A typical myzostomid has a flattened, rounded shape, with a thin edge drawn out into delicate radiating hairs called cirri. The dorsal surface is smooth, with five pairs of parapodia on the bottom surface. These parapodia are armed with supporting and hooked setae, by means of which the worm adheres to its host. Beyond the parapodia are four pairs of organs, often called suckers. These organs are probably of sensory nature, and are comparable to the lateral sense organs of capitellids. The mouth and cloacal opening are generally at opposite ends of the bottom surface. The former leads to a protrusible pharynx, from which the esophagus opens into a wide intestinal chamber with branching lateral diverticula. There appears to be no vascular system. The nervous system consists of a circumoesophageal nerve, with scarcely differentiated brain, joining below a large ganglionic mass, no doubt representing many fused ganglia. The dorsoventral and the parapodial muscles are much developed, while the coelom is reduced mostly to branched spaces in which the genital products ripen.
Full-grown myzostomids are hermaphrodites. Their internal organs consist of a branched sac opening to the exterior or each side. The paired ovaries discharge their eggs into a median chamber with side branches, often called the uterus, from which the ripe ova (eggs) are discharged by a mediar dorsal pore into the end of the rectum.