The animal lies head-down in the vertical limb of the U. It swallows sand from the base of the head shaft, which causes the column above to slump, forming the funnel at the surface. Digested sediment travels through the gut and is voided at the tail shaft, which the worm backs up periodically to deposit as the familiar cast.
The arrangement also handles respiration. Waves of body contraction pull water through the burrow from the tail end forward, oxygenating gills along the trunk and, incidentally, aerating the surrounding sand. On a densely populated flat the worms turn over a substantial volume of sediment each year.
A point of wider interest is the blood. Arenicola marina carries an extracellular giant haemoglobin, an erythrocruorin, that binds oxygen at high capacity and remains functional outside cells. It has been investigated as a candidate oxygen carrier for transfusion and organ preservation, with clinical evaluation still limited. The extent to which lugworm digging pressure affects local populations and shore ecology is debated among fisheries managers.