Kingdom Animalia: characteristics, classification and examples

The kingdom Animalia, also called Metazoo, is a large 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 move and reproduce sexually, however there are some exceptions, the which have contributed to the belief that some species, such as sponges and sea lilies, were considered plants.

We are going to see the characteristics that define this kingdom, with its particularities, and some of the groups that make it up.

• Related article: "The theory of biological evolution"

Characteristics of the Animalia kingdom

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

1. Multicellular

They are organisms made up 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 behaviors.

2. Heterotrophic nutrition

Unlike plants, which photosynthesize in order to generate their own food, animals depend on other organisms to survive. Basically, this type of nutrition involves transforming organic matter into the nutrients that the body needs.

The food source can be very varied depending on the species. There are herbivorous animals, such as cows, that feed exclusively on plants. Others, on the other hand, are carnivores like lions, feeding on other animals. If it eats 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), xylophogi (wood) ...

3. Aerobic metabolism

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

4. Reproduction

In most cases, animals reproduce sexually. This implies that there is a differentiation into two sexes: males and females. The advantage of this type of reproduction is that it allows for genetic variability, however it has the downside that the process is very slow, since the sexual partner must be found, the courtship must begin, fight for her if necessary and carry out the sexual act, all this requiring Energy.

There are some animals that reproduce asexually, like sponges or worms, which can be multiplied by bipartition, that is, breaking into two and creating a new individual from the original.

5. Sensory capacity

Thanks to the fact that organs can carry out different functions, animals have various types of senses. Apart from the five generally best known (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 prey. Some insects, such as moths, can sniff out pheromones and locate a possible reproductive partner several miles away. Sharks have electrical sensors and capture electrical fields, locating prey and perceiving how fast they are going or if they are camouflaging.

6. Symmetry

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

7. Locomotion

Representatives of the Animalia kingdom can move by themselves, and there are several ways they can do it. Some, like starfish, have hundreds of tiny feet on their limbs that allow them to move across the seabed. Others, like humans and other mammals, move by using their limbs, such as legs and fins.

Some animals, upon reaching 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. Chordates

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 phylum with the most described species, being mostly vertebrates (animals with internal skeletons), and more than half are fish.

Some groups of chordates are all vertebrates, such as mammals, fish with spines, reptiles, amphibians, and birds, as well as some marine species such as cephalochordates and witchfish.

2. Arthropods

They are invertebrate animals with exoskeletons and articulated appendages., and it is 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 multiple shapes and sizes, being one of the arthropods largest in history the case of the extinct meganeura or the giant dragonfly, which could reach 70 cm.

3. Poriferous

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

4. Mollusks

Mollusks include bivalves, squid, octopus, snails, and slugs. They are soft animals without an internal skeleton but they 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 they came to conquer terrestrial habitats. They have been found in places with extreme temperatures, such as in polar and tropical waters.

5. Annelids

They are ringworms, like leeches and earthworms. These rings are called metamers, which each have some repeated organs and which, if the worm divides, allow the survival of new individuals.

They have both sexual and asexual reproduction. In the case of the sexual, there are species that have two different 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.