The phylum Echinodermata includes animals that have an internal skeleton and a water-based pressure system for locomotion. Members of this phylum include sea stars, brittle stars, sea urchins, and sea cucumbers. The embryo of echinoderms is different from that of all other invertebrates, and its structure suggests an ancestry to the phylum Chordata.

All echinoderms have spiny skin that helps protect them from predators. All are marine creatures, and all are radially symmetrical as adults. A ring of nerve tissue surrounds the mouth, and nerves branch off from this tissue to extend throughout the body.

Echinoderms have the ability to reproduce by regeneration, a process in which a relatively small piece of the animal can regenerate an entire body. All echinoderms have an internal support system called an endoskeleton and a large body cavity containing a set of canals called a water vascular system.