The term fungus has more than one meaning. It is best limited to members of the kingdom Fungi - in which the normal trophic form is a system of filaments or mycelia and from which spores are occasionally produced. Feeding usually occurs through the mycelia, and the spores usually facilitate distribution and help the fungus colonize new habitats. The true fungi have their evolutionary origins within the chytrids (some taxonomists include these within the fungi). In addition to the true fungi, a number of other evolutionary lineages have produced fungus-like organisms. The most similar are the oomycetes, a lineage that is related to diatoms and brown algae - all being members of the stramenopiles. Other fungus-like organisms include amoeboid slime moulds. The true fungi are heterotrophic organisms. The cytoplasm is enclosed within a chitinous cell wall. While the majority of species grow as multicellular filaments called hyphae, with all of the hyphae together form a mycelium, some species (such as yeasts) also grow as single cells. Sexual and asexual reproduction of the fungi is commonly via spores, often produced on specialized structures (mushrooms). Some species have lost the ability to form specialized reproductive structures, and propagate solely by vegetative growth. Yeasts, moulds (molds), and mushrooms are examples of fungi. The fungi are more closely related to animals than plants, even though the discipline devoted to the study of fungi, known as mycology, often falls under botany. True fungi lack flagella, but the chytrid ancestors are unicellular organisms that swim using flagella. Occurring worldwide, most fungi are largely invisible to the naked eye, living for the most part in soil, dead matter, and as symbionts of plants, animals, or other fungi. They perform an essential role in many ecosystems in decomposing organic matter and are indispensable in nutrient cycling and exchange. Some fungi become noticeable when fruiting, either as mushrooms or moulds. Many fungal species have long been used as a direct source of food, such as mushrooms and truffles and in production of bread, and in fermentation of various food products, such as wine, beer, and soy sauce. Fungi are sources for antibiotics (such as penicillin) used in medicine and for various enzymes such as cellulases, pectinases, and proteases important for industrial use or as active ingredients of detergents. Many fungi produce bioactive compounds called mycotoxins, such as alkaloids and polyketides that are toxic to animals including humans. Some fungi are used for hallucinogenic effects. Several species of the fungi are significant pathogens of humans and other animals, and losses of crops due to fungal diseases (e.g., rice blast disease) or food spoilage caused by fungi can have a large impact on human food supply and local economies.