Animal Kingdom – Concept, origin, characteristics and examples

We explain what the animal kingdom is, its origin and characteristics. Also, how is its classification, taxonomy and examples.

animal Kingdom
About two million different species belong to the animal kingdom.

What is the animal kingdom?

The animal kingdom or animalia constitutes, together with the vegetable kingdom, fungi, protists and moneras, one of the possible ways in which biology classifies known life forms. It is one of the great traditional kingdoms, despite the fact that the classification has varied markedly over more than 200 years of classification attempts.

The creatures contained in this kingdom are called animals, and are characterized by having enormous ecological, morphological and behavioral diversity, since they are present throughout the planet. At the same time, they are distinguished from other eukaryotic kingdoms by lacking chlorophyll (they do not photosynthesize) and cell wall (present in plant and fungal cells), as well as by their almost entirely sexual reproduction and their capacity for autonomous and voluntary movement.

To the animal kingdom belong to around two million different species around the world, grouped into various taxa or phyla, and into two large categories: vertebrates and invertebrates. In this kingdom, in addition, the human being is also classified.

Origin of the animal kingdom

animal Kingdom
Some sea sponges could even date back 600 million years.

The first animals on the planet appeared during the so-called “Cambrian explosion” 540 million years ago, which consisted of an astonishing diversification and proliferation of life (especially multicellular life or metazoans) in the seas of the early Earth. The reasons that triggered such an evolutionary “boom” are unknown, but some possible culprits would be the proliferation of oxygen as a result of the dominant plant life, as well as the pressures exerted by volcanic and geochemical activity.

The first animal species were sea sponges, some of which could date as far back as 600 million years, according to ongoing studies. But since then animals have had a strong presence in the fossil record, as they proliferated in the seas and then on land and in the air.

Characteristics of the animal kingdom

Animal kingdom - ant
The metabolism of animals cannot produce its own food.

The fundamental characteristics of the animal kingdom can be summarized as follows:

  • They are polycellular and tissue eukaryotic organisms. This means that the bodies of animals are made up of tissues that, in turn, are made up of various types of cells organized among themselves. Even the smallest animals have a body made up of numerous cells, and these are of the eukaryotic type: they have a defined cell nucleus, in which the individual’s genetic information is contained. These cells also lack chloroplasts and cell walls.
  • They are heterotrophic and aerobic in metabolism. The metabolism of animals cannot produce their own food as plants do, so they must consume organic matter from other living beings to survive. This organic matter is digested until obtaining its essential nutrients and from it obtaining glucose, a biochemical molecule that will then be oxidized to obtain the energy (ATP) that sustains the body while walking. This oxidation occurs through respiration: oxygen is taken from the air or water (depending on the species) and CO2 is released.
  • They have their own mobility. This is one of the main distinctive features of animals: they can move at will, either in water, air or land, using specialized limbs: wings, fins, legs, legs. Thanks to this they can change their habitat and look for a more favorable one, escape predators or chase their prey.
  • They have symmetrical bodies. The bodies of animals can present two types of symmetry, that is, they can be divided into two identical halves. The first is bilateral symmetry (the body is divided longitudinally) and the second is radial symmetry (the body is divided based on its radius, since it is circular).
  • They practice sexual reproduction. With some specific exceptions, in the case of animals capable of parthenogenesis, animal species reproduce sexually, that is, through the copulation of two individuals of opposite sex (male and female) and the exchange of gametes or sex cells endowed with the half of the genetic load of the individual, and that they also have markedly different sizes and shapes.
  • Collagen structured bodies. Unlike other forms of life whose bodies are mainly composed of cellulose, animals have collagen as their structural protein.

Animal classification

animal Kingdom
Vertebrate animals are those that have a skull and a backbone.

In principle, the animal kingdom can be classified into two large groups: vertebrates (62,000 species) and invertebrates (95% of total species). As its name implies, vertebrates are those that have a skull and a vertebral column or spine, made up of vertebrae; while invertebrates are those that do not have an articulated internal skeleton.

Other forms of classification serve the specific habitat of the animals, being able to distinguish between marine animals (of the sea and the oceans), aquifers (of fresh water), terrestrial (of the mainland), flying (of the air), amphibians (of mixed life between water and land), parasitic (those that they live within the body of others) or urban (of the city).

Taxonomy of the animal kingdom

Animal kingdom - octopus
Mollusks have a soft body and their habitat is mainly aquatic.

The animal kingdom comprises a vast number of phyla or groups of species that share a well-defined body organization, among which the following stand out:

  • Poriferous. About 9,000 species of immobile sponges, benthic, and bodies with inhalant pores.
  • Cnidarians. About 10,000 species of simple, primitive aquatic animals, endowed with stinging tentacles and sac-like bodies.
  • Acanthocephalus. A phylum of 1,100 species of parasitic worms, whose bodies range from a few millimeters to 65 cm.
  • Annelids. About 16,700 species of worm-bodied invertebrate animals, segmented into rings.
  • Arthropods. A gigantic phylum of more than 1,200,000 described species of invertebrates endowed with chitin exoskeletons and jointed limbs, such as insects, crustaceans, arachnids and myriapods. They are the most numerous edge of the kingdom.
  • Brachiopods. Around 16,000 species of marine animals equipped with two shells or shells, with which they protect their soft bodies, similar to mollusks. They are usually immobile.
  • Bryozoans. A set of up to 5,700 species of marine animals (a few are freshwater) that live a fixed life and have a tentacular crown to capture food by filtering the water.
  • Chordates Around 65,000 species of vertebrate animals, possessing a dorsal cord of cells, of which the majority are fish, but which also include birds, mammals and reptiles.
  • Echinoderms. Marine and benthic animals, of which some 7,000 current species are known, include sea urchins, starfish and the like.
  • Mollusks. Another of the great phyla of the kingdom, includes 100,000 living species of invertebrate animals, soft-bodied and mainly aquatic habitat, among which are octopuses, clams, slugs, etc.
  • Nematodes. A phylum of worms that encompasses more than 25,000 species, commonly called round or cylindrical worms, and they account for 90% of life in the oceanic relief.
  • Flatworms. The so-called “flatworms” are about 20,000 species of hermaphroditic animals from aquatic or humid environments, many of which lead a parasitic life.

It would be necessary here to list many other phyla of numerous animal species, the differences of which can be very specific.

Importance of the animal kingdom

animal Kingdom
The animal kingdom is not only a source of food but also an object of study.

The animal kingdom is one of the most studied and one of the first to be formulated, since human interest in animals dates back to ancient times. Not only as a source of food, or of usable raw materials, but also as a source of biological knowledge that allows answering fundamental questions about the origin of life, the dynamics of the living body or the maintenance of the ecological cycle that allows a diverse world , vast and beautiful.

Examples from the animal kingdom

Some simple examples from the animal kingdom are:

  • Domestic animals: the dog, the cat, the hamster, the birds, the mice.
  • Insects and arthropods, from spiders, scorpions and centipedes, to crabs, scale insects, lobsters and the enormous diversity of insects: mosquitoes, flies, beetles, mantises, cockroaches, bees, etc.
  • Marine animals such as fish, dolphins, sea lions, whales, sharks, mussels, urchins, starfish, but also simpler ones such as zooplankton, jellyfish, etc.
  • Worms in their enormous variety: intestinal parasites, earthworms, crawling worms, etc.