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Some fungi are single-celled, while others are multicellular. Single-celled fungi are called yeast. Some fungi alternate between single-celled yeast and multicellular forms depending on what stage of the life cycle they are in. Fungi cells have a nucleus and organelles, like plant and animal cells do. The cell walls of fungi contain chitin, which is a hard substance also found in the exoskeletons of insects and arthropods such as crustaceans. They do not contain cellulose, which commonly makes up plant cell walls.
Multicellular fungi have many hyphae (singular: hypha), which are branching filaments. Hyphae have a tubular shape and are split into cell-like compartments by walls that are known as septa. These cells can have more than one nucleus, and nuclei and other organelles can move in between them. (There is some debate over whether multicellular fungi are truly multicellular, because organelles and cytoplasm can move from one cell to the other in a process called cytoplasmic streaming. They are commonly known as multicellular, but they are not multicellular in the same way as plants and animals, which have enclosed cells.) A fungus network of hyphae is called a mycelium.