56 JEAN DAWSON 



is spinning, the particles retain the same position relative to 

 one another throughout the process, thus showing that the 

 thread is not being stretched during spinning. (4) It may also 

 be noted that there is seen the same muscular activity of the 

 fore part of the foot that accompanies the flow of mucus under 

 other circumstances. 



The mucus, which is constantly secreted, passes backward 

 through the funnel formed by the curled foot and adds new length 

 to the thread as the snail goes up, just as the mucous track 

 lengthens as the snail crawls over the substratum. This process 

 may be made clear, perhaps, by referring again to the illustration 

 of the unrolling of the tape from the front end of the 

 hollow fiatiron. If the triangular face of the flatiron be curled 

 into a tube, as the iron moves over the substratum, the tape 

 instead of being formed into a ribbon is rolled into a thread. 

 As a rule the flow of the mucus is so continuous that the snail 

 glides gracefully through the water as though impelled by some 

 invisible power. Tye (1878) describes the process of spinning 

 in the following admirable way: " When a mollusk is forming 

 a thread the lateral margins of the foot are brought together, 

 forming a channel for the natural flow of mucus down the sides 

 of the foot to the tail; thus adding to the thread, which is grad- 

 ually extended." 



When the animal has come up on its thread so that its ten- 

 tacles touch the film, it usually stops, the shell is swung around 

 at right angles to the foot and the siphon elongates and opens 

 to take air on the film. Air being taken, the snail usually turns 

 about and begins to crawl down the thread head first. In turn- 

 ing, the foot may or may not come in contact with the film. 

 If it does so, the mucous thread adheres to the film and thus 

 the thread becomes fastened at its upper end. The film is 

 drawn down into a small concavity where the mucous thread 

 touches it. This is due to the fact that the thread is stretched 

 taut by the upward pull of the snail and presumably also by 

 the shrinkage of the mucus through exposure to the water. 

 Should the foot of the snail not touch the film as it turns to 

 descend, the thread is fastened at the lower end only. The 

 foot fails to touch the film oftener than it touches it, hence 

 there are more threads attached at the lower ends only than 

 there are attached at both ends. It may readily be seen that 



