On a somewhat related note, I've had swimming jellyfish in my tank a couple times. They disappear for months at a time, then out of the blue I'll see one. Pretty neat. They are small though -- about the size of a pea.
These guys are swimming like jellyfish or mollusks do. They react to stimuli by taking off the glass and swim away. Nice creatures. I only hope they are not dangerous (my favorite inhabitant - the spiny lobster - have recently died after a short and intense agony). Seeing some pics I now lean towards what yorte suggested - they are probably hydroid jellies.