A multicellular organism is an organism that is made up of more than one cell to form the body, as opposed to unicellular organisms. To form a multicellular organism, the cells must attach to one another. The first multicellular organisms were cyanobacteria, myxobacteria, and actinomycetes. After them, multicellular protists, fungi, plants, and animals came into existence.
Caenorhabditis elegans (a nematode)