Heteromastus filiformis is a long thin bright red worm. The animal is filled with hemoglobin, the same pigment which colors our blood red. We use hemoglobin to carry oxygen. The worm uses it for the same reason. This allows the worm to live in parts of the mudflats where there is very little oxygen. The worm burrows itself into a vertical tunnel, head first. It eats up sand containing bacteria, its main diet. The digested sand is excreted onto the flats and looks like black-gray pills.