Erm... ID Please???

[Coasting]

So I have 5 astrea snails in this tank.... 3 normal and 2 star. Last night I was checking my 2 star's out and noticed my bigger guy had a bunch of feather dusters attached to his shell, and my smaller guy had these wierd shell looking things, but the normals had nothing attached to them, just a little coraline on the 1.
Came home today, checked on the tank as usual, and my small star astrea was on the side of the glass, got a closer look at the shell things, and their alive!
...
What are these creepy things?
They havnt moved position since I saw the shells last night
They seemed to be trying to help him clean the glass.
Why is there odd stuff attatched to the star guys but not the normal ones?










And heres a Cowrie photo just because I can






Thanks for the help guys

[Nano]

maybe a parasite? (could be wrong)

[toytech]

they seem to be a type of limpet , i had some on my snails , kept there shell clean and never seemed to do any harm.

[reefgirl189]

I had one of these on an astrea snail too. I left it alone. Astrea is still alive and weird limpet thing went away.

http://youtu.be/R47t211KVmE

Turn your volume down, my dog starts barking in the video.

[ScubaSteve]

Looks to me like some sort of slipper snail (Credula sp.).