hitch hiker

I've just spotted this baby snail in my tank. I have no idea what it is. I have pest pond and rams horn snails but it's definitely not one of those. I put some assassin snails in less than 2 weeks ago so it's not one of those either. It has a long, conical shell. It's 'upright' sort of...

So yesterday while I was arranging some stuff around in the tank, after moving my tank lid I noticed some brown squishy thing siting there...I grabbed it on my finger and it was wiggling a bit and looked like it had an antennae or something coming out.  My first thought was, is this a shelled...