Jointed legs and an external skeleton are the two features that unite all arthropods. But the diversity goes a long way from there with semi-terrestrial amphipods bouncing around the drift grass, to gray sea roaches (isopods) scurrying up and down wharf pilings. Barnacles sweep their feather legs (cirri) through the water to snatch microscopic copepods. Best known are crabs, shrimps and lobsters. Small scavenging crabs and hermit crabs benefit a well balanced aquarium by cleaning out dead animals. Large predatory crabs are best kept in isolation. Shrimp do well in the aquarium if there is plenty of algae and NO LARGE FISH. All crustaceans thrive on a diet of chopped clams or fish.