Heteromastus filiformis (Capitellid Worm) is a species of segmented worms in the family Capitellidae. They are upward conveyors. Individuals can grow to 100 mm. They have parental care. Reproduction is semelparous, iteroparous, and gonochoristic. They are facultatively mobile burrowers.

Heteromastus filiformis is a long thin bright red worm. The animal is filled with hemoglobin, the same pigment which colors our blood red. We use hemoglobin to carry oxygen. The worm uses it for the same reason. This allows the worm to live in parts of the mudflats where there is very little oxygen.

Heteromastus filiformis is probably a complex of cryptic species belonging to the Capitellidae family. They lack protostomial or peristomial appendages, with branchiae absent or greatly reduced. The body has a distinct protostome, a thoracic version with a fixed number of segments, and a variable number of abdominal segments.