Animals are any of the species of organisms that are assigned to the taxonomic Kingdom Animalia, which contains groups broadly categorized as invertebrates and vertebrates. The latter contains the familiar types such as mammals, amphibians, birds, fish, and reptiles. The former, however, consists of those with or without an exoskeleton like the insects, crustaceans, jellyfish, worms, etc.
In the vastness and diversity of species belonging to the animal kingdom, we can generalize some aspects. With several exceptions, in particular parazoa, placozoa, and mesozoa, animals have a differentiated body in four distinct tissues: epithelial, connective, muscular and nervous. All animals have eukaryotic cells, surrounded by a characteristic extracellular matrix composed of collagen and elastic glycoproteins. This can be mineralized to form structures such as shells, bones, and spicules.
Different classifications of animals, as well as that of other kingdoms, have been proposed over the years. The first classifications were based mostly on morphological characteristics, taking into account, depending on the author a greater or lesser number of characters. Subsequently, the organisms were grouped also considering their embryonic development. In recent years, as is the case for the other realms, a classification based on molecular genetic studies is sought, based on the principle that certain genes are kept almost equal in the various groupings and the number of variations in the bases of DNA can be correlated with the time spent since leaving a common ancestor (Molecular Clock).
Chronologically the first taxonomic observations, collected in various scientific writings such as Historia animalium(History of Animals), De partibus animalium (Parts of Animals) and De Generatione Animalium (On the generation of animals) are traced back to Aristotle. Although he is often considered the founding father of modern Zoology, Aristotle never proposed an exhaustive and scientific taxonomic system. His studies were mostly scientific, physiological or ethological annotations, without applying, in any case, a true theoretical taxonomic project.
Carl Linnaeus created the first hierarchical biological classification for animals in 1758 with his Systema Naturae, which Jean-Baptiste Lamarck expanded into 14 phyla by 1809. In 1874, Ernst Haeckel divided the animal kingdom into the multicellular Metazoa (synonymous for Animalia) and the Protozoa, single-celled organisms no longer considered animals. In modern times, the biological classification of animals relies on advanced techniques, such as molecular phylogenetics, which are effective at demonstrating the evolutionary relationships between animal taxa.