Dude, haven't you been to your beaches? Go to Siesta Key and dig your toes into the sand in about a foot of water. You'll find these round hard exoskeletons. If it's still alive it'll be dark green, but the ones you usually find on the beach are bleached white from the sun.
IMO, you don't want to put one in your tank.
I've been lucky enough to run my toes into a couple live ones at Siesta. Of course, though, I left them there. Why would you want to put one in your tank? I don't know what good it'll do. There are other sand sifters that would probably be much hardier.
i think there was another post about sand dollars before, they dont live long in an aquarium and i think they eat the stuff you want in your sand(critters). Dont take my word for it, i am just goin by what i kinda remember.............do a search on them, you may find some stuff about them.........good luck!
I have had them, and they can do well. they are plantonic thoug, so you neen to make sure you supliment. The really dont do well in reef tanks because theres little surface area for them, and you have to watch the fish in a FOWLR to make sure there not eating them. Go out to a sand bar at the beach and sift through the sand........youl find them.