BIOLOGY OF PHYSA 49 



does not move from its position on the surface film. The minute 

 areas of muscular contraction may be seen in the fore part 

 of the foot, but not at all on the posterior and middle portion, 

 and it is worthy of note that the flow of mucus is seen only 

 in the parts where these contractions occur. The consequence 

 is that the mucus from the fore part of the foot does not leave 

 it as a flat ribbon stuck to the surface film, but flows back to 

 the somewhat hardened mass of mucus on the middle foot 

 and is pushed up over it. Thus, when the animal remains 

 feeding for some time in one place on the film, a mass of mucus 

 becomes heaped up in the center of the foot. 



II. Formation of an epiphragm. 



The dried mucus may form an epiphragm over the aperture 

 of the shell when the snail is exposed to drought. The epiphragm 

 consists of a thin sheet of dried mucus stretched usually just 

 within the aperture, so that it forms a thin partition between 

 the retracted snail and the dry substratum of air. The epiphragm 

 may be formed (i) when the animal is in the drying mud, (2) 

 on a grass stalk or other perpendicular solid, or (3) when it 

 is turned aperture up. In the early part of this research, a 

 series of swamps was studied for the purpose of learning some- 

 thing of the effect of desiccation on snails. Lymnaea palustris 

 was found to have formed an epiphragm in all three positions 

 above mentioned. To determine the way in which this epi- 

 phragm was formed, a number of Physa, Lymnaea palustris 

 and Lymnaea stagnalis were placed in a dry glass beaker and 

 observations made upon them from time to time. 



The snails crawled upon the dry substratum for an hour or 

 more. At first they moved with apparent ease with the foot 

 quite fully extended, but gradually the mucus on the foot began 

 to get drier and drier until further movement was impossible 

 and the snail finally remained quiet in its track. The foot 

 gradually retracted into its shell as movement ceased and finally 

 remained for some time just within its aperture. The foot 

 was contracted into the center of the aperture surrounded by 

 its retracted mantle, both together forming a smooth, plane 

 surface. The mucus could be seen to become dry and glassy 

 over the surface of both mantle and foot. Soon after this 

 condition was reached, gentle muscular contractions of foot 



