In the earliest classification systems, fungi were classified as plants. The first classification system to recognize fungi as a separate kingdom was proposed in 1784. Since that time, researchers identified four characteristics shared by all fungi: they lack chlorophyll; the cell walls of fungi contain the carbohydrate chitin (a tough, nitrogenous substance that can be found in the outer shells of some crabs or mollusks); they are not truly multicellular since the cytoplasm of one fungal cell mingles with the cytoplasm of adjacent cells; and they are heterotrophic eukaryotes (unable to produce their own food from inorganic matter, absorbing their food from whatever they are growing on), while plants are autotrophic eukaryotes.