Animals – the ultimate guide to the animal kingdom. On this page you’ll discover many different types of animal and how they’re classified – with awesome pictures and links to further information.
Active Wild Animal Index
Find out about animals from all around the world. These pages contain information, fun facts and awesome pictures:
- Rainforest Animals
- Endangered Animals
- African Animals
- Australian Animals
- Arctic Animals
- Antarctic Animals
- Animal Classification
- Mammals – The Ultimate Guide
- Visit the Active Wild Online Zoo
You can go straight to the section you need using the links below:
- Animals: The Ultimate Guide – Introduction
- Animal Classification
- The Animal Kingdom
- What Is An Animal?
- Types of Animal
- Sea Squirts
Animals – The Ultimate Guide: Introduction
From the smallest insects to the largest whales, deep sea fish to high-flying birds; animals come in many different shapes and sizes and are found in many different habitats throughout the world.
Animals play a big part in our lives. We all rely on animals for food (yes, even vegetarians; animals play a major role in the pollination of plants!). Animals also provide transport, security, entertainment and companionship. Without animals, we wouldn’t be able to survive.
Of course, we are animals ourselves. Humans are members of a group of animals known as primates. Our closest relations are chimpanzees and bonobos.
We are taught about animals from an early age, and many species are familiar to us. Even if we’ve never seen one in the wild, we can instantly identify animals such as tigers, orcas, elephants and pandas.
However, there are many animals that aren’t nearly as well-known, and some that don’t even look – or act – like animals at all. On this page we’ll explore the entire animal kingdom, and meet many different types of animal – some familiar, and some downright weird!
In this section we’re going to find out what makes an animal an animal!
In order to make sense of life on Earth, scientists separate all living things into groups. This is known as classification. All of the species in a particular group have certain things in common.
For example, all of the members of a group of animals called Mammalia (mammals) have hair, and feed their young with milk produced by the mother’s body.
All of the animals in a big group such as Mammalia can be further divided into smaller groups that contain animals that are even more closely related to each other. This continues until the groups can no longer be divided and all that is left are the individual species themselves.
(In actual fact, species can be divided into subspecies, but we won’t go into that here!)
The eight levels of classification are: domain, kingdom, phylum, class, order, family, genus and species.
The classification of living things is a science in itself, and is known as ‘taxonomy’.
- You can find out more about classification here: Animal Classification.
The Animal Kingdom
Some of the largest groups of living things are known as ‘kingdoms’. An animal is any member of the animal kingdom, Animalia.
Other kingdoms include Plantae (the plant kingdom) and Fungi.
As we’ve found, all of the species in any group must have certain things in common. So what characteristics link all of the members of the animal kingdom? What makes an animal an animal?
What Is An Animal?
Animals are incredibly varied, and have adapted to live in just about every environment on Earth. However, there are certain things that all animals have in common – things that make them animals, rather than plants, fungi, or any other type of organism.
Characteristics of Animals
There are a number of characteristics that define an animal. We’ll start off simple, then get down to the nitty gritty …
Animals can move
All animals are ‘motile’ for at least some of their lives. This means that they are able to move under their own steam, consuming energy as they do so.
Animals feed directly or indirectly on living things
All animals are heterotrophs, which means that they obtain nourishment from living things (or the products of living things). This separates animals from other organisms such as plants that obtain energy from sunlight.
Remember that a plant is a living thing, so even herbivores (plant-eating animals) are reliant on other living things for energy.
Animals are Multicellular
Cells are the ‘building blocks’ out of which living things are made. Some of the simplest organisms, such as bacteria and protozoa, are ‘unicellular’, which means they are made up of just a single cell.
Animals, on the other hand, are multi-celled (multicellular) organisms: they are made up of thousands, and usually millions upon millions, of cells.
Animals Have Eukaryotic Cells
Eukaryotic cells have a nucleus and other organelles that are enclosed within barriers called membranes.
(The nucleus contains the information that tells the organism how to grow. An organelle is a structure within a cell, such as a nucleus, that has a specific function.)
Both animals and plants have eukaryotic cells. Plant cells have certain features, such as a rigid cell wall and chloroplasts that enable photosynthesis, that are not present in animal cells.
Types of Animal
Before you read any further, think of an animal – any animal – and picture it in your head.
What did you come up with? A tiger? A dog? Perhaps you thought of a bird, or a frog, or even something like an octopus (you crazy independent thinker, you).
Even those animals, as varied as they may be, don’t represent the whole range of species within Animalia.
We’ll look at the familiar types of animal further down the page, but let’s start by looking at some less ‘typical’ animals.
Animals are often grouped into those that have backbones (vertebrates), and those that do not (invertebrates).
It’s probably because we’re vertebrates ourselves that when we’re asked to think of a ‘typical’ animal, it’s usually a vertebrate that pops into our heads.
However, in the grand scheme of things vertebrates are somewhat of a minority. Invertebrates – animals without backbones – make up as much as 97% of all the species in the animal kingdom.
The term ‘invertebrates’ isn’t actually used in ‘proper’ animal classification. This is because animals without backbones aren’t necessarily related to each other. In fact, some invertebrates are more closely related to vertebrates than they are to other invertebrates.
However, some of the weirdest, least animal-like animals are invertebrates, which is why we’ll start our exploration of the animal kingdom here.
Although seemingly more like plants than animals, sponges are nevertheless members of Animalia.
Sponges are very primitive animals, lacking organs and a nervous system. (A nervous system consists of nerves that transmit signals between different parts of the body, allowing them to ‘talk’ to each other.) Unlike other animals, the bodies of sponges are not symmetrical.
Although most sponges attach themselves to the sea bed and don’t move as adults, sponge larvae are able to swim.
Sea SquirtsSea squirts are tube-shaped sea animals, which, after a short time spent as free swimming larvae, fix themselves onto the sea bed. They filter particles of food from water that is passed through their bodies.
Although, like sponges, sea squirts look more like plants than animals, they have organs and a simple nervous system.
Lancelets are small fish-like animals with translucent bodies. Although they don’t have a backbone – and are therefore not vertebrates – they do possess a flexible central nerve cord called a notochord.
Lancelets are harvested for food in some Asian countries. They are also studied by scientists for clues as to how vertebrates (animals with backbones) evolved.
You might think of coral as the rocky substance that makes coral reefs. In fact, corals are invertebrate animals that, in their larval form, are able to swim. Most corals remain in their larval form for just a few days before settling in a suitable position.
The body form of a coral is known as a ‘polyp’. Polyps are tubular, with tentacles and a mouth at one end. The coral secretes a hard substance called calcium carbonate as a protective exoskeleton. Coral reefs are formed by the exoskeletons of coral colonies.
In case you were feeling pretty smug about being a mammal, here are some facts about insects that will wipe that self-satisfied smile off your face!
Insects are quite possibly the most successful animal groups ever to have existed. They’ve been around for 400 million years, appearing in the Devonian Period. (By comparison, modern humans only appeared a measly 200 thousand years ago.)
Over one million insect species have been identified – that’s more than half of all known animals. It is estimated that there may be as many as 10 million extant (living) species in total – so there are plenty more insects waiting to be discovered!
All insects share the same basic body plan. An insect’s body is divided into three parts: the head, thorax and abdomen. Their six legs are attached to the thorax, and the two antennae are attached to the head.
Most people know that crabs are crustaceans, but the group contains many other types of animal, including lobsters, shrimps and water fleas. Most crustaceans live in the sea, but some, such as wood lice, are adapted to living on land.
A crustacean’s body is protected by a hard exoskeleton. Many crustaceans have a carapace (hard shell), which is part of the exoskeleton. The crustacean has to shed the exoskeleton in order to grow.
Mollusks (Molluscs in British English)
The number of mollusk species living in the sea is greater than that of any other type of animal. The phylum Mollusk is extremely varied. It contains groups such as the bivalves (clams oysters and mussels); gastropods (slugs and snails); and cephalopods (squid and octopuses).
Most mollusks live in the sea, but some, such as snails and slugs, are also found on land.
A characteristic of all mollusks is a specialized fleshy skin part called a mantle. The mantle covers much of the body and forms a cavity which houses the mollusk’s vital organs.
Unlike invertebrates, vertebrates (members of the subphylum Vertebrata) form a scientific group in animal classification. Vertebrata includes all animals with a backbone (or rather, a vertebral column). According to the IUCN, there are 1,305,075 recognized vertebrate species.
As you probably know, humans are vertebrates. You can prove you’re a vertebrate right now by reaching round and feeling the ridges of your backbone running along the center of your back. (The backbone should actually be called backbones, because your spinal column consists of several separate bones called vertebrae).
Vertebrata are a varied bunch. The group includes animals such as fish, which have gills and live in the water, mammals which breathe air and mainly live on land, and amphibians, which do both (at different points in their lives).
Let’s meet some vertebrates!
Fish are aquatic animals with streamlined bodies, fins instead of limbs, and gills. Gills are organs that extract oxygen from the water and excrete carbon dioxide.
Fish were the first vertebrates to evolve. Today, around half of all vertebrate species are fish. There are two main types of fish: bony fish (Osteichthyes) and cartilaginous fish (Chondrichthyes).
The skeletons of bony fish are made of bone (duh), whereas those of cartilaginous fish are made of a flexible, bone-like substance called cartilage.
Bony fish are by far the largest group of fish. The group includes fish such as trout, goldfish, cod and marlin. Cartilaginous fish include the sharks, rays and skates.
Amphibians evolved from fish around 360 million years ago. They were the first vertebrates to live on land. Amphibians have moist skin, and don’t have protective scales.
Many amphibians begin life in water as larvae equipped with gills. After undergoing a process known as metamorphosis, amphibians develop lungs and are able to live on land. (Some amphibians, such as axolotls, don’t undergo metamorphosis.)
There are three main groups of amphibian: Anura (frogs and toads), Urodela (salamanders), and Apoda (caecilians).
Caecilians are burrowing animals that resemble worms or snakes. They are found both on land and in water.
Salamanders have long tails and cylindrical bodies. Most have four legs, but some species spend most of their lives in the water and only have two small front limbs.
Frogs have tail-less, muscular bodies, and many have well-developed hind legs for jumping and swimming.
Reptiles are members of the class Reptilia. They evolved from amphibians around 310 million years ago. Protected by scales, and able to lay eggs, reptiles were no longer reliant on water, and could occupy habitats unavailable to amphibians.
Reptiles rose to prominence during the Mesozoic Era – the ‘Age of the Reptiles’ – and, as dinosaurs, were the dominant land animals. One branch of dinosaurs evolved into birds, and some scientists consider birds to be part of Reptilia.
Reptiles are cold-blooded and most lay eggs, although some reptiles give birth to live young. Today’s reptiles include crocodilians (crocodiles, alligators and gharials), snakes, lizards and turtles.
Birds are now considered by many scientists to be ‘the last of the dinosaurs’. They evolved from dinosaurs around 150 million years ago, gradually losing their teeth and tails and developing beaks.
Birds are feathered, egg-laying vertebrates, most of which are able to fly. They are found on every continent and are able to survive in a wide range of conditions.
There are around 10,000 species of bird. More than half are passerines, or perching birds, members of the order Passeriformes. Passerine feet have three forward-facing toes and one backward-facing toe, an arrangement favorable to perching.
Other well-known bird groups include ratites (ostriches, cassowaries, kiwis, etc.), Psittaciformes (parrots), Sphenisciformes (penguins) and Accipitriformes (eagles, hawks and vultures).
Last but not least we come to the mammals, members of the class Mammalia. Mammals have hair, are warm-blooded, breathe air, and all but five species give birth to live young.
(The five species that don’t give birth to live young are the monotremes. Monotremes are egg-laying mammals. The group comprises the platypus and four species of echidna.)
One of the most important characteristics of mammalia is that female mammals have mammary glands. These are modified sweat glands that produce milk. This ready-made source of nourishment is highly advantageous to infant mammals.
The three main types of mammal are monotremes, marsupials and placental mammals.
Marsupials give birth to small, relatively undeveloped young, known as ‘joeys’. Joeys undergo further development within a special pouch in the mother’s body.
The developing fetus of a placental mammal receives nourishment from a special organ called a placenta while being carried in the mother’s womb.
- You can find out more about mammals here: Mammals: The Ultimate Guide
Animals – The Ultimate Guide: Conclusion
We hope that you have enjoyed reading about animals and the animal kingdom. On this page you’ve found out about how animals are classified and what makes an animal an animal. You’ve also met many of the groups within Animalia.
Now learn more about the incredible variety of animals by visiting one of these pages: