The classical definition of the metazoa is multicellular animals. As per the definition, these organisms exhibit differentiation of cells into many kinds of tissues and organs.  It is evident that there exists a major gulf between protozoans and multicellular animals. These latter organisms are primarily diploid and they develop from embryos. The gametes form within multicellular structures, never within single cells. In most cases we see differentiation into various organ systems including a gut.

The sudden appearance of creatures with differentiation into various specialized tissues and organs and organ systems is a major problem for evolution theory. That is why the Cambrian explosion attracts so much attention. This kind of coordination of embryonic development into specialized body plans which fulfil unique roles in nature, represents major inputs of information which a spontaneous process like evolution is unable to provide.