Myzostomids: Myzostomida

All myzostomids live on the bodies of sea lilies, sea stars, and brittle stars. Most species live as commensals on the outside of the bodies of sea lilies. Commensals (kuh-MEHN-suhls) are animals that live on or with other animals without harm to either one. Sea lilies and their relatives obtain food by catching floating food particles with short tentacles located along grooves on their arms lined with small hairlike structures called cilia (SIH-lee-uh). When myzostomids want to eat, they simply extend their mouthparts into this groove and suck up water and bits of food into their mouths.

A few species of myzostomids are parasites on sea lilies, sea stars, or brittle stars. They live inside the outer tissues of their hosts, or infest the body cavities, reproductive organs, or digestive systems.

Most species of myzostomids can function as both male and female. Smaller individuals function as males, but as they become larger, they also have female reproductive organs. Later, some older individuals may have only female reproductive organs. Reproduction takes place when one individual briefly comes into contact with another and attaches a sperm packet. Sperm from the packets penetrate the skin and fertilize mature eggs inside the body. The fertilized (FUR-teh-lyzed) eggs are later released into the water. The eggs hatch into free-swimming, unsegmented larvae (LAR-vee) with bands of cilia. Larvae are animals in an early stage that change form before becoming adults.