Myzostomid worm. Scanning electron micrograph (SEM) of the under surface of a myzostomid worm. The Myzostomida are an order of small marine polychaete worms, which are parasitic on crinoids.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. The mouth and cloacal opening are generally at opposite ends of the bottom surface (just visible middle left and right). Magnification: x70, when printed 10 centimetres wide.