 

 the lateral margins of this organ into contact the animal con- 

 structs a tube for inhaling and suddenly expelling the water 

 either upward or downward. Montagu stated, and the state- 

 ment has been repeated by Jeffrey, that the animal spins mucous 

 thread for letting itself down in the water and rising again for 

 respiration; but I have not succeeded in confirming and have 

 great doubts of its accuracy." Tate (1866) in speaking of Physa 

 says: " that it ascends at will to the surface in a direct line, or 

 returns to the bottom, or holds itself suspended in the water 

 with facility." " This little Physa can raise itself in the water 

 or sink at pleasure, without having resource to plants to assist 

 its progress; in descending through the water the movement is 

 gradual and uninterrupted, and is affected by means of a thread 

 affixed to the surface, a mode of progress analogous to that of 

 some of the land slugs which I have referred to as spinning a 

 line of gelatinous secretion from the skin." Tye (1878) did 

 more and better work than any one else on the subject of thread 

 spinning. He takes occasion to severely criticise Reeve for so 

 coolly doubting Montagu and overlooking Warington and says 

 that the mollusk is capable of " holding itself stationary at 

 different depths in water " when attached to a thread and that 

 no " hydraulic action " of the foot takes place. 



I have already quoted Tye's view that spinning is wholly 

 dependent upon the amount of air in the lung. He would there- 

 fore echo Warington 's statement that all fresh water snails 



