We explain what the animal kingdom is, what was its origin and how it is classified, along with its most important characteristics.
The Kingdom Animalia or Metazoa comprises a large group of organisms, and includes all the animals on the planet .
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 .
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:
Animals can be classified into two large groups, each comprising a varied number of classes.
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).
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:
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 .
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.
With the sole exception of sponges, animals exhibit some kind of morphological symmetry. This can be of two types:
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.
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.
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 .
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