Animal Classification Explained

Scientists have described more than 1.5 million species of animals, and millions more may still be waiting to be discovered. They range from sponges that never move to cheetahs that sprint at over 100 kilometres per hour, and from microscopic water bears to the blue whale, the largest animal that has ever lived. Classification brings order to this incredible diversity. In this guide you will learn how the animal kingdom is organised, the key features of each major group and how to tell a reptile from an amphibian or an insect from a spider.

What Makes an Animal an Animal?

All members of the kingdom Animalia share these features:

• They are multicellular eukaryotes.

• Their cells have no cell walls, which allows flexible body shapes and movement.

• They are heterotrophs: they cannot make their own food, so they eat other organisms.

• Most can move from place to place at some stage of their life.

• Most have a nervous system to coordinate their actions.

• They store carbohydrate as glycogen.

• Most reproduce sexually, developing from a fertilised egg.

The Taxonomic Hierarchy for Animals

Like plants, animals are placed into a hierarchy of groups: Kingdom, Phylum, Class, Order, Family, Genus and Species. As you move down the levels, the groups get smaller and the members become more similar.

Notice that humans and lions share the same kingdom, phylum and class, because both are mammals, but they separate at the order level.

Binomial names for animals

Each species has a two-part scientific name made of its genus and species, a system created by Carl Linnaeus. The tiger is Panthera tigris, the domestic cat is Felis catus and the Asian elephant is Elephas maximus. Related animals share a genus: the lion (Panthera leo), tiger (Panthera tigris) and leopard (Panthera pardus) are all "big cats" in the genus Panthera.

What is a species?

A species is a group of organisms that can interbreed to produce fertile offspring. A horse and a donkey can breed to produce a mule, but mules are usually infertile, so horses and donkeys are separate species.

How Scientists Decide Which Group an Animal Belongs To

Zoologists use several key features to classify animals:

• Backbone: vertebrates have one; invertebrates do not.

• Body symmetry: radial symmetry (like a wheel, as in jellyfish) or bilateral symmetry (a left and right side that mirror each other, as in humans).

• Body layers: two layers of cells (diploblastic) or three (triploblastic).

• Body cavity: whether there is a fluid-filled space (coelom) around the organs.

• Segmentation: whether the body is divided into repeating segments.

• DNA evidence: modern classification compares DNA and protein sequences to reveal true evolutionary relationships. This has moved some animals into new groups.

Invertebrates: Animals Without a Backbone

Invertebrates make up about 97 percent of all animal species. Here are the main phyla you need to know.

Porifera (sponges)

Sponges are the simplest animals. They have no true tissues, no organs and no symmetry. Their bodies are full of pores through which water flows, and they filter tiny food particles from it. They live attached to rocks, mostly in the sea.

Cnidarians (jellyfish, corals and sea anemones)

• Radial symmetry.

• Two body layers and a single opening that acts as both mouth and anus.

• Tentacles with stinging cells (cnidocytes) for catching prey.

• Examples: jellyfish, Hydra, sea anemones and corals. Coral reefs, built by tiny coral animals, are among the most biodiverse habitats on Earth.

Flatworms (Platyhelminthes)

• Flat, soft, unsegmented bodies with bilateral symmetry.

• Only one body opening.

• Many are parasites, such as tapeworms and liver flukes. Free-living planarians can regrow a whole body from a small piece.

Roundworms (Nematodes)

• Long, thin, unsegmented, cylindrical bodies tapered at both ends.

• A gut with two openings, a mouth and an anus.

• Extremely common in soil. Some are parasites, such as Ascaris and hookworms, which infect millions of people.

Annelids (segmented worms)

• Long bodies divided into many ring-like segments.

• A moist skin used for gas exchange.

• Bristles called chaetae on most segments help them move.

• Examples: earthworms, leeches and ragworms. Earthworms improve soil by mixing and aerating it.

Molluscs

• Soft, unsegmented bodies, usually with a muscular foot for movement.

• Most have a hard shell made of calcium carbonate.

• A mantle, a fold of tissue that produces the shell.

• Examples: snails, slugs, mussels, clams, oysters, squid and octopuses. The octopus is considered one of the most intelligent invertebrates and can solve puzzles.

Echinoderms

• Marine animals with spiny skin.

• Adults show five-part radial symmetry.

• A unique water vascular system that powers tiny tube feet.

• Examples: starfish (sea stars), sea urchins, sea cucumbers and brittle stars. Surprisingly, DNA shows that echinoderms are more closely related to vertebrates than to other invertebrates.

Arthropods: the largest animal phylum

Arthropods make up more than 80 percent of all known animal species. They share these features:

• A hard exoskeleton made of chitin, which they must shed (moult) to grow.

• Segmented bodies.

• Jointed legs. The word arthropod means "jointed foot".

Arthropods are divided into four main classes, a favourite exam topic:

Many insects also have one or two pairs of wings. There are more species of beetles than of any other animal group.

A common mistake is to call a spider an insect. Spiders have eight legs and two body parts, so they are arachnids.

Chordates and Vertebrates: Animals With a Backbone

Vertebrates belong to the phylum Chordata. All chordates have, at some stage of life, a flexible rod called a notochord, a hollow nerve cord along the back, gill slits and a tail. In vertebrates the notochord is replaced by a backbone (vertebral column) made of vertebrae, which protects the spinal cord.

Vertebrates also have an internal skeleton (endoskeleton) of bone or cartilage, a well-developed brain inside a skull and a closed circulatory system.

The five classes of vertebrates at a glance

Ectotherms (often called cold-blooded) rely on their surroundings to control their body temperature, while endotherms (warm-blooded) generate heat internally to keep a steady temperature.

Fish

Fish live in water, breathe using gills and swim with fins. Most have a streamlined body covered in scales, and many have a swim bladder to control buoyancy. There are two main groups: cartilaginous fish, such as sharks and rays, with skeletons made of cartilage, and bony fish, such as salmon and tuna. Fish are the largest vertebrate group, with over 30,000 species.

Amphibians

The name amphibian means "double life". Most amphibians begin life in water as larvae (such as tadpoles) that breathe with gills, then undergo metamorphosis into adults that live on land and breathe with lungs and through their moist skin. Because their skin must stay moist and their eggs have no shells, most amphibians live near water. Examples include frogs, toads, newts, salamanders and legless caecilians. Amphibians are declining rapidly worldwide because of habitat loss, pollution and a fungal disease called chytridiomycosis.

Reptiles

Reptiles have dry, scaly skin that prevents water loss, allowing them to live in dry places. They breathe with lungs and lay eggs with leathery shells on land. These amniotic eggs contain membranes and fluid that protect the embryo, a key adaptation that freed reptiles from needing water to reproduce. Examples include snakes, lizards, crocodiles, alligators, turtles and tortoises. Dinosaurs were reptiles, and their descendants are the birds.

Birds

Birds are the only animals with feathers. They have wings, a beak with no teeth, hollow lightweight bones and a very efficient breathing system with air sacs. They are endotherms and lay hard-shelled eggs, usually incubated in nests. Not all birds fly: penguins, ostriches, emus and kiwis are flightless. There are around 11,000 bird species.

Mammals

Mammals have hair or fur, and females feed their young with milk from mammary glands. They are endotherms, have a diaphragm for breathing, different types of teeth and the largest brains relative to body size. Mammals are divided into three groups:

• Monotremes: egg-laying mammals, such as the platypus and echidna.

• Marsupials: give birth to tiny, undeveloped young that continue growing in a pouch, such as kangaroos and koalas.

• Placental mammals: the young develop inside the mother, nourished by a placenta. This includes humans, dogs, whales, bats and elephants.

Remember that whales and dolphins are mammals, not fish: they breathe air with lungs, give birth to live young and feed them milk. Bats are the only mammals capable of true flight.

Tricky Animals That Confuse Students

• Starfish are not fish; they are echinoderms.

• Jellyfish are not fish; they are cnidarians.

• Spiders are not insects; they are arachnids.

• Whales and dolphins are mammals, not fish.

• Penguins are birds, even though they cannot fly.

• Turtles are reptiles, not amphibians, even though many live in water.

• Seahorses are real fish, despite their unusual shape.

Why Animal Classification Matters

Classifying animals correctly helps scientists protect endangered species, track the spread of diseases carried by insects such as mosquitoes and ticks, and discover new medicines from animal venoms.

Using a Dichotomous Key for Animals

• 1a. Has a backbone: go to 2. 1b. No backbone: invertebrate.

• 2a. Has feathers: bird. 2b. No feathers: go to 3.

• 3a. Has hair or fur: mammal. 3b. No hair: go to 4.

• 4a. Has fins and gills as an adult: fish. 4b. No fins: go to 5.

• 5a. Dry scaly skin: reptile. 5b. Moist skin without scales: amphibian.