And to say that they're really not suited to aquariums is an understatement. They're not at all suited to home aquariums.
They feed on mostly worms and mollusks that live in the sand and they can only feed while they're actually walking. As a result, it takes a very large, well-established sandbed just to produce enough food and browsing room for even a small one. As they grow, it would take a tank of literally several thousand gallons to keep them since as Tom pointed out, they get over a foot long. Of course, in captivity they're usually going through the long process of starving to death, so it's rare to see more than an inch or so of growth before they die.