Animalia Kingdom

Alexander Pearson
Animalia Kingdom

What is the Animalia kingdom?

The reino Animalia It is the kingdom of nature where all animal organisms are included, that is, they are made up of animal cells. Insects, fish, mollusks, humans, dogs, cats, frogs, crocodiles, turtles and many others belong to this kingdom..

It is one of the most abundant and diverse kingdoms in nature, including a huge number of organisms, large and small, and even microscopic. Organisms belonging to this kingdom are characterized by their ability to move, especially in relation to other organisms such as plants..

Some species of the Animalia kingdom

Animals correspond to all the insects that we see in the garden, including the worms that we find in the ground or under rocks. Corals and anemones, sea sponges and all fish and large cetaceans such as whales are also animals. Of course, the most popular animals are mammals, birds, reptiles, and amphibians..

Most animals require oxygen to breathe, they need to feed on other living things for nutrients and energy, and they excrete to the environment where all the waste materials they produce are found..

Man is classified together with animals and this, throughout its history, has taken advantage of the help of different animals in innumerable ways to plant, transport, build and keep company.

Characteristics of the Animalia kingdom

The Animalia kingdom has a series of characteristics that differentiates it from the rest of the kingdoms of living beings:

They are made up of animal cells

Animal eukaryotic cell

All animals are made up of animal cells, which have a number of important characteristics that distinguish them from other cells such as plants, fungi and bacteria..

As eukaryotic cells, some of the fundamental characteristics of animal cells are: the plasma membrane and the absence of a cell wall, the nucleus, lysosomes, the cytoskeleton, peroxisomes, mitochondria, the endoplasmic reticulum, the Golgi complex, the centrosomes and centrioles, etc..

On the other hand, although there are notable exceptions, most animals are made up of more than one cell, that is, they are multicellular organisms, and these cells are in permanent communication with each other..

They feature specialized fabrics

It is not true for all animal groups, but generally organisms belonging to the animal kingdom have groups of cells that “assemble” together to form tissues, which fulfill special functions for animal life..

In "more complex" animals, these tissues can form organs, so that in their bodies the physiological tasks are delimited by these organs and the systems they make up. In humans, for example, the nervous, respiratory, circulatory, digestive, excretory, etc. systems are distinguished..

They are heterotrophic organisms

A fundamental characteristic that differentiates animals from organisms like plants, for example, is their form of nutrition. Animals are heterotrophic organisms and must obtain their food from other animals. On the contrary, plants can produce their own food.

They can move

A sloth moving across a branch

Although some adult animals are sessile (unable to move from one place to another), most animals have, at least in one of their stages of the life cycle, the ability to move and move..

This differentiates animals from plants and fungi, to mention two groups of mostly sessile organisms..

They reproduce sexually

Some animals, especially the simplest and smallest ones, generally reproduce by asexual strategies, such as binary fission, for example, which depends on a mitotic division..

However, most organisms in the animal kingdom reproduce sexually, which means that they necessarily go through a period of meiotic cell division, where the genes of two organisms of different sexes mix..

Animal classification

Organisms belonging to the animal kingdom can be classified in many ways: according to their habitat, according to their reproduction, according to their diet, etc., but one of the most popular is according to whether they have bones and backbones or not..

Thus, two large groups of animals have been defined in nature: invertebrates and vertebrates. The first ones do not have bones, but the second ones do and also have a spinal column, just like the one that human beings have in the back.

Invertebrate animals

Scorpions are invertebrate animals

Invertebrate animals are animals that do not have vertebrae - hence their name - or internal bones. They represent one of the most abundant and diverse groups in the animal kingdom, and generally do not reach large sizes, with the exception of some that live under water..

Although it is a surprising figure, these animals represent more than 90% of all animal species on the biosphere, which means that they are extremely important for the dynamic balance of practically all terrestrial ecosystems..

The classification of these animals is somewhat complex, as there are too many and all very different, however, many authors consider that 21 phyla are the most representative of the entire group, namely:

  1. Porifera (the poriferous or sea sponges)
  2. Placozoa (placozoans; flat and simple looking marine animals)
  3. Ctenophora (the ctenophores; some animals, also marine, similar to jellyfish)
  4. Cnidaria (the cnidarians; the group of true jellyfish and anemones)
  5. Nemertea (the nemerteans or marine worms)
  6. Platyhelmithes (flatworms; a parasite or free-living aquatic or semi-aquatic flatworms)
  7. Gnathostomulida (gnathostomulids; other small soil worms)
  8. Nematomorpha (the nematomorphs; a group of small worms, usually parasites)
  9. Nematoda (the nematodes; the group of cylindrical worms)
  10. Kinorhyncha (the quinorhincs, also known as marine microbes)
  11. Gastrotricha (the gastrotricos; cylindrical aquatic organisms covered with cilia)
  12. Rotifera (rotifers; aquatic or semi-aquatic organisms similar to insects)
  13. Entoprocta (entoprocts; other microscopic aquatic animals similar to anemones)
  14. Acanthocephala (the acantocephalus; a group of parasitic worms of vertebrate animals)
  15. Mollusca (mollusks: snails, squid, slugs, octopuses, etc.)
  16. Arthropoda (arthropods: the largest group of invertebrates; includes insects, arachnids, myriapods, crustaceans, and others)
  17. Onychophora (the onychophores; animals similar to cylindrical worms but with legs)
  18. Annelida (the annelids; a group of segmented worms, marine or terrestrial)
  19. Bryozoa (bryozoans; sessile aquatic animals)
  20. Priapulida (the priapulids; marine worms that inhabit the bottom of the sea)
  21. Hemichordata (the hemichordates; marine worms with signs of a backbone)

Vertebrate animals

Frogs are vertebrate animals

Humans and other mammals, birds, reptiles, fish and amphibians belong to the group of vertebrate animals.

All these have in common the presence of a spinal column and a system of bones that supports their bodies, which is why they are also distinguished from most invertebrates by the large sizes they can reach. These organisms are classified within the Chordata phylum, that is, the chordates.

One of the most special characteristics of these animals, shared by all, is that their bodies are well defined in a head, a trunk or thorax and a tail..

Among vertebrates there are 7 different classes. Let's see what they are:

  1. Agnata class: agnate - "jawless fish" - are those aquatic animals known as "witches" and lampreys
  2. Class Condrichthyes: Also known as the "cartilaginous fish", organisms in this group include sharks and stingrays
  3. Osteichthyes class: called “bony fish”, they include most of the fish that we know, both freshwater and saltwater
  4. Class Amphibia: Amphibians are cold-blooded animals that live between water and land; examples of these are toads, frogs, and caecilians; all with wet skins
  5. Class Reptilia: other cold-blooded animals with dry skins covered with scales; examples of reptiles are snakes, turtles and crocodiles
  6. Class Aves: warm-blooded flying animals, with fur covered with feathers and legs with scales; good examples of birds are chickens, macaws, pigeons, eagles and hawks, ostriches, condors, among others
  7. Mammalia class: Mammals are warm-blooded animals that feed their young through milk that they produce in specialized glands; their skin is covered by hair and they can reproduce by eggs or by viviparism. Examples of mammals are man, cows, dogs, giraffes, elephants, lions, etc..


Classification of animals according to their diet

As we discussed earlier, animals are heterotrophic organisms, which means that they obtain the nutrients and energy they need to survive by eating other living organisms..

According to what type of organisms serve as food, animals can be herbivores, carnivores or omnivores:

  • Herbivores: are all animals that feed on organisms of plant origin, either plants themselves or other organisms formed by plant cells. Cows, horses, and elephants are examples of animals that only eat plants.
  • Carnivores: they are all animals that feed on the meat of other animals, either alive or dead (scavengers). Lions, tigers, sharks, and eagles are examples of carnivorous animals.
  • Omnivores: they are the animals that feed indistinctly on plants and other animals. Humans, pigs, chickens, and raccoons are examples of omnivorous animals..


Reproduction in the animal kingdom is highly varied: some animals reproduce asexually, while others do so sexually.

For those who reproduce sexually, the fertilization of female sex cells by male can be of two types: internal or external, depending on where both cells are obtained during the reproductive act, within a structure of the female or outside her, respectively.

In addition, in the same group of animals with sexual reproduction there may be different strategies, according to which the following categories of animals have been proposed:

  • Oviparous: those that reproduce through eggs like all birds, many fish, reptiles and insects
  • Viviparous: those who give birth to live young, such as mammals
  • Ovoviviparous: those that reproduce through eggs, but these are retained inside the mother until the young mature and hatch, so they "give birth" to live young. Many insects, fish, and reptiles are ovoviviparous

Examples of species from the Animalia kingdom

African elephant (Loxodonta africana)

African elephant (Loxodonta africana)

It is a herbivorous mammal that lives in Africa, being the largest land animal, being able to measure up to 4 meters in height. It is distributed in 23 African countries, inhabiting forests, grasslands or savannas.

The giraffe (Giraffa camelopardalis)

Giraffe (Giraffa camelopardalis)

It is a herbivorous animal that lives in Africa, being the tallest land animal. They are characterized by their long neck that allows them to feed on leaves from the top of the trees, by their spots to camouflage themselves and by their osicones, also called horns..

The chuck (Mandrillus sphinx)

A male chuck

It is a primate that lives in several countries in Africa, especially in groups and in tropical forests. They have an omnivorous diet, consisting mostly of fruits and insects. They are characterized by their hairless face, their elongated muzzle with a middle parting and their red lips.

Chironex fleckeri or the sea wasp

The sea wasp (Chironex fleckeri)

It is a good example of a marine invertebrate. It is a cnidarian (jellyfish) characterized by its small size and its lethal venom, considered one of the most lethal on the planet..

It lives mainly in Australian waters and is a carnivorous animal that feeds on small fish, turtles and other aquatic animals..

Lumbricus terrestris or the earthworm

Earthworm (Lumbricus terrestris)

A species of invertebrate animal belonging to the group of annelids, widely distributed throughout the world. They are reddish worms and can be found in gardens.

Osmia lignaria, the bricklayer bee or the blue orchard bee

The bricklayer bee 

This is a species of blue insect, very similar in size and appearance to honey bees. They are important pollinators of native crops and are capable of building nests within branches and trunks of trees..

Birostris blanket or the giant manta ray

Giant stingray (Manta birostris)

It is a species of vertebrate belonging to the group of cartilaginous fish (like sharks). It is one of the largest species of stingrays, but it differs from others in that it does not have a poisonous stinger on its tail..

Eptatretus stoutii or the Pacific witch fish

Pacific witchfish (Eptatretus stoutii). Source: Stan Shebs, CC BY-SA 3.0 , via Wikimedia Commons

It is a kind of agnate (vertebrate), which means that it lacks jaws. It lives in the deep waters of the Pacific Ocean and has the appearance of a prehistoric animal; they look like eels.

Lithobates catesbeianus or the bullfrog

Bullfrog (Lithobates catesbeianus)

It is a very large amphibian that can weigh up to 1 kg and measure up to 20 cm long. Its introduction in regions where it is not native has caused significant ecological damage, since they do not have natural predators, they reproduce rapidly and their larvae produce toxic substances for the larvae of other organisms..

Phoenicopterus roseus or the common flamingo

Common Flamingo (Phoenicopterus roseus)

It is a large species of bird, typical of the African, European and Asian continents. Individuals of this species have pink beaks and legs, with light pink plumage on the body and dark pink on the wings..

Felis silvestris catus or the domestic cat

Domestic cat (Felis silvestris catus)

It is a small feline, adapted to life as a human pet. They come in different sizes, features and colors.

Mesocricetus auratus or the syrian hamster

Syrian hamster (Mesocricetus auratus)

It is a species of small mammal, of the group of rodents. Very common as a pet or laboratory animal, but apparently endangered in the wild.

Aldabrachelys gigantea or the Aldabra giant tortoise

Aldabra giant tortoise (Aldabrachelys gigantea)

It is a species of large land reptile - it can weigh more than 200kg - characterized by its incredible longevity, since it has been estimated that they can live for more than 150 years.


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