BIOLOGY OF PHYSA 71 



retain the faeces of other snails." This scouring habit of snails 

 is well known and on account of it they are commonly put in 

 aquaria \\ith other animals. The use of the mucus in collecting 

 food may be clearly shown if the animal is on the surface film 

 by sprinkling some fine bread crumbs on the surface of the water. 

 The crumbs collect in masses in the floating ribbons of mucus 

 and soon the snail begins to eat crumbs and mucus together. 

 Gradually the ribbon is pulled into the mouth and thus the 

 snail is enabled to gather food from distant parts. The animal 

 however, scrapes off a great many microscopic plant and animal 

 forms from stones and plants in its habitat, independently of the 

 mucus. The collecting power of the mucus may be satisfac- 

 torily demonstrated by examining it under the microscope. 

 Quantities of small animal and plant forms are stuck to it and are 

 thus devoured by snails. In the strict sense of the word, there- 

 fore, even Planorhis trivolis cannot be said to be a vegetarian. 

 It may be seen also that a great deal of non-nutritious substance 

 is taken into the body of the snail in this wholesale house- 

 cleaning process. Indeed, it is difficult to find clean mucus 

 even when the animal is kept for some time in clear water, as 

 waste epithelial cells and other waste products of the body 

 are thrown off with the mucus. 



As has been shown above, the more hungry a snail becomes, 

 the less slime it secretes. At first the slime becomes thick 

 and, as the fasting proceeds, it ceases to flow freely. Conse- 

 quently one cannot pick up a ribbon of mucus from the surface 

 film behind a fasting snail. The lack of mucus may be also 

 noticed when such a snail moves upon the substratum in that 

 the snail sticks to the surface of the smooth aquarium with 

 difficulty and thus is floated many times to the surface of the 

 water. This would be an advantage to the snail in a region 

 where food is scarce, in quiet water, in that it might feed more 

 often from the surface film, and, in moving water it would be 

 transported to another locality with perhaps a more abundant 

 food supply. 



Although Physa eats its own and other snail's slime, yet I 

 have seen it while fasting leave its egg masses unharmed, sticking 

 to the sides of the aquarium. Occasionally I have seen a snail 

 while feeding from the sides of a vessel apparently try to in- 

 gest an egg mass that chanced to be in its path. Failure to 



