A phylum of extremely simple and cryptic marine animals, for which only two species have been described, Trichoplax adhaerens and Treptoplax reptans. These tiny animals were discovered in Europe in the late 1800's living on the glass walls of an aquarium. Since then, most of what has been learned about their biology has come from studying cultures kept in various laboratories around the world. Their bodies are made up of only a few thousand cells of just four types. They also have the smallest amount of DNA measured for any type of animal. They lack tissues, organs and organ systems.They reproduce asexually, but it is not known if they can also reproduce sexually. Practically nothing is known about them in nature. Some scientists believe they emerged early in metazoan evolution, either before or just after the sponges (Porifera). However, recent DNA studies lead others to believe they emerged after the Cnidaria. Of the two species described, Treptoplax reptans has never been seen since its description in 1896, causing some to doubt its existence. However Trichoplax adhaerens has been reported from the Mediterranean and many tropical and subtropical locations around the world. It may be that Trichoplax adhaerens actually consists of more than a single species.