We explain what the animal kingdom is, what was its origin and how it is classified, along with its most important characteristics.

What is the animal kingdom?

The Kingdom Animalia or Metazoa comprises a large group of organisms, and includes all the animals on the planet .

The creatures contained in the animal kingdom are characterized by a gigantic morphological and behavioral diversity , but are distinguished from the other kingdoms by their absence of chlorophyll and cell wall, their multicellular, eukaryotic and heterotrophic nature , as well as their sexual reproduction and their mobility autonomous.

About two million different species belong to this kingdom , grouped into various taxa (phyla) and into two large categories ( vertebrates and invertebrates ). This kingdom also includes the human being .

Origin of animals

Most of the phyla of the animal kingdom appear in the fossil record during the Cambrian period , in a so-called "explosion" of life that took place in the oceans , around 540 million years ago.

However, there is no adequate evolutionary explanation to account for the origin of animal life, although three different theories are used:

  • the colonial. The animals were born from cell colonies that grew in complexity.
  • the symbiont. Animals were born from symbiotic processes of protist organisms.
  • The cellularization. Animals were born from the development of the cell nuclei of certain primitive organisms.

Animal classification

Animals can be classified into two large groups, each comprising a varied number of classes.

  • Vertebrates. Those that possess a backbone or backbone, such as birds , fish , reptiles , amphibians , mammals , chondrichthyans, and lampreys.
  • Invertebrates. Those that do not have it, such as arthropods ( insects , arachnids, crustaceans ), molluscs , porifera, echinoderms, annelids, nematodes, cniaria and flatworms.

Animal biodiversity

There are almost a million and a half different animal species in total , distributed throughout the planet . Of all classes of animals, the most numerous in species are arthropods (1,200,000 species), molluscs (93,000 species), chordates (64,700 species), nematodes (25,000 species), and flatworms (20,000 species).

Animal feeding

Most animals do not absorb the nutrients they need to survive from the environment . So they must eat them. According to their food preferences, animals can be classified into:

  • herbivores. They feed on plants .
  • Carnivores. They prey on other animals.
  • omnivores. They feed on both plants and animals.
  • Detritivores. They eat decomposing organic matter.
  • parasites They feed on other living things , without killing them.

Animal mobility

One of the essential conditions of the animal kingdom is the mobility of its individuals, as opposed to the stillness of plants and fungi , but shared by other beings of the protist kingdom. Only a few animal species remain fixed throughout their lives .

Cellular Characteristics of Animals

The cells of all animals are eukaryotes, that is, they have a defined nucleus where the genetic material of the cell is housed ; a characteristic that accuses its common origin with plants and fungi. However, unlike these, their cells do not have a cell wall.

Animals are multicellular and have a high level of specialization in highly differentiated tissues , whose structural protein is collagen. Its structure is more flexible than that of plant beings, which allows it a certain margin of cellular reorganization.

Animal symmetry

With the sole exception of sponges, animals exhibit some kind of morphological symmetry. This can be of two types:

  • Radial. In the case of cnidarians, ctenophores and some sponges. It is considered much more primitive. The living being has a circular shape and can be divided into two symmetrical parts in any desired radial direction.
  • Bilateral. As in the rest of the animals. The living being can be divided into two symmetrical parts based on a vertical body axis, which splits it into two identical halves.

Animal breath

Regardless of their habitat ( water , air , land or subsoil), animals breathe, that is, they consume oxygen from the environment and release carbon dioxide (CO 2 ) . Some even achieve gas exchange through the skin , such as amphibians.

Reproduction and embryonic development of the animal

Animal reproduction is typically sexual (barely some due to parthenogenesis), presenting gametes of very different sizes and development through a zygote, a blastula (which is reached by mitosis) and the formation of an embryo.

Some invertebrates can reproduce asexually under specific conditions, especially those that constitute colonies, such as coral.

Animal nervous system

Another important condition of animals is that, in their extremely high cellular specialization, they constituted nerve cell systems that control not only their movements, but also allow them to register and determine the environment (senses) and whose evolutionary maximum point is the human brain , capable of of self-awareness, abstract and complex thought , generation of sign systems and long-term memory retention .

The above content published at Collaborative Research Group is for informational and educational purposes only and has been developed by referring to reliable sources and recommendations from technology experts. We do not have any contact with official entities nor do we intend to replace the information that they emit.


Veronica is a culture reporter at Collaborative Research Group, where she writes about food, fitness, weird stuff on the internet, and, well, just about anything else. She has also covered technology news and has a penchant for smartphone stories. .

Leave a reply

Your email address will not be published. Required fields are marked *