When fully grown, the lugworm of the coasts of Europe is up to long and in diameter. Other species on the North American coast range from . The body is like that of any typical annelid: ringed or segmented. Its head end, which is blackish-red and bears no tentacles or bristles, passes into a fatter middle part which is red. This in turn passes into a thinner yellowish-red tail end. The middle part has bristles along its sides and also pairs of feathery gills. There is a well-developed system of blood vessels with red blood rich in the oxygen-carrying pigment, haemoglobin.