<P> The variations in natural language definitions of what constitutes an organ, their degree of precision, and the variations in how they map to ontologies and taxonomies in information science (for example, to count how many organs exist in a typical human body) are topics explored by writer Carl Engelking of Discover magazine in 2017 as he analyzed the science journalism coverage of the evolving scientific understanding of the mesentery . He explored a challenge now faced by anatomists: as human understanding of ontology generally (that is, how things are defined, and how the relationship of one thing to another is defined) meets applied ontology and ontology engineering, unification of varying views is in higher demand . However, such unification always faces epistemologic frontiers, as humans can only declare computer ontologies with certainty and finality to the extent that their own cognitive taxonomy (that is, science's understanding of the universe) is certain and final . For example, the fact that the tissues of the mesentery are continuous was something that was simply not known for sure until it was demonstrated with microscopy . Because humans cannot predict all future scientific discoveries, they cannot build a unified ontology that is totally certain and will never again change . However, one of the points made by an anatomist interviewed by Engelking is that, finality aside, much more could be done even now to represent existing human knowledge more clearly for computing purposes . </P> <P> The organ level of organisation in animals can be first detected in flatworms and the more derived phyla . The less - advanced taxa (like Placozoa, Sponges and Radiata) do not show consolidation of their tissues into organs . </P> <P> Complex animals are composed of organs and many of these organs evolved a very long time ago . For example, the liver evolved in the stem vertebrates more than 500 million years ago, while the gut and brain are even more ancient, arising in the ancestor of vertebrates, insects, and worms more than 600 million years ago . </P> <P> Given the ancient origin of most vertebrate organs, researchers have looked for model systems, where organs have evolved more recently, and ideally have evolved multiple times independently . An outstanding model for this kind of research is the placenta, which has evolved more than 100 times independently in vertebrates, has evolved relatively recently in some lineages, and exists in intermediate forms in extant taxa . Studies on the evolution of the placenta have identified a variety of genetic and physiological processes that contribute to the origin and evolution of organs, these include the re-purposing of existing animal tissues, the acquisition of new functional properties by these tissues, and novel interactions of distinct tissue types . </P>

Where are the organs located in the body