Echinoderms are a type of invertebrate found in marine or ocean environments. Common echinoderms include the sea star, sea urchin, sand dollar and sea cucumber. Most echinoderms have arms or spines that radiate from the center of their body. The central body contains their organs, such as mouth for feeding and anus for expelling waste.
Many forms of echinoderms look quite different from each other. Most sea stars have five arms arranged symmetrically around the central body. If you turn the sea star over, you will see the mouth and anus on the underside. Although the sea urchin is also an echinoderm, its appearance is quite different from the sea star. The sea urchin has many long pointed spines radiating from the central body. Some sea urchin spines, such as those of the fire urchin, are poisonous. The sand dollar is flat with many short spines, called tube feet, along the bottom of its body. The sea cucumber looks very much like a cucumber, with its mouth at one end and anus on the other end, and many small tube feet on the underside of the body.