A Great Water Snail in the pond.
You might not know this, but although they can extract some oxygen directly from the water, they also possess a primitive lung system. They make their way to the surface frequently and suck air in through that open tube.
Also unlike land snails, their eyes are not on the end of the stalks. If you look carefully at this one, where that paler line joins the head, ( just above the bubbles), you might be able to see a faint black spot, that is one of its eyes.
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