Concept preview

What you will be able to do

1. 01Explain the features that define an animal and set animals apart from plants and fungi.
2. 02Name the major invertebrate phyla and describe the key body plan of each.
3. 03Compare how different invertebrates are supported, how they move, and how they feed.
4. 04Explain why the arthropods are the most successful group of animals on Earth.

An animal is a multicellular, eukaryotic organism that survives by ingesting other organisms — it is a heterotroph that takes food into its body and digests it internally, rather than making food from sunlight as a plant does. Animal cells are distinctive because they lack the rigid cell walls found in plants and fungi, so animal bodies are held together instead by flexible protein frameworks and specialised tissues. Almost every animal is capable of movement at some stage of its life, powered by muscle tissue and coordinated by nerve tissue, and most develop from a fertilised egg through a series of embryonic stages. These few shared traits — many cells, no cell walls, eating other organisms, and usually moving — unite creatures as different as a coral, an earthworm and an elephant into a single kingdom.