Nematodes are common and widespread metazoa. Most are 2-3 mm long and have a long thin shape and a slightly blunt anterior end, resembling earthworms. They are rather stiff and can move either by writhing or gliding through a substrate. At high magnification, a strong muscular pharynx is apparent near the front and egg-bearing ovaries near the back end. Nematodes occur in lightly loaded plants operated at a low F/M (food to mass) ratio. They are commonly found in old activated sludge with ample dissolved oxygen and in biofilm reactors. They can be used as a type of bioindicator for determining when toxins have been introduced into a system. Their burrowing action promotes floc health by allowing oxygen to penetrate the larger pieces of floc.