Introduction

Echinoderms are radially symmetrical ocean animals. It is the only group that consists only of marine animals. All of the animals in this group are invertebrates (they do not have backbones). There are about 6,000 species. Echinoderms usually have arms or tentacles, but some do not. Some of the animals in this category are the sea stars (a.k.a. starfish), sea urchins, sand dollars, and sea cucumbers.