Snails without shells?


I picked up a piece of life rock today from my LFS. The rock was some that one of the employees had traded for a clam. Anyhow this rock was covered with 2-3 different types of shrooms but along with the rock has come some critters. They look just like my cleaner snails but they dont carry the shell. They are out when the lights are off and seem to be cleaning the glass? Anyone got any ideas of what they are? Will they hurt anything or just clean? Also my cleaner shrimp likes to pick at them if need be i can get some pictures of the critters tomorrow. Any advice would be greatly appreciated.


wablondie sounds like shes right on, especially if they have the humps on the back.
Slugs wouldn't have that little shell.


How big are they?I have seen these,or something similar,in my tank for a year now.The couple I see now are about the size of a nickle.They have grown a grey colored disc looking shell on thier backs.Hmmm?