Phylum Echinodermata: Spiny-skinned animals

This is a huge assemblage of animals, and the second largest phylum (after Arthropoda). Among its many members are the sea stars ("starfishes"), urchins, and sea cucumbers. They are characterized by having a radial body plan, feet that use hydraulic action, and radiating arms. They tend to feed on slow moving prey such as clams, oysters, corals, and other echinoderms.

Sea cucumbers are considered a food delicacy by many people, but other echinoderms have limited commercial use. Sea stars are standard fare in shell shops around the world, and in many places they threaten local coral reefs. The spines of many sea urchins can penetrate human skin and may cause serious wounds.

The larval stage of the echinoderm resembles a tadpole, having bilateral symmetry, a pronounced tail, and is free swimming. The embryo derives from a developmental process also seen in chordates. For these reasons echinoderms are thought by many biologists to be closely related to chordates.