We explain what the animal kingdom is, its characteristics, and examples. We also explain its classification, taxonomy, and origin.
What is the animal kingdom?
The Animalia kingdom, or animal kingdom, is one of the main groups into which biology classifies living beings, along with the plant kingdom (Plantae), the fungi kingdom (Fungi), the protist kingdom (Protista), and the monera.
The species grouped in the Animalia kingdom are called animals, and are characterized by being multicellular organisms (having more than one cell), eukaryotes (their cells have a nucleus), and heterotrophs (they feed on other living beings). You must read about Stalking once.
In addition, 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), jellyfish, anemones, and hydras (cnidarians), flatworms (platyhelminthes), and roundworms (nematodes). Mollusks (such as octopuses and clams), annelids (such as earthworms), arthropods (such as spiders, shrimp, and ants), echinoderms (such as starfish), and finally chordates are also classified. All vertebrate animals, including humans, belong to this last category.
The animal kingdom includes approximately 1.5 million recorded species, of which only 3% are vertebrates. Some species are very ancient; for example, sponge species have been recorded that are 600 million years old. Maybe you should definitely read about Crush once.
Characteristics of the animal kingdom
The fundamental characteristics of the animal kingdom can be summarized as follows:
- They are multicellular eukaryotic organisms: Animal cells are eukaryotic cells (they have a nucleus and organelles inside). Animals, in turn, are composed of more than one eukaryotic cell. Unlike plant cells, animal cells lack chloroplasts and cell walls. They 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.
- They have aerobic metabolism: Animals obtain chemical energy through cellular respiration, a metabolic process that involves the consumption of oxygen and the release of CO2 as a byproduct.
- They have independent mobility: Animals can move voluntarily, at least at some stage of their life. They have specialized structures that they use to move in water, air, or land (such as wings, fins, legs, and feet). This characteristic allows them to change habitats and seek a more suitable one, escape predators, or pursue prey.
- They 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).
Animal Classification
In biology, the animal kingdom is usually classified into two large groups:
- Vertebrates (those with a backbone or vertebral column)
- Invertebrates (those without a backbone)
The vast majority of animals are invertebrates, as they make up 97% of all known animal species.
Other forms of classification, less commonly used in the scientific field, focus on the specific habitat of animals. For example, marine animals (of the sea and oceans), aquatic animals (of freshwater), terrestrial animals (of dry land), flying animals (of the air), amphibian animals (of mixed life between water and land), parasitic animals (those that live inside the bodies of others), or urban animals (of the city).
Taxonomic Classification of the Animal Kingdom
The Animalia kingdom comprises a vast number of phyla, or groups of species.
The main ones are:
- Porifera: This group is made up of sponges. They typically live in saltwater, 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 with sack-like bodies and tentacles to capture their prey. There are about 10,000 species.
- Platyhelminthes: This group is made up of “flatworms.” Some are free-living, while others are parasitic. They can be hermaphroditic. There are about 20,000 species.
- Nematodes: This group is made up of “roundworms.” They have cylindrical bodies and are found in a wide variety of habitats. There are more than 25,000 species.
- Mollusks: This group includes octopuses, clams, and snails, among others. They have soft bodies and often a shell or valve. There are about 100,000 species.
- Annelids: This group includes worms with cylindrical bodies segmented into rings. It includes earthworms and leeches. It is estimated that there are about 16,700 species.
- Arthropods: This group includes insects, crustaceans, arachnids, and myriapods. They have a chitinous exoskeleton and jointed limbs. It is a very large group that includes more than 1,200,000 described species.
- Echinoderms: This group includes sea urchins, starfish, and other similar organisms. They have radial symmetry and are exclusively marine. Their skeleton is made up of rigid plates beneath the skin. There are about 7,000 extant species.
- Chordates: This group includes vertebrates, amphioxus, and tunicates. They all share a notochord (an embryonic structure that plays a special role in the development of the nervous system). There are more than 65,000 species.
Examples from the Animal Kingdom
Some simple examples from the animal kingdom are:
- Yellow tube sponge (Aplysina fistularis): It lives in the Caribbean and is one of the most abundant marine sponges. It is characterized by having a tubular structure similar to some corals.
- Portuguese man-of-war (Physalia physalis): It is a jellyfish with venomous tentacles that can be toxic to humans.
- Freshwater planarian (Planaria torva): It is a small flatworm that has the ability to regenerate a part of its body if it is fragmented.
- Garden snail (Cornu aspersum): It is a land snail known as the garden snail. It has male and female sexual organs.
- Earthworm (Lumbricus terrestris): It is an earthworm with a body divided into segments. It feeds on organic matter.
- Yellow scorpion (Buthus occitanus): This is a type of scorpion that lives in arid areas and is nocturnal.
- Red-tipped starfish (Protoreaster linckii): This is a starfish that lives on the African continent.
- Great white shark (Carcharodon carcharias): This is a shark that lives in the oceans within the continental shelves and feeds on marine mammals.
- Panther (Panthera tigris sumatrae): This is a feline native to the island of Sumatra. Among the tigers, it is the smallest. It is an endangered species.
- Domestic horse (Equus ferus caballus): This is a domesticated mammal. It is believed that primitive horses were much smaller than modern ones.
- Common frog (Pelophylax perezi): This is a green or brown amphibian with black spots. Females are larger than males, who are usually slimmer.
- Emperor penguin (Aptenodytes forsteri): This is a bird native to Antarctica. It can grow to over 1.2 meters in height.
- European rabbit (Oryctolagus cuniculus): This is a rodent that lives near the sea. Due to its high reproductive capacity, it can easily become a pest.
- Red kangaroo (Macropus rufus): This 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.
- Northern flying squirrel (Glaucomys sabrinus): This is a gliding mammal. It has a fur-covered membrane that extends for flight. It lives in 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, as human interest in animals dates back to ancient times.
In ecosystems, animals contribute to the balance of biodiversity. They can be predators, prey (a 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 humans, animals are a food source and the basis of numerous industries related to livestock, beekeeping, fishing, and tourism. Furthermore, the study of animals has provided answers to 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 early Earth.
The reasons that triggered such an evolutionary explosion are not entirely clear, but it is known that oxygen production (a result of dominant plant life) and significant volcanic and geochemical activity played key roles.
The first animal species are believed to have been sea sponges; some species may date back 600 million years.
References
All the information we offer is supported by authoritative and up-to-date bibliographic sources, which ensure reliable content in line with our editorial principles.
- 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. Editorial Médica Panamericana (2008).
- Global Biodiversity Information Facility. gbif.org
