Invertebrates
Animals without backbones make up most species.
About this topic
Invertebrates have no backbone and make up about 97% of animal species. Main phyla: sponges, cnidarians (jellyfish), flatworms, roundworms, annelids (earthworms), molluscs (snails, squid) and arthropods (insects, spiders, crustaceans), the biggest group, with a segmented body, jointed legs and an exoskeleton.
Arthropods are classified by body plan: insects have six legs and three body parts, arachnids eight legs and two body parts, crustaceans many legs and two pairs of antennae, and myriapods (centipedes, millipedes) many segments. Their hard exoskeleton protects them but must be shed in moulting for growth.
Invertebrates are vital to ecosystems. Bees and other insects pollinate crops, earthworms aerate and enrich soil, corals build reefs and many are food for other animals. Pests such as locusts and mosquitoes harm crops and carry disease. Molluscs include octopuses, which are intelligent and can solve puzzles.
Do not assume invertebrates are all small or simple: the giant squid can reach many metres in length. Another misconception is that spiders are insects; spiders have eight legs and are arachnids. Classification is a way of grouping animals by shared features.
Key ideas
- Arthropods, molluscs, annelids
- Exoskeletons
- Insects are the largest group
Quick check
Use cases: where Invertebrates helps
- Homework: review the key idea behind Invertebrates before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Invertebrates without notes.
- Visual learning: play with the interactive lab to see how Invertebrates changes when you adjust its inputs.
- Teaching a friend: explain Invertebrates in your own words, then compare your version with the key ideas.
Biology underpins medicine, farming, conservation and biotechnology.
Frequently asked questions
What is Invertebrates?
Animals without backbones make up most species.
What will I learn in Invertebrates?
Arthropods, molluscs, annelids. Exoskeletons. Insects are the largest group.
Is Invertebrates part of Biology?
Yes. It is one of 150 Biology topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Invertebrates lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Invertebrates?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.