52 JEAN DAWSON 



fresh water pulmonates, but it is certain that the viscosity of 

 the mucus in any given snail varies from time to time according 

 to the amount of food taken. The following experiment will 

 serve to show the changes: Mucus was gathered from the foot 

 of a poorly fed snail while it was crawling on the surface film. 

 Lymnaea stagnalis furnishes a larger mass than a Physa and 

 hence is better for the purpose of demonstration. The mucus 

 was found to be jelly-like and, when attempts to stretch it were 

 made, broke in thick chunks with but short connecting strings. 

 If this same snail was well fed for 20 to 30 minutes and its mucus 

 examined, it was found to be much thinner and stretched into 

 longer strings Vvithout being lumpy. Indeed, this difference 

 may be seen by trying to hold back a snail by its ribbon when 

 it is crawling on the film, when the animal is well fed and again 

 when it is. hungry. The poorly fed snail may be drawn in any 

 direction one pleases by pulling back on the ribbon of mucus, 

 but this same snail well fed \xi\\ not be much affected, because 

 it is not adhering strongly to its less viscous ribbon and glides 

 easily from under it. This difference in viscosity ma 3^ also 

 be plainly noted by causing poorly fed and well fed snails to 

 move over the powdered substratum. If the thick mucus 

 of the hungry snail is laid down in a homogenous trail, it shrinks 

 away from its path on both sides while a like ribbon of thinner 

 mucus remains intact. 



This change in the viscosity of the mucus due to the physio- 

 logical conditions of the snail has been described for Limax 

 arborum by Harte (1862-3) and his observations agree 

 with those for the fresh water snails. He says that it has 

 been observed that when this species is gorged with food, its 

 slime is thin and watery and unable to sustain its weight, but 

 that, after the process of digestion has been performed, the 

 mucus again becomes thick and tenacious. 



When Physa is taken from food and placed in conditions 

 where it must fast for some days, its mucus passes from a thin 

 watery consistency to that of a thick jelly. As the flowing 

 mucus becomes thicker it diminishes in volume and finally 

 after four or five days there is no visible flow. On account 

 of this condition of its mucus the animal in the fasting condition 

 has some difficulty in remaining attached to the substratum 

 when its lung is full of air. 



