Animals

An animal is a term used to refer to a living organism that feeds on organic matter. Because of this feeding habit, all animals can be called “heterotrophs,” which means that they ingest other organisms or these organisms’ products in order to sustain life. Animals usually have a nervous system and a set of specialized sense organs that allow them to respond immediately to stimuli. In the strict scientific sense, animals are multicellular and eukaryotic organisms belonging to the kingdoms Animalia and Metazoa. Many animals undergo a process of metamorphosis in their lives, with their body plans eventually becoming fixed as they go along the developmental stage. Most animals can move spontaneously and independently. The term animal comes from the Latin word “animalis,” which literally translates to the phrase “having breath.” When used colloquially, the term animal refers to mammals and vertebrates, but the biological definition includes creatures such as insects, sponges, jellyfish, and even humans. Animals are eukaryotic and have multiple cells in their bodies, traits that clearly separate them from other organisms such as bacteria and protists. When they eat, animals typically digest their food in an internal chamber, and this separates them from algae and plants. Animals also have bodies differentiated into separate tissues, including muscles and nerve tissues (however, the sponges and Placozoa are notable exceptions). All animals have eukaryotic cells, which can be calcified to from organic structures such as bones, shells, and spicules. The groups of animals are as follows: Porifera, Radiata, and basal Bilateria (sponges, corals, sea anemones, jelly fish, and echinoderms), Deuterostomes (starfish, sea urchins, sea cucumbers, and even vertebrates such as fish, amphibians, birds, mammals, and reptiles), Ecdysozoa (insects, spiders, and crabs), Platyzoa (flatworms), and Lophotrochozoa (snails, clams, squids, earthworms, and leeches). These groups are further divided into class, order, family, genus, and species. This section contains scientific information about animals, which is the term used for multicellular and eukaryotic organisms belonging to the kingdoms Animalia and Metazoa.