They have "fans" on their front appendages, tiny little hairs that filter the water for food and infusoria. These 2 are about 3 inches, head to tail, but I've read they can get up to 6.5 inches. They were a little expensive, $12 each.
"... if any one specie does not become modified and improved in a corresponding degree with its competitors, it will soon be exterminated." - Charles Darwin