Multicellular organisms that develop from embryos; one of the three kingdoms of multicellular organisms (the other two being Fungi and Plantae, the plants). The gametes form within multicellular sex organs and never within unicellular structures. The kingdom includes all animals other than protozoons (some of which are colonial); Porifera (sponges) are sometimes excluded because their structure differs markedly from that of other animals. Animals first appeared in the Precambrian, the sponges (Porifera) from one kind of protist forebear, and all other animals from another (or possibly more than one other) protist. The oldest fossils are burrows of a coelomate in rocks rather less than 700 Ma old.