Heteromastus filiformis is a long thin worm. Adult specimen can grow eight to eighteen centimeters long while the diameter is never more than one millimeter. The worm is blood red in color. This comes from the pigment haemoglobin. The excrement look like black pills. The worm burrows itself, head first, down to 40 centimeters in fine sand and mud in tidal areas, coastal waters and estuaries. The animal lives on bacteria from the oxygen depleted sections of the bottom. The adult heteromastus filiformis forms food for the Nephtys hombergii, flatfish such as plaice and flounder and the bar-tailed godwit. Young specimen form prey for flatworms, crabs, shrimp and benthic animals.