Once considered to be a class under Annelida, and even a separate phylum, Haplodrili is now widely accepted to be an order under Polychaeta.

Saccocirrus, which also lives in sand, and more closely resembles the Polychaeta, has throughout the greater length of its body on each segment a pair of small uniramous parapodia bearing a bunch of simple setae.

The body is small and resembles polychaete larvae; the epidermis is ciliated; the number of segments varies from five and up, or can be completely absent; small prostomium with or without appendages; parapodia absent; septa reduced or absent; the nervous system consists of a brain and longitudinal ventral nerve cords closely connected with the epidermis (without distinct ganglia); this is widely separated in Saccocirrus, closely approximated in Protodrilus, and fused together in Polygordius; the coelom is well developed and the dorsal and ventral longitudinal mesenteries are complete; the nephridia are simple, and open into the coelom.

Polygordius differs from Protodrilus and Saccocirrus in the absence of a distinct suboesophageal muscular pouch, and in the absence of a peculiar closed cavity in the head region, which is especially well developed in Saccocirrus, and probably represents the specialized coelom of the first segment.

In the male, every genital segment has a right and a left protrusible penis, into which both the nephridium and a sperm-sac open.