IGCSE Biology glossary
Invertebrate
Biology editorial review is supported by the named review panel. Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Definition
An animal without a backbone. Invertebrates make up over 95 percent of all animal species and include arthropods (insects, arachnids, crustaceans), molluscs, and annelids.
What to include in an exam answer
Arthropods are the most commonly examined invertebrate group. Know that insects have three body segments, three pairs of legs and often wings; arachnids have two body segments and four pairs of legs.
Example in context
Identify whether a spider is an insect or an arachnid. Give a reason (2 marks): A spider is an arachnid because it has four pairs of legs and two body segments, whereas insects have three pairs of legs and three body segments.
Related syllabus topic
Characteristics & classification of living organisms