To us the Animals are the most familiar Eukaryotes. All fish, reptiles, birds, mammals - and humans - are here. Yet these belong to just one group among the 33 phyla that comprise Kingdom Animalia.

Most Animals are unfamiliar and remain virtually unknown to non-scientists. Over 100,000 species making up 25 phyla are for the most part small worms. Several hundred thousand tiny insect-like species fill Phylum Arthropoda. Over 800,000 Animal species have been identified.

All Animals are multicellular. None have cellular walls. The cells are held together by intracellular protein fibers. Animals reproduce sexually, with an egg cell fertilized by a smaller sperm cell, both with one set of chromosomes (haploid). A rear flagella propels the sperm, characteric of the Ophistokont super-kingdom. All Animals eat, most through a mouth cavity of some kind.

Originally thought of as more advanced than the single-celled Protozoa ("first-animals") they were originally named Metazoa ("higher-animals").