A starfish is not a fish. A sand dollar is not a form of money. And a sea cucumber is not a vegetable. All of these misleadingly named animals are echinoderms, or "spiny-skinned" marine animals that live on the ocean floor.
The echinoderms are not closely related to any of the other invertebrates. And in one very obvious way, the creatures in this phylum differ from other animals, vertebrates or invertebrates. Most animals, including human beings, have bilateral symmetry—their bodies can be divided into more-or-less-identical right and left halves. Echinoderms, by contrast, are endowed with radial symmetry. Their bodies are built on a circular, or radial, plan. In the center of the body is the mouth. From that hub, the arms or other structures extend outward at regular intervals, much like the spokes of a wheel.
Biologists find the larval forms of echinoderms particularly fascinating because of their close affinities with the larvae of the protochordates—tunicates and other primitive animals whose ancestors gave rise to backboned animals.