A typical myzostomid has a flattened, rounded shape, with a thin edge drawn out into delicate radiating hairs called cirri.

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.

[2] Fridtjof Nansen wrote in 1885 the thesis Bidrag til myzostomernes anatomi og histologi[3] on the Myzostomida.

In the past Myzostomida have been regarded as close relatives of the trematode flatworms or of the tardigrades, but in 1998 it was suggested that they are a sub-group of polychaetes.