Many of the numerous invertebrates on Earth live in the
ocean. Some, such as polyps and jellyfish, have radial
symmetryΩthat is, their bodies are structured around an
axis. A typical echinoderm such as the starfish has tiny, flexible,
tube-shaped legs arranged like the spokes of a wheel. The animal
uses them to hold onto surfaces and to move. Sponges, on the
other hand, are very simple, multiple-celled animals, with
many small pores that they use to feed.