This small phylum contains only fifty species in two Classes, the Rhombozoa and Orthonecta. The Rhomboza are parasites of Cephalopod Molluscs. They range in size from 0.5 - 7.5 mm, and have only 20 - 30 ciliated cells around one single axial cell. Their life-cycle is not completely known. The Orthonecta are parasites of marine invertebrates. They are free-living in the sexual stage. The germinal cells invade their host, then multiply to form a multinucleate plasmodium that can reproduce asexually producing other plasmodia. When single-sex cells are produced they leave the host to reproduce sexually. It is not known whether the two Classes are truly related, not has their taxonomic position been settled, as some authorities believe they are more closely related to the ciliated Protozoa.

Rhopalura sp., right, is a parasite of Platyhelminthes, Mollusca and Annelida. Its body is simply a layer of epithelial cells, many with cilia, surrounding a group of sex cells.