An invertebrate is an animal without an internal backbone which includes animals such as insects, crustaceans, worms, jellyfish and sponges. The diversity of invertebrate species far outnumbers that of the vertebrate animals. One group of invertebrates called Arthropods, which includes insects, spiders and crustaceans, contains almost 80% of all animal species. The remaining invertebrates constitute between 12-15% of all animal species.
The first invertebrates are believed to have evolved between 650 and 540 million years ago from single-celled organisms similar to certain cells found in sponges. Since then, they have diversified and spread into almost all environments on Earth and have evolved into some very sophisticated animals. Invertebrates use a range of methods for reproduction, sourcing food and surviving – the success of their methods is evident by the sheer number of invertebrates that exist on Earth today.