What is the animal kingdom?

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

1. What is the animal kingdom?

The animal or animal kingdom 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, although the classification has varied markedly throughout more than 200 years of classification attempts.

The creatures contained in this kingdom are called animals, and they are characterized by having an enormous ecological, morphological and behavioral diversity , since they are present throughout the planet. At the same time, they are distinguished from the other eukaryotic kingdoms by lacking chlorophyll (they do not do photosynthesis ) 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 .

Around two million different species belong to the animal kingdom around the world , grouped into several taxa or edges, and in two broad categories: vertebrates and invertebrates . In this kingdom, in addition, the human being is also classified.

2. Origin of the animal kingdom

The first animals on the planet appeared during the so-called “Cambrian explosion” 540 million years ago , which consisted of an amazing 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 ignored, but some possible perpetrators would be the proliferation of oxygen resulting from 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 even 600 million years ago, according to ongoing studies. But since then the animals have a strong presence in the fossil record, as they proliferated in the seas and then on the mainland and through the air .

3. Characteristics of the animal kingdom

The fundamental characteristics of the animal kingdom can be summarized in the following:

• They are polycellular and tissue eukaryotic organisms. This means that animal bodies 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 eukaryotic: 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 metabolism. The metabolism of animals cannot produce their own food as plants do, so they must consume organic matter from other living things to survive. Said 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 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 distinguishing 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 habitat and find a more conducive one, escape from predators or chase their prey.
• They have symmetrical bodies. Animal bodies can have 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 intercourse of two individuals of opposite sexes (male and female) and the exchange of gametes or sex cells endowed with the half of the individual’s genetic load, and they also have notoriously different sizes and shapes.
• Bodies structured by collagen. Unlike other life forms whose bodies are mainly composed of cellulose, animals have collagen as their structural protein .

4. Classification of the animal kingdom

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

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

5. Taxonomy of the animal kingdom

The animal kingdom comprises a vast number of edges or groups of species that share a well-defined body organization, among which are:

• Porifers Nearly 9,000 species of immobile, benthic and body sponges with inhaling pores.
• Cnidarians Around 10,000 species of simple, primitive aquatic animals, equipped with stinging tentacles and sac-shaped bodies.
• Acanthus. An edge of 1,100 species of parasitic worms, whose bodies range between a few millimeters and 65 cm.
• Annelids Around 16,700 species of invertebrate animals with worm bodies, segmented into rings.
• Arthropods A gigantic edge of more than 1,200,000 described species of invertebrates endowed with chitin exoskeleton and articulated 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 endowed with two leaflets or shells, with which they protect their soft body and similar to mollusks. They are usually motionless.
• Bryozoans A set of up to 5,700 species of marine animals (a few are freshwater) that have a fixed life and have a tentacular crown to capture food by filtering the water.
• Ropes Around 65,000 species of vertebrate animals, possessing a dorsal cord of cells, of which the majority are fish, but also includes birds, mammals and reptiles.
• Echinoderms Marine and benthic animals, of which about 7,000 current species are known, include sea urchins, starfish and the like.
• Mollusks Another of the great edges of the kingdom, includes 100,000 living species of invertebrate animals, soft body and mainly aquatic habitat, among which are octopus, clams, slugs, etc.
• Nematodes An edge of worms that covers more than 25,000 species, commonly called round or cylindrical worms, and represent 90% of life in the oceanic relief.
• Platelmintos The so-called “flat worms” are about 20,000 species of hermaphroditic animals from aquatic or humid environments, many of which lead a parasitic life.

It would be missing here to list many other edges of numerous animal species, whose differences can become very specific.

6. Importance

The animal kingdom is one of the most studied and 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 us to answer 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.

7. Examples

Some simple examples of 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, damp piglets, lobsters and the enormous diversity of insects: mosquitoes, flies, beetles, mantis, cockroaches, bees, etc.
• Marine animals such as fish, dolphins, sea lions, whales, sharks, mussels, sea urchins, starfish, but also simpler ones such as zooplankton, jellyfish, etc.
• The worms in their enormous variety: intestinal parasites, earthworms, crawling worms, etc.