Others relate mesozoa to a group including annelids, planarians, and nemerteans. Geoplanidae is a family of flatworms known commonly as land planarians or land flatworms. Planariidae is a family of freshwater planarians. Obama is a genus of land planarians from South America. Some studies have begun the investigation of the evolutionary ecology of these invasive planarians. Some may even feed on other land planarians. Some species of land planarians have become invasive pest species. Cannibalism has been observed in land planarians. Girardia is a genus of freshwater planarians belonging to the family Dugesiidae. Australoplana is a genus of land planarians from Australia and New Zealand. Similar invasions of other terrestrial planarians are occurring in many other parts of the world. As in most land planarians, mating occurs by internal fertilization when two individuals meet. In some species of planarians, sexual behaviour takes the form of penis fencing. The third type occurs for example in planarians. Some land planarians show hunting behaviour, using chemical signals to detect their prey.