A metazoan is any member of the animal kingdom (Animalia or Metazoa) usually considered to include multicellular, heterotrophic eukaryotes in which (unlike plants) the cells lack cell walls. The term metazoan is less commonly used now than it was because it stems from a classification in which there were two main divisions within the animal kingdom: the multicellular animals, or metazoa, and the single-celled protozoa. The protozoa are no longer included in the animal kingdom, so only the metazoa remain.
Metazoans include everything from sponges and jellyfish to insects and vertebrates. It is used to be thought that sponges evolved from a different single celled organism to higher animals, so that the the Metazoa were considered to be a polyphyletic taxon. But recent molecular phylogenetic evidence indicates this is not the case. The metazoans are considered to be monophyletic, all of them having evolved from a single unicellular choanoflagellate ancestor, sometime during the Ediacaran period.
A metazoan has a body composed of eukaryotic cells that are differentiated into tissues, organs, and usually a digestive cavity. It develops from an embryo and is characterized by having two layers of cells in the gastrula stage. Unlike other metazoans, sponges do not have muscles or nerves and therefore do not move except at the level of the individual cell.