LYMN^ID^E OF NORTH AMERICA. 43 



"Lymnceus clodes (palustris) feeds readily on dead flies, tadpoles 

 and its own eggs, as well as on other snails when their shells have 

 been crushed. It was also seen to engulf and retain the fasces of other 

 snails. Brockmeier's observations of 'plankton fishing' in the case of 

 Limnaus peregra were confirmed. 'Plankton fishing' may be described 

 as follows : While the snail is floating attached to the surface film 

 of the water it will sometimes depress its head-end a little below the 

 water's surface, form a scoop out of the foot, and by means of this 

 collect the microscopic organisms and other floating material at the 

 posterior end, in a trap formed by the mucus which continues to flow 

 out backward, even after the animal itself has become stationary. As 

 soon as sufficient food has been collected, the snail twists its head 

 around to the posterior end of the foot and quickly engulfs the 'catch', 

 mucus and all. Turning now to the matter of plant food, snails were 

 seen rasping off bits of Lemna and they seemed as well satisfied with 

 dead or etiolated portions as with those which were still green. The 

 way in which pond snails plow a clean path across the sides of an old 

 aquarium that has become green with algal growths, is well known 

 and a microscopic examination of the faeces of snails reveals the 

 presence of a large amount of plant food, green cells, diatoms, etc. 

 Rathay found that Pleurococcus can pass through a snail unassimi- 

 lated. A couple of green apples which had been bitten into and 

 thrown into the 'pump pool' were later found covered, wherever the 

 skin was broken, with snails that were evidently feeding. The char- 

 acter of the faeces of these snails furnished confirmatory evidence 

 that they had eaten greedily of the apples. It is reasonable, therefore, 

 to conclude from such observations that snails are omnivorous, al- 

 though they are probably most attracted to plant food. The part they 

 play as natural scavengers renders their presence in water troughs 

 and other sources of drinking water highly desirable. 



"Some rough estimate of the amount of food ordinarily eaten by 

 snails may be seen from the following experiment. A single snail, 

 8 mm. in shell length, was isolated in a dish containing the usual food 

 supply. After 48 hours its faeces, which are cylindrical and approxi- 

 mately of equal diameter, were measured and found to give a total of 

 230 mm. This means that every 24 hours a snail gets rid of over 

 fourteen times its own length in faeces. 



"There seems to be no evidence of any struggle for the possession 

 of food among these animals. Snails will gather around a dead tad- 

 pole, for example, until all the 'standing room' is taken and those 

 that arrive late at the feast are unable to do anything aggressive. Un- 



