ID of something ?

[glarior]

My friend found these things in his SW tank. The outside shell is hard and underneath its soft. Any idea what they are?
http://img.tapatalk.com/d/14/06/18/zyba6yvu.jpghttp://img.tapatalk.com/d/14/06/18/8upa6ybe.jpghttp://img.tapatalk.com/d/14/06/18/yruja5yg.jpg

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[jroescher]

Limpet. A type of snail. They are good to have.

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[Mike]

Yep +1

[mtfish]

Looks like a slipper limpet.