48 JEAN DAWSON 



then in no case an epiphragm formed. The animal did not 

 protect itself by secreting more mucus to form a second 

 epiphragm, but retreated into its shell as drying occurred, and 

 remained with the aperture of its shell fully exposed. Walter 

 (1906, p. 7) makes the following remarks upon the relation 

 of the flow of the mucus and locomotion: " When a snail has 

 been allowed to leave the water and to crawl around upon an 

 absorbent surface until its mucous supply is temporarily ex- 

 hausted, it is unable to resume locomotion, on being returned 

 to the water." 



To get a reliable record of the flow of the mucus when the 

 animal is moving and at rest, the following observations and 

 experiments were made: Some neutral non-soluble powder 

 such as chalk or talcum was thoroughly mixed with water and 

 then allowed to settle on the bottom of a dish. Snails were 

 allowed to crawl over this substratum. The powder incor- 

 porated with the mucus caused it to become instantly visible. 

 Walter (1906, p. 7) used carmine particles for this purpose. 



The edges of the ribbon are formed by the glands in the broad 

 anterior part of the sole and are not at any time covered by the 

 tapering posterior portion. Thus, if one is looking down upon 

 the moving snail, this edge of the ribbon may be studied as 

 fast as it leaves the glands of the foot. The mucus covers the 

 path which the snail has plowed, through the powder, and 

 when loosened from the substratum, looks like a white gauzy 

 ribbon. The center of the ribbon may be much thicker 

 than either of the two edges and vice versa. It may appear 

 mottled, i. e., irregularly thick and thin, and there may be places 

 in the snail's path where no mucus has been deposited at all. 



When a snail stops on the substratum, even for some time, 

 there is no accumulation of mucus formed where it rested on 

 the powder. This would seem to be evidence that mucus stops 

 flowing with the cessation of the movement of the foot. An 

 interesting case of the secretion of mucus without the forma- 

 tion of a ribbon may be seen when a snail is quietly feeding 

 on the surface film. This happens \^ith Physa and other fresh- 

 water pulmonates observed, but is especially well shown in 

 Lymnaea stagnalis. The fore part of the foot stretches gently 

 back and forth while the snail is feeding, but the foot as a whole 



