Starfish belong to the phylum Echinodermata, which is a Greek term referring to the spiny texture of many species in the phylum. Echinodermata is the largest phylum with no freshwater or terrestrial representatives, and it includes starfish, sea urchins and sea cucumbers.
The phylum Echinodermata includes about 7,000 living species. Members of Echinodermata are often called echinoderms. They are invertebrate animals that have radial symmetry, which means they have similar structures in several directions around a central point. In most starfish, this radial symmetry is consistent in five directions. The only large exception are sea cucumbers, which have bilateral symmetry.