Introduction to animals

Animals are a diverse group of organisms that make up the kingdom known as Animalia. They are a very familiar group of organisms and are the very kingdom of life that humans belong to. In this introduction to animals we will cover the difference between vertebrate and invertebrate animals and introduce a range of animals from both groups.

The first animals are thought to have evolved over 550 million years ago. Over the last 550 million years the animal kingdom has become incredibly successful and diverse with many million species now inhabiting Earth.

The general description of an animal is a multicellular, heterotrophic organism.The term multicellular simply means that an animal is made from more than one cell. A human, for example, is made from around 30 trillion cells.

Introduction to animals - vertebrate and invertebrateHeterotrophic means that animals are unable to make their own food and can only obtain energy by eating other organisms.  Animals are also eukaryotes because their cells contain a nucleus and organelles.

Key to an introduction to animals is the distinction between vertebrate and invertebrate animals. Vertebrate animals are those that have an internal backbone such as humans, cats and dogs. An invertebrate is any animal that doesn’t have an internal backbone. These include animals such as insects, spiders, jellyfish, crabs and even sea anemones.


Invertebrates are animals that are missing a backbone. They are an incredibly diverse group and include over 90% of all animal species. Many invertebrates we know very little about or are even yet to be discovered.

The evolution of invertebrates was the Earth’s introduction to animals. The most ‘primitive’ invertebrates can easily be mistaken for plants or other types of organisms because they look so different to the animals we are familiar with.

CoralThese ‘primitive’ animals include organisms such as sponges, corals and anemones. Sponges are thought to be one of the first animals to have evolved. They retain some of the single-celled ancestry that animals evolved from. Sponges are able live and reproduce as a single-celled organism for a short period. Corals and anemones are slightly more advanced than sponges and belong to the same group of animals as jellyfish.

Insects, spiders and crustaceans all belong to a group of invertebrates called arthropods. Many of these invertebrates are very advanced and display complex behaviors and body types. Arthropods have complex bodies with hard external skeletons and jointed limbs.

Many species from these group show advanced behaviors. Bees for example communicate with each other by wagging their backside in various ways. Spiders produce complex webs to catch prey and stake out until an unsuspecting insect gets caught.

SpiderArthropods are arguably the most successful group of animals on Earth. They are an incredibly diverse group and estimates of the total number of arthropod species is well over a million. Insects are particularly diverse and account for over half of all animal species.

Between jellyfish and insects there is a wide range of many other invertebrates. Some examples include worms, millipedes, centipedes, starfish, urchins, squid, octopi, oysters and snails.


A vertebrate animal is any animal with an internal backbone. These include many of best known animals such as cats, dogs, horses, birds, lizards, snakes and elephants. The largest animals on Earth are all vertebrates but many vertebrate animals can also be relatively small such as mice, skinks and some birds. Vertebrates are also the most intelligent of all organisms, although an octopus is also pretty darn smart.

Vertebrate animals can be split into four groups: fish, amphibians, reptiles (and birds), and mammals. The remainder of this introduction to animals will introduce these four different vertebrate groups.


Giant trevallyFish were the first vertebrates to appear on the scene. The first fish evolved from an invertebrate ancestor and has since diversified into more than 30,000 species. All other vertebrate animals have since evolved from prehistoric fish.

A fish is an aquatic animal that has gills and limbs without fingers or toes. There are four different groups of fish. The first group includes fish without true bones known as cartilaginous fish which includes sharks and rays. We also have bony fish, jawless fish and hagfish.

Living in water has its issues. Fish have evolved various adaptations to help them with aspects of life such as movement, buoyancy and avoiding being eaten by bigger fish.


Amphibians helped vertebrate animals to move onto land. This group of vertebrates include animals such as frogs, toads, salamanders and newts. Most amphibians start life out as a larvae living into water and then undergo metamorphosis (transform) into an air-breathing adult.

Amphibians have been around for over 360 million years. Despite their long survival, many species of amphibians are under serious threat of extinction due to problems such as habitat loss and predation from invasive species.

Reptiles and Birds

A reptile species known as a green tree geckoReptiles are cold-blooded, egg laying animals that have scales or scutes, rather than hair. They were the first group of vertebrate animals to live entirely on land and some species have since moved back into the water. Some different reptiles include lizards, snakes, crocodiles and turtles.

Birds are a class of reptiles that are distinct from all other animals because they have wings and feathers. Birds are renowned for their incredibly ability to fly. Many species perform incredible migrations each year, flying thousands of miles across Earth. While birds are famous for flying, some species have lost the ability to fly such as penguins and ostriches.


Mammals are a group of warm-blooded animals that have hair and mammary glands. Many mammals species are much-loved and adored animals. Animals such as cats, dogs, monkeys, sheep, cows, horses, giraffes, elephants, dolphins, whales, beers, goats, deer and pigs are all mammals.

Humans also belong to this group of animals. More specifically we belong to a group of mammals called primates. Primates include animals such as monkeys, baboons, gorillas, chimpanzees and orangutans.

ChimpanzeeMammals are most definitely the smartest of all the animal groups. Dolphins and monkeys are considered two of the smartest animals in the world. And then there are humans who are undoubtedly more intelligent than any other organism on Earth.

Last edited: 22 October 2015

For more information on animals, check out these resources:
Animals | A-Z Animals | Arkive