Any of various small wormlike animals that are parasites of marine invertebrates and have an inner layer of one or more cells surrounded by an outer layer of ciliated cells.
Any of a variety of very small parasitic marine animals of the group Mesozoa. Mesozoans resemble worms and have simple bodies divided into two layers. The outer layer is covered with cilia, and there are no internal organs. The taxonomic status of the Mesozoa is unclear, as some scientists believe the mesozoans to be an ancient and primitive group and others regard them as degenerate forms of more complex animals.