Parazoa of the phylum Placozoa contains only one known living species Trichoplax adhaerens. A second species, Treptoplax reptans, has not been observed in more than 100 years. Placozoans are very tiny animals, about 0.5 mm in diameter. T. adhaerens was first discovered creeping along the sides of an aquarium in an amoeba-like fashion. It is asymmetrical, flat, covered with cilia, and able to adhere to surfaces. T. adhaerens has a very simple body structure that is organized into three layers. An upper cell layer provides protection for the organism, a middle meshwork of connected cells enable movement and shape change, and a lower cell layer functions in nutrient acquisition and digestion. Placozoans are capable of both sexual and asexual reproduction. They reproduce primarily by asexual reproduction through binary fission or budding. Sexual reproduction occurs typically during times of stress, such as during extreme temperature changes and low food supply.