The Animal Kingdom, or Animalia Kingdom, as the name suggests, is home to animals: eukaryotic, multicellular beings with heterotrophic nutrition, that is, they feed by ingestion. In the five kingdom system, this taxa is the most representative.
Animals vary greatly in terms of size, shapes, colors and habitats. Many have bilateral symmetry, which helps with balance and locomotion – the latter is a characteristic found in most of them. In addition, the body of many is already organized into systems. The nervous system and sensory organs, for example, greatly help animals in the task of relating to the environment in which they live, facilitating the capture of food, protection from predators, among other behaviors. As for reproduction, the most common form is sexual, and there are those who reproduce both asexually and sexually.
The study of animals in Biology is called Zoology. In basic education, it has as its object of study the nine main phyla, in terms of diversity: the porifera, cnidarians, flatworms, roundworms, molluscs, annelids, arthropods, echinoderms and chordates.
To further facilitate the study, Zoology is usually, didactically, subdivided into Invertebrate Zoology and Vertebrate Zoology. The first is responsible for the study of animals that do not have a backbone and braincase, that is: the first eight listed in the previous paragraph; arthropods being the most representative. As for Vertebrate Zoology, it is responsible for the specific contents of the Phylum Chordata, which is divided into three subphyla: that of the urochordates, cephalochordates and craniates.
Although the name suggests, it is incorrect to say that all vertebrates have vertebrae, since some chordates do not have such a structure, such as the representatives of the first two mentioned subphyla (urochordates and cephalochordates); and those of the Myxine Class, of the craniates subphylum, represented by the hagfish.