The blow lugworm (Arenicola marina), also known as sandworm, is a large species of marine worm.

Its coiled castings are a familiar sight on a beach at low tide but the animal itself is rarely seen except by those who, from curiosity or to use as fishing bait, dig the worm out of the sand.

Its head end, which is blackish-red and bears no tentacles or bristles, passes into a fatter middle part which is red.

The middle part has bristles along its sides and also pairs of feathery gills.

Lugworm blood has a large oxygen carrying capacity which may have medical applications.