I brought 10 home about two weeks ago to help me deal with some algae build up on my sand bed. 8 of which are doing an amazing job and are cleaning up relatively all left over food from the fish. I have two that have not moved nor burrowed into the sand in over a week. I don’t know how to tell if they’re dead and if they are, do I remove them from the tank? Pictures of them are posted. EDIT: After taking the picture of the second I realized there’s absolutely nothing inside of the shell. Now I’m wondering what caused this and how do I prevent it?!