What is This?

[Brian5000]

I never really though much of this guy until I saw it "out" for the first time the other day. It appears to be a snail of some sort whose shell is embedded in the live rock. What is it? Is this thing supposed to be stuck like that? Or is this some ridiculous and cruel circumstance? Here is a pic:
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BMW 320

 

[hdtran]

Can you get a better close-up? At this distance, I'm going to speculate either abalone or limpet.

 

[Brian5000]

It's small (I'd say that door it about 3mm). It's dark with white freckles. It only opens up as far as depicted, far enough to stick out its antennae and snout. I can see its mouth opening and closing (like a snail) as it appears to be sucking in water, and I saw a peice of floating dead algae get sucked into it's mouth. Here is another attempt at a pic:
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Honda GL500

 

[Brian5000]

Can ayone tell me what this is? I don't think it's an albalone or alimpet. From what I've seen looking both those up, a limpet doesn't really have a shell and both are at least motlie. This thing's shell is perminently fused to the live rock. It also think it may not have a foot. It's hard to tell, but it never exposes one. Otherwise it has the exact morphology of a snail.
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Mercedes 220