Myzostomida

a subclass of annelid worms of the class Polychaeta (some zoologists classify Myzostomida as a separate class). Myzostomes are commensals or parasites of echinoderms (sea lilies, starfish, and ophiurids). Their parasitic way of life has led to the simplification and alteration of body structure (as compared with the structure of typical annelids). The oval or discoid body is from 0.5 to 12.5 mm long. On the ventral side of the body are five pairs of processes, or modified parapodia. The processes terminate in hooklike bristles, by means of which the worms attach themselves to the animal host. Myzostomes have a poorly developed brain and no eyes. The excretory organs are metanephridia (usually one pair). There are no circulatory and respiratory systems. Myzostomes are hermaphroditic: the male sex glands mature first, and the female glands later. A larva, the trochophore, emerges from the fertilized egg.

There are seven families of myzostomes, comprising about 120 species, distributed primarily in tropical and subtropical seas. In the USSR six species are found in the Barents, Kara, and Bering seas, as well as in the Sea of Okhotsk and the Sea of Japan. Some myzostomes are motile and crawl rapidly on the body of the animal host; others remain near the host’s mouth opening or penetrate the intestine. Some of the worms are found in the integuments of the arms and disk of the host, forming characteristic cysts.