

A Peculiar Planorbis 



CHARLES M. BREDER, JR 



A tank containing among other things 

 a number of local pond snails (Planorbis 

 Sp.) of rather small size, was taken by 

 the writer as a prize, during December, 

 1917, at a drawing held by the Essex 

 County Aquarium Society. These mol- 

 luscs were very prolific, the aquarium 

 soon becoming the home of numerous off- 

 spring. But having on hand more atract- 

 ive species with which to grace this tank, 

 they were pooled with some Jap snails, 

 Viviparus malleatus, in a large aquarium 

 used for rearing young exotic fishes, and 

 no notice was taken of them for more 

 than a year. 



Recently one rather odd individual 

 drew attention. Apparently the common 

 condition that is responsible for the ero- 

 sion of the shells of snails had eaten a 

 hole completely through the centre of the 

 shell, and the animal was still alive and in 

 good health. Further examination dis- 

 closed that half of the individuals of the 

 species were in the same condition ; liter- 

 ally living doughnuts ! The perforations 

 varied from pinholes to ones making the 

 animal suggest the mentioned pastry. 



It is reasonable to believe that as the 

 snails grow and the shells increase in size, 

 the water becomes deficient in lime. Co- 

 incidentally the water dissolves parts of 

 the shells previously built up, usually the 

 oldest portion — the spire — which for 

 some reason is less resistant to its action 

 than the more recently formed body 

 whorl. It should follow that the greater 

 the number of snails to a given body of 

 water, the greater the degree of erosion. 

 From the observations of the writer this 

 is generally substantiated. Be the cause 



what it may, there is evident weakness or 

 lack of resistance at the apex of the shell, 

 and this has been noted in a large number 

 of species. Such snails have invariably 

 died when the etching perforated the shell 

 and thus exposed the body of the animal, 

 this being especially the case with the red 

 variety of Planorbis corneus. The un- 

 usual feature of the eroded local Planor- 

 bis is that the animals live and appear 



to enjoy perfect health. Can it be that a 

 race of snails is thus developing with 

 power to render innocuous this ordinarily 

 fatal condition? Experiments in artifi- 

 cial selection along this line would be in- 

 teresting, and some sort of result could 

 be expected when it is considered how 

 readily these univalves adapt themselves 

 to changed conditions. 



The following experiment will serve to 

 show the effect of environment on a pond 

 snail. Eggs of Physa heterostropha were 

 allowed to hatch and develop in a small, 



