BIOLOGY OF PHYSA 47 



face film it does not come into direct contact with the air. The 

 phenomenon of the mucus flowing from the wider fore part 

 of the foot resuhs in a mucous ribbon being laid down the width 

 of this part. Perhaps the relation of the snail to its mucous 

 ribbon may be made clearer by the following illustration. A 

 hollow triangular flatiron is made to move forward with its 

 broad end in advance, thus crudely imitating the foot of Physa. 

 This hollow flatiron is filled with tape the width of a slot which 

 extends across the whole broad end of the iron. The tape 

 passes out of the slot and extends under the iron and as fast 

 as the iron is moved forward the tape from within is unwound. 

 Now if we imagine the tape adhering to both the solid beneath 

 and the iron above, while the iron is being pushed over the upper 

 side of the ribbon by muscular force while the lower side of the 

 ribbon is left adhering to the substratum, we have in a very 

 crude way imitated the relation the snail bears to its ribbon 

 of secreted mucus, except that in the snail the mucus does not 

 flow from a slot or linear area as in the case of the illustration. 

 The flow of the mucus attends the movement of the snail forward 

 whether it be on solid, film or thread. 



If the animal is moving forward there may be seen upon 

 close examination small areas of muscular contraction over 

 the whole sole of the foot, but more numerous and plainer on 

 the border of the fore part of it. This is especially well shown 

 on the foot of Lymnaea siagnaUs while crawiing on the surface 

 film. These muscular contractions are not in regular waves 

 as they so plainly are in Helix, but are small and irregular, and 

 result in parts of the foot being momentarily slightly depressed 

 here and there over the surface. These momentary slight 

 depressions of the surface of the foot cause its mucus to reflect 

 the light (as may any slight roughness of a bright surface), 

 and they are thus made visible. This muscular activity always 

 accompanies the flow of mucus as it always does locomotion. 

 Whether it plays a part in locomotion or in the flow of mucus 

 is not known. If a snail retracts its foot into the aperture 

 of its shell this muscular activity is not seen, nor is mucus seen 

 to flow (though the foot is still visible). In the experiment 

 described later to show the mode of forming an epiphragm, 

 the mucus was gently removed from the contracted foot which 

 would otherwise have formed an epiphragm, and there was 



