Vertebrate and Invertebrate Animals

One of the simplest and oldest ways scientists sort the animal kingdom comes down to a single question: does it have a backbone? Animals with a vertebral column are vertebrates; animals without one are invertebrates. That one distinction separates fish, birds and mammals from the vast world of insects, spiders, snails, worms and crustaceans. This guide explains what each group is, how they differ, why invertebrates matter so much, and how the topic is taught in schools.

The headline fact surprises most people: about 97% of all known animal species are invertebrates. Described vertebrate species number only around 66,000 to 70,000 worldwide, a tiny fraction of animal diversity, while invertebrate species run into the millions with many more still undiscovered. The animals we notice most — our pets, zoo animals, wildlife documentaries — are nearly all vertebrates, which gives a misleading impression of how the animal kingdom is actually made up.

How Scientists Classify Animals by the Backbone

Modern classification uses many traits, from genetic sequences to developmental patterns, but the presence or absence of a vertebral column remains one of the clearest teaching tools. There's an important asymmetry to it, though. Vertebrates form a single, well-defined scientific group — the subphylum Vertebrata within the phylum Chordata — sharing a common ancestor. "Invertebrates" is not a scientific group at all; it's a convenient catch-all for every animal that happens to lack a backbone, spanning dozens of unrelated phyla with completely different evolutionary histories and body plans.

The idea has deep historical roots. Carl Linnaeus established the foundations of modern taxonomy in 1735, and Jean-Baptiste Lamarck, working in the early 1800s, helped formalise the split between animals with and without backbones — he coined the term "invertebrate" for the latter. The core contrast is structural: vertebrates have an internal skeleton built around a spine of bone or cartilage, which supports complex organ systems and larger body sizes, while invertebrates either wear an external skeleton (the hard shells of crabs and insects) or have no rigid support at all (jellyfish and worms).

What Are Invertebrates?

Invertebrates are animals that lack a backbone of bone or cartilage — a definition that stretches from insects and spiders to snails, jellyfish, corals, worms, sea stars and the remarkably intelligent octopus. Their dominance in biodiversity is hard to overstate: insects alone make up more than half of all known animal species, and roughly two million invertebrate species have been named so far, with millions more likely waiting to be described.

Without a backbone, invertebrates have evolved several different solutions for body support. Soft-bodied animals like jellyfish and many worms rely on internal fluid pressure to hold their shape. Insects, crabs and millipedes wear an exoskeleton of chitin that protects them and anchors their muscles, which is exactly why the specialist supplies used to house invertebrate pets focus so much on calcium and humidity. Others again — snails, clams and corals — build hard shells of calcium carbonate. Between them, invertebrates occupy nearly every habitat on Earth, from deep-sea hydrothermal vents to mountain streams, deserts and city gardens, and they support a thriving UK community of keepers sourcing species from specialist invertebrate suppliers.

The Major Invertebrate Groups

Invertebrates are spread across many phyla, each a distinct body plan refined over hundreds of millions of years. The ones students most often meet are:

• Arthropoda — the largest animal phylum, covering insects (ants, beetles, butterflies), arachnids (spiders, scorpions) and crustaceans (crabs, shrimp, lobsters). All share jointed legs and a segmented, externally-skeletoned body — and within the crustaceans sit the isopods, closer relatives of crabs than of insects, which most people know as woodlice.
• Mollusca — snails and slugs, bivalves like clams and mussels, and the cephalopods (octopus, cuttlefish, squid) that show striking intelligence for invertebrates.
• Annelida — segmented worms, including soil-improving earthworms, medicinal leeches and colourful marine bristle worms.
• Echinodermata — the five-part symmetry of sea stars, brittle stars, sea urchins and sea cucumbers, with their unique water-vascular systems.
• Cnidaria — jellyfish, sea anemones, corals and freshwater hydra, all armed with specialised stinging cells.
• Porifera — the sponges, which filter food from seawater and, on fossil evidence, have existed for more than 600 million years, making them among the oldest animal groups.

Typical Characteristics of Invertebrates

For all their diversity, invertebrates share some broad tendencies. The defining one is the absence of an internal backbone. Most are small, though there are spectacular exceptions — giant squid can reach around 13 metres, and the Japanese spider crab has a leg span of nearly 4 metres. Many have an open circulatory system, in which a blood-like fluid called haemolymph bathes the organs directly rather than staying inside vessels; isopods and other crustaceans depend on adequate calcium for their exoskeletons to stay healthy.

Breathing varies widely: crabs and marine worms use gills, insects pipe air directly to their tissues through tubes called tracheae, and earthworms simply absorb oxygen through moist skin. Reproduction is often fast — fruit flies go from egg to adult in about ten days at 25°C, and many temperate butterflies complete their whole life cycle within a year. Terrestrial isopods breed readily too, which is part of why keepers weigh up the positives and negatives of isopods in bioactive enclosures.

What Are Vertebrates?

Vertebrates belong to the subphylum Vertebrata and all share a vertebral column protecting the spinal cord — a spine of repeated segments (vertebrae) made of bone in most species, or cartilage in others such as sharks and rays. That internal skeleton allows larger body sizes and more complex movement. Familiar examples span the major classes: mammals (humans, lions, whales), birds (eagles, robins, penguins), reptiles (snakes, lizards, turtles), amphibians (frogs, salamanders) and fish (from salmon and clownfish to sharks and rays).

Despite being vastly outnumbered — described vertebrates make up less than 5% of known animal species — they tend to be larger and more familiar to us. Our pet dogs and cats are mammals; the animals we watch in zoos and documentaries are overwhelmingly vertebrates. They're simply the animals that most closely share our own body plan.

The Major Vertebrate Groups

School science traditionally divides vertebrates into five classes:

• Fish — gill-breathing, mostly aquatic animals such as salmon, clownfish and sharks. Sharks and rays have cartilage skeletons rather than bone but still count as vertebrates because of their vertebral column.
• Amphibians — frogs, toads, salamanders and newts, which often begin life in water with gills before developing lungs, and usually return to water to breed.
• Reptiles — snakes, lizards, crocodiles and turtles, typically with scaly skin and leathery-shelled eggs, often thriving in drier places than amphibians can.
• Birds — warm-blooded, feathered, beaked animals from tiny sparrows to ostriches; most fly, though penguins use their wings to swim.
• Mammals — warm-blooded animals that usually have hair or fur and feed their young milk, including humans, elephants, bats and dolphins.

Typical Characteristics of Vertebrates

Vertebrates are built around an internal skeleton with a skull and spine. The vertebral column protects the spinal cord while staying flexible, and an enclosed brain sits inside a protective skull. Most have a closed circulatory system, with blood kept inside vessels and hearts ranging from two chambers in fish to four in birds and mammals. Their nervous systems and sensory organs tend to be more complex than those of most invertebrates, supporting elaborate behaviours from hunting to social life. Reproduction is usually sexual — internal fertilisation in reptiles, birds and mammals; external fertilisation in many fish and amphibians — and parental care ranges from none at all to years of nurturing in mammals.

Key Differences Between the Two Groups

The backbone is the simplest dividing line, but the groups differ in several linked ways. Skeleton is the fundamental one: vertebrates have an internal skeleton that grows with them, while invertebrates either shed and replace an external skeleton as they grow or have no hard structure at all. Size and mobility follow from this — vertebrates reach far larger sizes (the blue whale, at around 30 metres, is the largest animal ever known) and often move faster and farther, whereas invertebrates stay small but compensate with overwhelming numbers.

Their internal systems differ too. Vertebrates' closed circulation efficiently supplies large, high-energy bodies, while invertebrates' open systems suit smaller ones. And vertebrate nervous systems centre on well-developed brains, though octopus and cuttlefish are famous exceptions that rival some vertebrates for intelligence. A few contrasts capture it: a human's internal skeleton of 206 bones versus a boneless but extraordinarily flexible octopus; an eagle's hollow flight bones versus a butterfly's exoskeleton; a shark's cartilage spine versus a crab's hard outer shell.

What the Two Groups Share

For all their differences, both are part of the animal kingdom and share its core traits. Both are multicellular and heterotrophic — they must eat other organisms rather than make their own food. Both grow, reproduce and pass on genes, respond to their surroundings, and maintain internal balance through homeostasis. Even parental care, often thought of as a vertebrate speciality, appears widely among invertebrates: spiders guard their egg sacs, some beetles provision their larvae, and octopus mothers famously tend their eggs for months without eating. Complex traits, in other words, have evolved independently on both sides of the divide.

Why Invertebrates Matter

Because they're so numerous and diverse, invertebrates quietly hold up much of the living world. Pollinators — bees, butterflies, moths, flies and beetles — help pollinate an estimated 75–80% of flowering plant species, underpinning both wild ecosystems and the crops that feed billions of people. Decomposers and recyclers, from earthworms to dung beetles to woodlice, break down dead leaves, waste and organisms and return nutrients to the soil; seeing what woodlice eat and how they feed is a good illustration of that role in action.

People benefit directly, too — honey, silk, and shellfish such as mussels, shrimp and crabs all come from invertebrates, and species like the fruit fly Drosophila melanogaster and the nematode Caenorhabditis elegans have driven over a century of breakthroughs in genetics and neuroscience. The hobby side of this has grown into a real community as well, gathering at events like invertebrate and reptile shows.

Conservation Challenges

Many invertebrate populations are declining through habitat loss, pesticides, pollution and climate change, yet they attract far less attention and funding than charismatic vertebrates like pandas or elephants. Bee and butterfly numbers have fallen across North America and Europe, and warming, acidifying oceans are driving coral bleaching that threatens reef ecosystems this century. The encouraging part is how much small actions help: planting native flowers, cutting pesticide use, and reducing night-time outdoor lighting all support local pollinators and nocturnal insects, and projects like monarch-butterfly recovery and urban pollinator corridors show targeted effort can stabilise declining populations.

How Vertebrates and Invertebrates Depend on Each Other

The two groups are woven together through food webs and partnerships. Food chains show it plainly: leaves feed caterpillars, caterpillars feed small birds, and birds feed hawks — remove the caterpillars and the whole chain above them suffers. In the ocean, phytoplankton feed krill (themselves invertebrate crustaceans), which feed fish, which feed seals. The dependence runs indirectly too, since many plant-eating vertebrates rely on invertebrate pollinators for the fruits and seeds they eat. And some relationships are openly cooperative — cleaner shrimp pick parasites from reef fish, and oxpecker birds eat ticks off African mammals — proof that very different body plans can still benefit one another.

Teaching Vertebrates and Invertebrates in the Classroom

This classification is a staple of primary and lower-secondary science — UK Key Stage 2, US grades 3–5 and equivalents — because it introduces systematic thinking in a concrete, observable way. Sorting exercises with picture cards or toy animals let students practise the distinction, and pairing skeleton or x-ray images with photos of familiar animals makes the internal-skeleton idea visual. A simple "mini-safari" in a schoolyard turns it into fieldwork: most outdoor spaces hold ants, spiders, beetles, snails and worms to record and compare against local birds and squirrels, and older students can take it further by building a bioactive vivarium with its own cleanup crew.

The topic also lends itself to differentiated teaching: large, clear images help visual learners, repeating key vocabulary (backbone, skeleton, exoskeleton) builds familiarity, and physical models or labelled drawings add tactile and written reinforcement. As an applied extension, students can research the best isopod species for a vivarium as a hands-on example of invertebrate diversity.

Frequently Asked Questions

What is the main difference between vertebrates and invertebrates?

Vertebrates have an internal backbone (a vertebral column) made of bone or cartilage; invertebrates do not. Vertebrates also tend to be larger with more complex organ systems, while invertebrates are usually small but far more numerous and diverse.

What percentage of animals are invertebrates?

About 97% of all known animal species are invertebrates. Described vertebrates number only around 66,000–70,000 species — less than 5% of the total.

Are isopods vertebrates or invertebrates?

Invertebrates. Isopods (including woodlice) are crustaceans within the phylum Arthropoda — relatives of crabs and shrimp — with an external skeleton rather than a backbone.

Give two examples each of vertebrates and invertebrates.

Vertebrates: a human and a frog. Invertebrates: a spider and a snail.

What is the difference between an internal and external skeleton?

An internal skeleton sits inside the body and grows with the animal, as in vertebrates. An external skeleton is a hard outer covering that must be shed and regrown as the animal grows, as in insects and crustaceans.

Is "invertebrate" a scientific group?

Not really. Vertebrates form a single related group (subphylum Vertebrata), but "invertebrate" is just a catch-all for every animal without a backbone, spanning many unrelated phyla with different evolutionary histories.