Animals are eukaryotic, multicellular organisms that form the biological kingdom Animalia. With few exceptions, animals are motile (able to move), heterotrophic (consume organic material); they reproduce sexually, and their embryonic development includes a blastula stage. The body plan of the animal derives from this blastula, developing specialized tissues and organs as it develops; this plan eventually becomes fixed, although some undergo metamorphosis at some stage in their lives.
Over 66 thousand vertebrate and over 1.3 million invertebrate species currently exist.
Zoology is the study of animals. Classification of animals into groups (taxonomy) is accomplished using either the hierarchical Linnaean system; or cladistics, which displays phylogenetic trees called cladograms to show relationships. Some recent classifications based on modern cladistics have explicitly abandoned the term “kingdom”, noting that the traditional kingdoms are not monophyletic, i.e., do not consist of all the descendants of a common ancestor.