what species of Starfish/snail/sand sifter of any sort?

[intarsiabox]

I'm not sure of their real name but I usually see them in dealers tanks listed as "white sand sifting starfish" for around $15. I've heard they can do too good of a job though and desimate your sand of all life. I'm not sure if this is exaggerated or not as I don't have sand in my tank so have never owned one.