Vertebrates and Invertebrates: What the Difference Is

Quick answer

A vertebrate is an animal with a backbone; an invertebrate is an animal without one. That single split puts fish, amphibians, reptiles, birds and mammals on one side and everything else on the other: insects, spiders, snails, worms, jellyfish, corals, octopuses and the crustaceans, which include the woodlice I breed. Around 97% of the animal species described so far are invertebrates. Spiders and snails are invertebrates. Fish are vertebrates. Isopods, millipedes, springtails, cockroaches and beetles are all invertebrates, and all of them can be kept at home.

The one question that sorts the animal kingdom

Vertebrates share a vertebral column, a spine of repeated bony or cartilaginous segments protecting the spinal cord, and they belong to one scientific group, the subphylum Vertebrata. "Invertebrate" is not a group in the same sense. It is a word for everything that is not a vertebrate, coined by Jean-Baptiste Lamarck around 1800, and it covers dozens of unrelated phyla with nothing in common beyond the missing backbone. A jellyfish and a beetle are less closely related to each other than you are to a shark.

What invertebrates have instead of a spine varies. Arthropods (insects, spiders, crustaceans, millipedes) wear a jointed external skeleton of chitin that they shed to grow. Molluscs such as snails build a shell of calcium carbonate. Worms and jellyfish hold their shape with fluid pressure. That is why the animals I keep need calcium in the enclosure rather than in their bones: an isopod's skeleton is on the outside, rebuilt at every moult, and the calcium guide for isopods explains where it comes from.

The vertebrate groups

Five classes, the way schools teach them: fish, amphibians, reptiles, birds and mammals. Sharks and rays have cartilage rather than bone but still have a vertebral column, so they count. Vertebrates are the animals we notice, as pets, livestock and the stars of wildlife films, which makes it easy to forget that fewer than 5% of described animal species are vertebrates. The estimate sits around 70,000 species.

The invertebrate groups

• Arthropods - the largest phylum by far. Insects (beetles, cockroaches, butterflies), arachnids (spiders, scorpions), myriapods (millipedes and centipedes) and crustaceans (crabs, shrimp, woodlice). Springtails sit beside the insects as hexapods rather than inside them.
• Molluscs - snails and slugs, mussels and clams, and the octopus and cuttlefish.
• Annelids - earthworms, leeches and marine bristle worms.
• Echinoderms - starfish, sea urchins and sea cucumbers.
• Cnidarians - jellyfish, anemones and corals.
• Sponges - the simplest animals of all, filtering seawater for something like 600 million years.

Are spiders invertebrates?

Yes. Spiders are arthropods in the class Arachnida: eight legs, two body sections, no antennae and no backbone. They are not insects, which have six legs and three body sections, but they are every bit as much invertebrates. I do not keep spiders. The nearest thing to one in my collection is the white spotted assassin bug in the other invertebrates collection, an ambush predator with venom of its own.

Are snails vertebrates or invertebrates?

Invertebrates. A snail is a mollusc, and the shell that makes people ask the question is on the outside, built from calcium carbonate by the mantle, with a soft body and no bone underneath. Slugs are the same animals with the shell reduced or gone. Land snails are among the easiest invertebrates to keep, which is why giant African land snails turn up in my range whenever I have them.

Are fish invertebrates?

No. Fish are vertebrates: bony fish like salmon and goldfish have a bony spine, and sharks and rays have one of cartilage. The confusion comes from names. A jellyfish, a starfish and a cuttlefish are not fish at all; they are a cnidarian, an echinoderm and a mollusc, all invertebrates, named centuries before anyone sorted the sea by skeleton. Woodlice have relatives in the water too: the aquatic isopods are crustaceans living the same life as their land cousins, backbone-free in both cases.

Are isopods vertebrates or invertebrates?

Invertebrates, and crustaceans specifically, which puts woodlice closer to crabs and shrimp than to any insect. The terrestrial isopods, the suborder Oniscidea, are the woodlice, pill bugs and roly-polies of gardens everywhere, and the animals most of this site is about. They breathe through modified gills on the underside, which is why they need moisture, and they wear their skeleton outside, which is why they need calcium. The beginner's guide to keeping isopods in the UK covers keeping them, and the best isopod species for a vivarium narrows the choice down.

Why invertebrates matter more than their size suggests

Insects and other invertebrates pollinate the great majority of flowering plants, including most of the crops that need a pollinator at all. They break down dead leaves, wood and waste and return the nutrients to the soil, which is exactly the job a cleanup crew does in a bioactive vivarium; the bioactive cleanup crew guide shows how isopods and springtails divide that work between them. They feed almost everything above them in the food chain, caterpillars to birds and krill to whales. And they are declining, through habitat loss, pesticides and light pollution, with far less attention than the vertebrates that depend on them.

Which invertebrates can you keep at home?

More than most people expect, and several make better first pets than a hamster. The line between "a bug" and "a pet" is mostly husbandry, and the guide to invertebrates as pets for children is honest about which ones suit a household. What I breed and sell, captive-bred unless a listing says otherwise:

• Isopods - over 200 species and morphs, from hardy Dairy Cows to the premium Cubaris.
• Springtails - the half of a cleanup crew that grazes soft mould and fine debris, and one of the cheapest additions to a humid enclosure.
• Millipedes - slow, gentle detritivores; the best beginner millipedes guide picks the forgiving ones.
• Cockroaches - display species like the Madagascar hisser and the Emerald, and feeders for reptile keepers.
• Beetles - rainbow stag and flower beetle larvae, which spend most of their lives as grubs in flake soil.

Every one of them is an invertebrate, every one lives in a tub on a shelf, and none of them will ever need a walk.

Frequently asked questions

What does invertebrate mean?
An animal without a backbone. The word covers about 97% of described animal species, from insects and spiders to snails, worms, jellyfish and octopuses.

What is the main difference between vertebrates and invertebrates?
Vertebrates have an internal spine of bone or cartilage; invertebrates do not, and support themselves with an external skeleton, a shell or fluid pressure instead. Vertebrates tend to be larger; invertebrates are far more numerous.

Are spiders invertebrates?
Yes. Spiders are arachnids, a class of arthropods, with an external skeleton and no backbone. They are not insects.

Are snails vertebrates or invertebrates?
Invertebrates. Snails are molluscs; the shell is external and there is no bone inside.

Are fish invertebrates?
No. Fish are vertebrates. Jellyfish, starfish and cuttlefish are invertebrates that happen to carry the word "fish" in their names.

What percentage of animals are invertebrates?
About 97% of the animal species described so far. Vertebrates number around 70,000 species.

Are isopods vertebrates or invertebrates?
Invertebrates. Isopods are crustaceans, relatives of crabs and shrimp, with an external skeleton that they shed as they grow.

Is "invertebrate" a scientific group?
No. Vertebrates are a single related group, the subphylum Vertebrata. "Invertebrate" is a label for everything else, and it spans many unrelated phyla.

Which invertebrates make good pets?
Isopods, springtails, millipedes, cockroaches, beetle larvae and land snails are all kept in the UK hobby, and the hardy isopods and millipedes are the easiest starting points.