Animal Kingdom (Animalia)

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

What is the animal kingdom?

The kingdom Animalia or animal kingdom is one of the main groups in which biology classifies living beings along with the plant kingdom (Plantae), the fungal kingdom (Fungi), the protista kingdom (Protista) and the monera kingdom.

The species grouped in the kingdom Animalia are called animals, and they are characterized by being multicellular organisms (they have more than one cell), eukaryotes (their cells have a nucleus) and heterotrophs (they feed on other living beings).

Furthermore, animals are mobile, at least at some stage of their life, and most reproduce sexually.

The most common classification of animals divides them into vertebrates and invertebrates according to whether or not they have a backbone. These categories are used in biology to facilitate the study of the animal kingdom, although taxonomic classifications do not include them. The vast majority of animals are invertebrates.

Among the main groups of animals are sponges (porifera), the group of jellyfish, anemones and hydras (cnidarians), flatworms (flatworms) and round worms (nematodes). Also distinguished are mollusks (such as octopuses and clams), annelids (such as earthworms), arthropods (such as spiders, shrimp and ants), echinoderms (such as starfish) and finally chordates. All vertebrate animals belong to this last category, including humans.

They belong to the animal kingdom approximately 1.5 million registered species of which only 3% are vertebrates. Some species are very old, for example, sponge species that are 600 million years old have been recorded.

• See also: Biological kingdoms

Characteristics of the animal kingdom

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

• are multicellular eukaryotic organisms. Animal cells are eukaryotic cells (they have a nucleus and organelles inside). In turn, animals are composed of more than one eukaryotic cell. Unlike plant cells, animal cells lack chloroplasts and a cell wall.
• are heterotrophic organisms. Animals cannot produce their own food like plants do, so they must consume organic matter from other living beings. Organic matter is digested inside the body until the necessary nutrients are obtained.
• have aerobic metabolism. Animals obtain chemical energy through cellular respiration, a metabolic process that involves consumption of oxygen and release of CO₂ as a byproduct.
• have their own mobility. Animals can carry out voluntary movements, at least at some stage of their lives. They have specialized structures that they use to move in water, air or land (such as wings, fins, legs, legs). This characteristic allows them to change habitat and look for a more favorable one, escape from predators or chase prey.
• have sexual reproduction. Most animals reproduce by generating two types of gametes or sex cells. However, there are some groups of invertebrate animals that have asexual reproduction mechanisms (for example, flatworms).

• Reproduction in animals
• animal cell

Classification of animals

In biology, the animal kingdom is usually classified into two large groups:

• Vertebrates (those that have a vertebral column or spine)
• Invertebrates (those without a spine)

The vast majority of animals are invertebrates, making up 97% of the total known animal species.

Other forms of classification, less used in the scientific field, take into account the specific habitat of the animals. For example, we distinguish marine (from the sea and oceans), aquifers (from fresh water), terrestrial (from solid land), flying (from the air), amphibians (of mixed life between water and land), parasitic (those who live inside the bodies of others) or urban (of the city).

• Classification of animals

Taxonomic classification of the kingdom Animalia

The kingdom Animalia comprises a vast number of phyla or groups of species.

The main ones are:

• Poriferous. This group is made up of sponges. They usually live in salt water, although there are also freshwater species. They can reproduce asexually. There are about 9,000 species.
• Cnidarians. This group is made up of hydras, anemones and jellyfish. They are simple aquatic animals, which have a sac-shaped body and use tentacles to capture their prey. There are about 10,000 species.
• Platyhelminths. This group is made up of “flatworms”. Some are free-living and others are parasitic. They can be hermaphrodites. There are about 20,000 species.
• Nematodes. This group is made up of the “roundworms”. They have a cylindrical body and are found in a large number of habitats. There are more than 25,000 species.
• Mollusks. This group is made up of octopuses, clams and snails, among others. They have a soft body and often a shell or valve. There are about 100,000 species.
• Annelids. This group is made up of worms that have a cylindrical body segmented into rings. Includes worms and leeches. It is estimated that there are around 16,700 species.
• Arthropods. This group is made up of insects, crustaceans, arachnids and myriapods. They have a chitin exoskeleton and articulated limbs. It is a very extensive group that includes more than 1,200,000 described species.
• echinoderms. This group is made up of hedgehogs, starfish and other similar organisms. They have radial symmetry and are exclusively marine. Their skeleton is made up of rigid plates under the skin. There are about 7,000 current species.
• Chordates. This group is made up of vertebrates, amphioxus and tunicates. They share the existence of a notochord (an embryonic structure that has special relevance in the development of the nervous system). There are more than 65,000 species.

• Classification of the animal kingdom

Examples from the animal kingdom

Some simple examples from the animal kingdom are:

1. yellow tube sponge (Aplysina fistularis). Vive in the Caribbean and is one of the most abundant marine sponges. It is characterized by having a tubular structure similar to some corals.
2. Portuguese man-of-war (Physalia physalis). It is a jellyfish, with venous tentacles that can be toxic to humans.
3. freshwater planaria (Planaria torva). It is a small flatworm that has the ability to regenerate a part of its body if it is fragmented.
4. garden snail (Cornu aspersum). It is a land snail known as a garden snail. Has male and female sexual organs
5. earthworm (Lumbricus terrestris). It is an earthworm, with the body divided into segments. It feeds on organic matter.
6. yellow scorpion (Buthus occitanus). It is a type of scorpion that lives in arid places and has nocturnal habits.
7. red pointed star (Protoreaster linckii). It is a starfish that lives on the African continent.
8. white shark (Carcharodon carcharias). It is a shark that lives in the sea within continental shelves and feeds on marine mammals.
9. Panther (Panthera tigris sumatrae). It is a feline native to the island of Sumatra. Among the tigers, it is the smallest. It is an endangered species.
10. domestic horse (Equus ferus caballus). It is a domesticated mammal. It is believed that primitive horses were much smaller than today's horses..
11. common frog (Pelophylax perezi). It is a green or brown amphibian with black spots.. Females are larger than males, who are usually thinner.
12. emperor penguin (Aptenodytes forsteri). It is a bird native to Antarctica. It can exceed 1.2 meters in height.
13. European rabbit (Oryctolagus cuniculus). It is a rodent that lives in soils near the sea. Due to its great reproductive capacity, it can easily become a pest.
14. red kangaroo (Macropus rufus). It is a marsupial native to southern Australia. The development of the young begins in the uterus and ends in an external pouch, called a marsupium.
15. northern flying squirrel (Glaucomys sabrinus). It is a gliding mammal. It has a membrane covered with hair that extends to allow it to fly. It lives in the regions near the north pole.

Importance of the animal kingdom

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.

In ecosystems, Animals participate in the balance of biodiversity. They can be predators, prey (food source for other species) or both at the same time. Some animals, such as bees and hummingbirds, play an important role in plant reproduction because they act as pollinators.

For human beings, animals are a source of food and the basis of numerous industries linked to livestock, beekeeping, fishing and tourism. Furthermore, the study of animals allowed us to answer fundamental questions about the origin of life, the dynamics of the human body and interactions in ecosystems.

Origin of the animal kingdom

The first animals on the planet appeared during the so-called “Cambrian explosion.” 540 million years ago, there was a diversification and proliferation of life (especially multicellular life or metazoans) in the seas of the early Earth.

The reasons that triggered such an evolutionary explosion are not entirely clear, but it is known that the production of oxygen (a result of the dominant plant life) and the great volcanic and geochemical activity played fundamental roles.

It is believed that the first animal species were sea sponges: some species could date back 600 million years.

References

• Audesirk T., Audesirk G., Bruce E., BIOLOGY 3: Evolution and ecology. 6th edition. Pearson education (2003).
• Curtis H., Barnes N., Massarini A., Schnerck A., BIOLOGY. 7th Edition. Panamericana Medical Editorial (2008).
• Global Biodiversity Information Facility. https://www.gbif.org/