This document provides an overview of invertebrates, describing their key characteristics and how they are classified. It notes that invertebrates do not have an internal skeleton, and explores some of the main types including sponges, jellyfish, worms, arthropods, and molluscs. Arthropods are described as the largest and most important group of invertebrates, having an external skeleton and many legs. Within arthropods, insects are discussed in more detail, noting their three-part body structure.