Phyllodoce mucosa is a long and slender worm, growing to a maximum length of about 15 cm (6 in) and some 275 segments.

In the posterior part of the animal these tend to coalesce into a continuous dark band; additionally there are a pair of small yellow spots on each segment of this region.

[3] Like other members of its family, P. mucosa has a reversible pharynx which can be turned inside out and which is used to catch prey or engulf food fragments.

It is present on intertidal sand and mud flats, stones and shelly gravels, at depths down to about 20 m (70 ft).

There the worms suck at the dead tissue and may increase their body weight by a third before retreating back into the sediment.