Echinoderms

Echinoderm, pronounced ih KY nuh durm, is the general name of certain spiny-skinned sea animals. There are about 6,000 kinds of echinoderms. Starfish, brittle stars, sand dollars, sea urchins, and sea cucumbers are among the most common kinds. All echinoderms have an internal bony skeleton. Their spines are a part of the skeleton. The echinoderm phylum (large animal group) is the only major phylum made up entirely of sea animals.

Adult echinoderms have radial symmetry. Their body parts are arranged around the center of the animal like the spokes of a wheel around the hub. Echinoderm bodies are usually divided into five sections with the mouth in the center.

Echinoderms are the only animals that have many tiny tubelike structures called tube feet. The tube feet project from the body in rows. Echinoderms use the tube feet for moving, feeding, breathing, and sensing. The outer tip of each tube often forms a suction disk for gripping hard surfaces. Within the echinoderm's body, a tiny bulb attached to the tube foot forces water into it to make it lengthen. An internal system of water-filled canals connects the tube feet to each other and to a sievelike plate that usually opens to the sea water. The entire system of tube feet and canals is called the water vascular system.

Echinoderms reproduce by laying eggs that develop into larvae and swim freely. The larvae have bilateral symmetry (two similar halves). The larvae sink to the ocean bottom and change into the adult, radial form.

Scientific classification. Echinoderms make up the echinoderm phylum, Echinodermata.