Spirorbis corallinae is a very small (1-2 mm) coiled polychaete that lives attached to seaweed in shallow saltwater. It has a smooth, white or semi-translucent, sinistral (left-handed) coiled shell encasing an orange body about 1.5 mm in length. The worm has a short abdominal region and a slightly broader thorax terminating in colourless tentacles, used to filter food from the water. One of the tentacles is slightly larger than the rest and shaped like a saucer, which is used as an operculum. This seals the opening of the shell and serves to protect the worm from predators and desiccation when out of water.