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The Class Asteroidea ("star form") includes animals commonly called starfish or sea stars. Most forms have a flattened and flexible body with a varying number (often five) arms, or rays, arranged around a central disc. The oral surface (the side with the mouth) faces the substrate. The "top side" (aboral surface) contains the anus and the entrance to the water-vascular systems called a madreporite. Water enters the madreporite and is delivered through a system of canals to the tube feet, which are equipped with suckers that can adhere to the substrate or their prey (often bivalves). Although beautiful, some starfish are regarded as serious pests to the oyster, clam and mussel industry, and the crown-of-thorns starfish (Acanthaster planci) can severely damage Pacific coral reefs grazing
on the tiny animals that deposit the calcium carbonate exoskeletons that form
the reef. |
