Class Asteroidea

Class Asteroidea

Class Asteroidea consists of the sea stars. They usually have five arms, but can have as many as 10-42 arms! A sea star has suction cup like tube feet which extend at ambulacral grooves through the skin and are used to pull the sea star along the floor. The mouth of a sea star is on the undersurface, and the anus is on the top. A sea star can eat a bivalve like an oyster by grabbing the shell with the tube feet and positioning its mouth in the opening between the hinged jaws. The stomach of the sea star comes out of its mouth and enters the oyster to digest the soft parts before returning to its original spot. Because the digestive tract and coelom of a sea star develop from the same part of the gastrula as in a lancelet, a chordate, sea stars are thought to be very closely related to our phylum.

Class Asteroidea is in the Phylum Echinodermata. The species that make up this phylum do not show body segmentation, and are radially symmetrical when fully grown but bilaterally symmetrical in the larvae stage. Almost all of the species are marine, although a few can live in brackish water. The coelom of the animals in this phylum is made from the digestive tube, not from cell masses. Therefore, echinoderms are deuterosomes. Echinoderms have an endoskeleton, made of 95% calcium carbonate. Another hallmark of the echinoderms is hard, spiny skin. This is a common feature, but not always apparent in echinoderms. The uniting feature of echinoderms is a water-vascular system. This is a system of canals branching throughout the body that branch into many sections called tube feet. There are at least 2,000 tube feet, which can penetrate the body wall and skeleton in places called ambulacral grooves, in most echinoderms. These tube feet, and in many echinoderms arms and even organs, can be regenerated. There are five major classes in the phylum Echinodermata.

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