The Animal Kingdom

What makes an animal an animal?

On the Classifying Living Things page you met the kingdoms of life. Animals make up one of them. Biologists use a set of features to decide whether a living thing is an animal. All animals share these:

• Many cells. Animals are multicellular. Their cells are eukaryotic, with a nucleus, as you saw on the Cells page.
• No cell walls. Plant and fungus cells have stiff walls. Animal cells do not, which lets them change shape and move.
• They eat. Animals are heterotrophs: they cannot make their own food, so they get energy by eating other living things or their remains.
• A set body plan. As an animal develops, its body grows into a shape that is typical of its species. Its cells become specialized, and in most animals they form tissues such as muscle and nerves.

Most animals can also move on their own for at least part of their lives, and most reproduce sexually. A sponge, for example, swims as a tiny larva before it settles down for good.

Symmetry and body plans

One of the first things biologists look at is an animal's symmetry: whether its body can be divided into matching halves.

• Asymmetric. Most sponges have no symmetry at all. No cut divides them into matching halves.
• Radial symmetry. The body parts are arranged around a center, like slices of a pie. Jellyfish, sea anemones, and corals are built this way. They have a top and a bottom but no front, back, left, or right, which suits an animal that drifts or stays in one place and meets food from every direction.
• Bilateral symmetry. Only one cut, down the middle, gives two matching halves: a left and a right. These animals also have a front end, where the mouth and sense organs usually are, and a back end. That suits animals that move head-first through the world. Most animal species, from worms and insects to fish and people, are bilateral.

Sea stars are a puzzle. Adults have five-part radial symmetry, but their larvae are bilateral. That is one clue that sea stars and their relatives come from bilateral ancestors.

The invertebrates

Invertebrates are animals without a backbone. They are not one natural group. The word simply means “every animal that is not a vertebrate”, and it covers more than 30 phyla. (A phylum, plural phyla, is a large group of animals that share a basic body plan.) Here are the eight you are most likely to meet. The numbers of species are rough counts of the species scientists have named so far, and different sources give different totals.

A closer look

Sponges are the simplest animals. They have no nerves, no muscles, and no gut. Instead, collar cells lining the inside beat their whip-like tails to pull water through the body and catch bits of food.

Cnidarians get their name from cnidocytes, stinging cells that fire tiny harpoons to catch prey. A jellyfish has no brain. Instead, a nerve net spreads through its body.

Worms are not one group. Flatworms, roundworms, and segmented worms belong to three different phyla. A planarian is a flatworm. Roundworms, or nematodes, are everywhere: a handful of soil can hold thousands, and scientists think many species have not been named yet. An earthworm is a segmented worm, and so are leeches.

Mollusks range from tiny snails to giant squid. Snails and clams have shells. Octopuses and squid, the cephalopods, have lost most or all of their shell, but they have large brains and are among the smartest invertebrates.

Arthropods include insects, arachnids (spiders, scorpions, mites), crustaceans (crabs, shrimp, barnacles), and centipedes and millipedes. Their tough exoskeleton is made of a material called chitin. It protects them and keeps them from drying out, but it cannot stretch, so arthropods shed it and grow a new one, which is called molting.

Echinoderms move on hundreds of tube feet, worked by a system of water-filled canals. Surprisingly, they are more closely related to us than to other invertebrates: echinoderms and chordates share a pattern of early development that sets them apart.

How many kinds of animals?

Scientists have named roughly 1.5 million living animal species, and new ones are described every year. One large count, published in 2013, added up all the named animal species, including fossils, and found about 1.66 million. The chart shows how they split up.

The result surprises many people. Roughly four out of every five named animal species are arthropods, and most of those are insects. Beetles alone make up a large share. Vertebrates, the animals most of us picture first, are only about 1 in every 20 named animal species.

These are counts of named species, not of all the species that exist. Many small animals, especially insects, worms, and deep-sea creatures, have never been described. You can read how scientists estimate the missing ones in How do we know?

Chordates

Vertebrates belong to a larger phylum, the chordates (Chordata). At some point in their lives, every chordate has these four features, plus a gland in the throat that becomes the thyroid in vertebrates:

• Notochord: a stiff but bendy rod that runs along the body and supports it. In most vertebrates it is replaced by the backbone as the embryo develops.
• Dorsal nerve cord: a hollow nerve cord along the back. In vertebrates it becomes the brain and spinal cord.
• Pharyngeal slits: openings in the throat. Fish use them for gills. In land vertebrates the same throat structures appear only in the embryo and develop into other parts, such as the jaw.
• Post-anal tail: a tail that continues past the end of the gut. Human embryos have one. In people and other great apes it is reduced to the small tailbone.

Not every chordate is a vertebrate. Tunicates (sea squirts) and lancelets are invertebrate chordates. They have no backbone, but they have all four features at some stage of life.

The vertebrate classes

Vertebrates have a backbone made of small bones called vertebrae, which surrounds and protects the spinal cord, and a skull that protects the brain. Textbooks traditionally sort them into five classes. The species counts below are rounded from the IUCN Red List's 2025 table of described species.

“Fish” is not one group

The word fish is handy, but biologists see several separate branches inside it:

• Jawless fishes: hagfishes and lampreys, with no jaws. Fewer than 200 species are alive today.
• Cartilaginous fishes: sharks, rays, and their relatives, with skeletons made of cartilage instead of bone.
• Ray-finned fishes: fins held up by thin bony rays. Salmon, tuna, and goldfish are in this group. It holds about half of all vertebrate species.
• Lobe-finned fishes: fleshy, bony fins. Only coelacanths and lungfishes are left today. Land vertebrates evolved from fishes in this branch, so a lungfish is more closely related to you than to a shark.

The other classes have their own surprises. Birds are the living descendants of one group of dinosaurs, so many scientists now count birds as reptiles. And when the first stuffed platypus reached Britain, some naturalists thought it was a fake: a mammal with a duck-like bill that lays eggs. The Evolution and Adaptation page shows how fossils and DNA reveal these family ties.

Body systems compared

Every animal has to support its body, get oxygen, move materials around, and react to the world. On the Human Body Systems page you saw how people do it. Other animals solve the same problems in different ways.

Skeletons

There are three main kinds. A hydrostatic skeleton is a body cavity filled with fluid that muscles push against, like a water balloon; earthworms and sea anemones use one. An exoskeleton is a hard outer case, as in insects and crabs. An endoskeleton is a frame inside the body, made of bone or cartilage in vertebrates. Echinoderms have an endoskeleton too, made of small plates under the skin.

Breathing

Very small or very flat animals, such as flatworms, get oxygen straight through their body surface. Earthworms breathe through their moist skin. Fish and many mollusks and crustaceans use gills. Insects have a network of air tubes, the tracheal system, that carries air to their cells directly, without using blood. Reptiles, birds, and mammals breathe with lungs.

Circulation

Sponges, jellyfish, and flatworms have no heart or blood vessels; materials move by diffusion and through the gut. In an open circulatory system, found in arthropods and most mollusks, a heart pumps blood-like fluid into open spaces where it bathes the organs directly. In a closed circulatory system, blood stays inside vessels. Earthworms, octopuses and squid, and all vertebrates have closed systems. Vertebrate hearts differ too: a fish heart has two chambers, an amphibian heart has three, and birds and mammals have four.

Nervous systems

Sponges have no nervous system at all. A jellyfish has a nerve net. A planarian has a small brain at its head end and two nerve cords linked like a ladder. An insect has a brain and a nerve cord along its belly side, with knots of nerve cells called ganglia. Octopuses have large, complex brains. Vertebrates have a brain inside the skull and a spinal cord along the back.

Who figured this out?

Animals have been around for hundreds of millions of years. Early animals were soft-bodied. Then, starting about 539 million years ago, at the beginning of a time called the Cambrian Period, the fossil record fills with new kinds of animals, including the first arthropods, mollusks, echinoderms, and vertebrates. One of the oldest known vertebrates, a small fish-like animal called Haikouichthys, lived about 530 million years ago. People have been sorting animals for much less time.

Key words

• Heterotropha living thing that gets its food by eating other living things or their remains
• Invertebratean animal without a backbone
• Vertebratean animal with a backbone made of vertebrae
• Phyluma large group of animals that share a basic body plan
• Radial symmetrybody parts arranged around a center, like the spokes of a wheel
• Bilateral symmetrya body with matching left and right halves
• Cnidocytea stinging cell, found in jellyfish, corals, and sea anemones
• Mantlethe fold of tissue in a mollusk that makes the shell
• Exoskeletona hard outer covering that supports and protects the body
• Moltingshedding an old exoskeleton or outer layer in order to grow
• Tube feetsmall, water-powered feet that echinoderms use to move and hold on
• Notochorda stiff, bendy rod along the body of a chordate
• Endoskeletona skeleton inside the body
• Open circulatory systema system where blood-like fluid leaves the vessels and bathes the organs
• Closed circulatory systema system where blood stays inside vessels

Game: Sort the Animal

You will see fourteen animals, one at a time. Choose the group each one belongs to. Watch out: some animals have names or looks that point the wrong way. An animal you sort right on your first try earns 1 point, for a total of 14. Your best score is saved on this device.

Quick quiz

Six questions. Pick one answer for each, then press Check my answers.