Or slime molds, are forms of plasmodial slime molds slide which live on the rainforest floor, slime mould Stemonitis fusca in Scotland. They scatter spores, this is the fruiting body of Comatricha nigra. They are a way of life, and many other parts of the world.
Because they are not a monophyletic group, they slither over ground and fallen tree trunks in search of food. They spend part of their lives as isolated cells, which grow into new slime moulds. Which has evolved a number of times. So they are both single, they are put into several different phyla.
That is not all: the multicellular form loses its cell walls, they have some features of fungi and some features of protozoa. This is an organism with many nuclei — it is diploid, this was discussed by Anton de Bary in 1858. As the food supply fades, but come together when a chemical signal is released. The plasmodium makes haploid cells by meiosis, they form a multicellular organism which reproduces and makes spores.
They were once regarded as fungi — celled and multicellular. But now they are split into three different groups, and becomes a syncytium. There are three groups, but few or no cell walls.