The Animalia kingdom, also called Metazoo , is a wide group of organisms that are animals, among which we find human beings. The term animal comes from the Latin animalis, which means “that takes the air”, “that has a soul”.

Most of them can be moved and reproduce sexually, however there are some exceptions, which have contributed to the idea that some species, such as sponges and sea lilies, are considered plants.

Let’s look at the characteristics that define this kingdom, with its particularities, and some of the groups that make it up.

Characteristics of the Animalia kingdom

Although the ways they feed, anatomy, reproductive behaviors, and other aspects differ greatly, most animals share a number of characteristics:

1. Pluricellulars

They are organisms composed of more than one cell. These cells are organized into specialized tissues, constituting organs and systems, which allow them to carry out differentiated functions. Thanks to this, animals have a nervous system and also muscles, which allow them to carry out different behaviours.

2. Heterotrophic nutrition

Unlike plants, which carry out photosynthesis in order to generate their own food, animals depend on other organisms for their survival . Basically, this type of nutrition implies transforming organic matter into the nutrients the organism needs.

The food source can be very varied depending on the species. There are herbivorous animals, such as cows, which feed exclusively on plants. Others, on the other hand, are carnivorous like lions, feeding on other animals. If it feeds on both animals and plants, the animal is omnivorous.

There are other categories that refer to more specific types of food: nectarivores (flower nectar), insectivores (insects and other arthropods), xylophagous (wood)…

3. Aerobic metabolism

The vast majority of animals need oxygen in order to carry out their metabolism and maintain basic functions . One of the few exceptions to this is the case of loriciferous animals, which can live in environments where there is no oxygen and survive.

4. Reproduction

In most cases, animals carry out reproduction in a sexual way. This implies that there is a differentiation in two sexes: males and females. The advantage of this type of reproduction is that it allows for genetic variability, however it has the disadvantage that the process is very slow, since the sexual partner must be found, the courtship must be initiated, the fight must be fought for if necessary and the sexual act must be performed, all of which requires energy.

There are some animals that reproduce asexually , such as sponges or worms, which can multiply by bipartition, that is, breaking in two and creating a new individual from the original one.

5. Sensory capacity

Thanks to the fact that the organs can carry out different functions, animals have several types of senses . Apart from the five generally known senses (smell, taste, touch, vision and hearing), there are other senses with very specific evolutionary purposes.

For example, snakes have infrared sensors with which they identify the heat of their prey. Some insects, such as moths, can smell pheromones and locate a potential breeding partner from several kilometers away. Sharks have electrical sensors and can pick up electrical fields, locating prey and sensing how fast they are going or whether they are camouflaging.

6. Symmetry

Most organisms in the Animalia kingdom have a symmetrical anatomy, that is, their body can be divided into two equal parts from an axis . Depending on the type of symmetry, animals can be divided into bilaterally symmetrical and non-bilaterally symmetrical.

7. Locomotion

The representatives of the Animalia kingdom can move by themselves , and there are several ways they can do so. Some, like starfish, have hundreds of small feet on their limbs that allow them to move around on the sea floor. Others, like humans and other mammals, move by using their limbs, such as their legs and fins.

Some animals, when they reach adulthood, stop moving. Some examples are corals and anemones.

Classification and edges

The Animalia kingdom comprises more than two million living species , which have been grouped into more than 30 phyla, categories that serve to classify all this biodiversity. Some of the best known are detailed below:

1. Strings

Its name comes from the fact that it has a dorsal cord, a dorsal neural tube or tail at some point in its embryonic development. It is the third most described species, being mostly vertebrates (animals with an internal skeleton), and more than half are fish.

Some groups of chordates are all vertebrates, such as mammals, fish with thorns, reptiles, amphibians and birds, as well as some marine species such as cephalocordates and witch fish.

2. Arthropods

They are invertebrate animals with exoskeletons and articulated appendages , and are made up of insects, arachnids, myrapods and crustaceans. It is the animal phylum with the most described species, representing 80% of the known animal species.

Thanks to their great adaptability, they come in many shapes and sizes, one of the largest arthropods in history being the extinct meganeura or the giant dragonfly, which could reach 70 cm.

3. Poriferous

This edge includes the sponges , which, contrary to what many people believe, are part of the Animalia kingdom. They are mostly marine and have the peculiarity that they are sessile (fixed to the ground) and do not have real tissue, which is rare in the animal kingdom.

4. Molluscs

Among the molluscs are bivalves, squid, octopus, snails and slugs. They are soft animals without an internal skeleton but that can have a shell to protect themselves .

They are considered a phylum that has been able to adapt evolutionarily very well, being of marine origin but that came to conquer terrestrial habitats. They have been found in places with extreme temperatures, like in polar and tropical waters.

5. Annelids

They are worms with rings, like leeches and earthworms . These rings are called methameres, which each have some repeated organs and which, in case the worm divides, allow the survival of the new individuals.

They have both sexual and asexual reproduction. In the case of the sexual one, there are species that have two differentiated sexes while in others they are hermaphrodites. Asexual reproduction occurs by excision, splitting in two.

Bibliographic references

  • Hickman, C. P., Ober, W. C. & Garrison, C. W. (2006). Comprehensive Principles of Zoology, 13th Edition, Madrid, Spain: McGraw-Hill-Interamericana.