The coexistence of two species of organisms, in which one species (parasite) damages the other species (the host), is called parasitism. Often this biotic relationship leads to death of the host in the medium term. Therefore, the life cycle of the parasite is often associated with a host change. In rotifers you can sometimes find foreign bodies in the body cavity or on the organs. It seems likely that these are (developmental stages of) parasites. The taxonomic classification, biology and ecology of these parasites is unclear in many cases.