Snail in a hermit crab shell?!

[Mwag]

I picked up my clean up crew on Saturday and everyone seems to be doing fine. It I noticed TWO snails at the top of the tank in hermit crab shells. They are alive and indeed snails. I don't know what type of snails they are but was wondering if snails ever change shells. I have a picture of one because the other one is behind the heater ATM. My CUC contains
Cleaner Shrimp
Peppermint Shrimp
3 Red Legged Hermit Crabs (who are still in their own shells)
2 Nassarius Snails
1 Turbo Snail
2 Astrea Snail
2 Trochus Snail
2 Snails in Hermit Crab shells
Is this normal? They were like this when I got them.

 

[Mwag]

Not the best picture but the best I could do

 

[ahiggins]

Looks like cerith snails to me.
Snails don't change shells, their shells grow with them but hermits constantly change. It's almost always that hermits inhabit snail shells, they like to kill snails for that very purpose (some species more aggressively than others)