Endless Forms in my Pond

This little snail obviously has a niche in my pond, but I didn't know they made them so small. I have seen others about 10mm in length but have no idea if they are all the same species.This little fellow was found in the pond water in a glass with the duck weed growing in it. There are also many other small animals darting back and forth,too fast to study them. The picture below is taken from its underside