Invertebrates Explained: Animals Without Backbones
Invertebrates lack a backbone and make up about 97% of animal species — insects, mollusks, worms, jellyfish, spiders, and more. Definition, characteristics, and major groups.
Global Animal Guide · July 10, 2026
Quick answer
Invertebrates are animals without a vertebral column. They are not a single evolutionary clade but an informal umbrella covering sponges, cnidarians, worms, arthropods, mollusks, echinoderms, and dozens of other phyla — roughly 97% of described animal species. Insects alone account for about a million named species.
Last updated: July 2026.
Definition (and a caveat)
Invertebrate is a useful teaching word, not a formal taxon like Mammalia. French naturalist Jean-Baptiste Lamarck popularised it for “animals without vertebrae.” Modern biology still uses it as a practical umbrella for everything outside Vertebrata.
Major invertebrate groups
Porifera (sponges) — Cellular organisation; filter feeders; no true tissues in the classic sense.
Cnidaria — Jellyfish, corals, sea anemones, hydroids — radial symmetry and stinging cells (cnidocytes).
Platyhelminthes — Flatworms, including free-living planarians and parasitic flukes/tapeworms.
Annelida — Segmented worms such as earthworms and leeches.
Mollusca — Snails, clams, octopuses, squids — soft bodies, often with shells.
Arthropoda — Insects, spiders, crustaceans, millipedes — jointed legs and exoskeletons; the largest phylum. See arthropods explained.
Echinodermata — Starfish, sea urchins, sea cucumbers — pentaradial adults, water-vascular systems.
Dozens of smaller phyla (nematodes, tardigrades, bryozoans, and more) fill every habitat from deep sea to soil.
Shared patterns (with exceptions)
| Feature | Typical invertebrate pattern |
|---|---|
| Support | Exoskeleton, hydrostatic skeleton, or none |
| Circulation | Often open (haemolymph bathes organs) |
| Respiration | Gills, tracheae, book lungs, or skin |
| Digestion | Complete or incomplete gut |
| Nervous system | Nerve nets or ganglia — not always a brain |
Cephalopods break stereotypes: closed circulation, camera eyes, and problem-solving behaviour rival vertebrates.
Why invertebrates matter
- Pollination — Bees, butterflies, flies, beetles
- Decomposition — Earthworms, dung beetles, soil mites
- Food webs — Krill, copepods, insects feed fish, birds, and mammals
- Reef building — Corals create habitat for a quarter of marine species
- Bioindicators — Damselflies and mayflies signal freshwater quality
Losing insects and other invertebrates collapses ecosystems faster than losing a single charismatic mammal.
Related reading
Frequently asked questions
What are invertebrates?
Animals that lack a backbone. The term is informal — it groups many unrelated phyla that share the absence of vertebrae.
What percentage of animals are invertebrates?
About 95–97% of described animal species are invertebrates.
Is an octopus an invertebrate?
Yes. Octopuses are mollusks (cephalopods) with no backbone, despite advanced intelligence.
What is the largest invertebrate?
The giant squid and colossal squid are among the largest — tentacle spans can exceed 10 m.
Are insects invertebrates?
Yes. Insects are arthropods, the most species-rich invertebrate group.