Helisoma anceps, synonym: Planorbella anceps, is a freshwater snail from family Planorbidae. It is native to North America. It is sometimes kept in aquaria in Europe.
Planorbidae are breathing free air and their breathing pore is called pneumostome (it is the same as in land snails). The pneumostome is on the right side of the body, but it is close to the median line (= to the center).
The very large pneumostome is visible on this photo of juvenile Helisoma anceps:
Additionally Planorbidae have another breathing organ, that is used for breathing underwater. It is called pseudobranch and it is placed on the left side of the body. It evolved by extending of the anal region and therefore anus is always close to the pseudobranch. The pad-like pseudobranch of an adult Helisoma anceps is about 2 mm wide and 1 mm high. You can see the pseudobranch easily, if you will watch to the left side of your planorbid snails crawling in the water.
The following two photos shows the extended pseudobranch of the same specimen of juvenile Helisoma anceps: