Yes, seemingly a plethora of other classification listings exist—all depending on various criteria. For example, more informally, animals are often classified as the Metazoa sub-kingdom in the traditional two-kingdom system of classification (animals and plants). Thus, the Metazoa subkingdom is often considered to be synonymous with the Animalia kingdom. This subkingdom includes all animals except the protozoa (for more about protozoa, see the chapter “Bacteria, Viruses, and Protists”).

In yet another example, some biologists divide the Animalia kingdom into two sub-kingdoms: the parazoa (from the Greek para, meaning “alongside,” and zoa, meaning “animal”), which includes multicellular animals with a digestive tract (all animals except Porifera, or sponges) and the eumetazoa (from the Greek eu, meaning “true”; meta, meaning “later”; and zoa, meaning “animal”), which includes multicellular organisms with less specialized cells than the eumetazoa and includes the single phylum of Porifera. (For more about animals, see the chapter “Aquatic and Land Animal Diversity.”)