46 JEAN DAWSON 



layer of sediment, in which case the incorporation of particles 

 of sediment into the mucus at once renders it visible, as will 

 be shown by later experiments. The invisible trails of the 

 snail in the water may be detected by passing the hand across 

 the substratum over which the animal has crawled. ^ 



Fresh mucus is not soluble in water. This is easily demon- 

 strated by placing a mass of it in water and watching it from 

 day to day until disintegration takes place. It does, however, 

 undergo changes in the water. If fresh mucus be exposed to 

 water for a few moments it gradually becomes visible, taking 

 on an opalescent and later a blue-white hue. This occurs, how- 

 ever, when the mucus is in mass several mms. thick, and not 

 when it is, as stated above, in a thin ribbon adhering to a solid. 

 The mass of mucus soon shrinks and becomes less viscous, less 

 elastic and somewhat brittle. As the mucus thus becomes 

 more dense it loses part but not all of its elasticity as will 

 be shown by a later experiment. Fresh mucus exposed to 

 the air in sheets, dries, becomes brittle, and resembles in appear- 

 ance a thin sheet of mica or of dried collodion. 



Physa holds its head a millimeter or so from the substratum 

 or the surface film and is thus able to move it freely in all direc- 

 tions. The mucus forming the ribbon apparently begins to 

 flow on the broader anterior part of the foot and apparently 

 moves backward in straight lines from the point from which it 

 started.. Perhaps this is the only part of the foot that does form 

 the ribbon, since if mucous glands were situated over the middle 

 and posterior part of the tapering foot, the ribbon would be 

 unifonnly thicker in the center. The apparent flow of the 

 mucus backward, in straight lines may be easily verified by 

 placing small objects on the fore part of the foot and watching 

 them as the animal crawls on the surface film. As they pass 

 backward they always remain in the same position relative 

 to the plane of symmetry of the foot and also relative to the 

 borders of the ribbon of mucus. Their apparent progress back 

 over the foot in straight lines is due to the fact that the mucus 

 as fast as it is secreted depends from the surface film and re- 

 mains there without further movement, so that the snail crawls 

 ahead under the mucous ribbon as it is laid down. Thus the 

 ribbon has the appearance of moving back over the foot. Since 

 the mucous ribbon is depending from the under side of the sur- 



