The starfish and their relatives form a peculiar phylum of animals. Nearly all of them have some kind of spines on their bodies. They also have organs that are found in no other animals.
Starfish and their relatives have an amazing ability to grow new parts. You can cut off an arm and the fish will soon grow a new one. You can cut the animal in two and it will soon become two starfish.
The structure of the starfish; a starfish has no head. It has a circle of arms that come together at the center of the body. At this center there is a stomach that opens through a small mouth on the underside. There are many short spines all over the body.
Inside the animal is a body cavity that extends out into the arms. The liquid in this cavity is mostly sea-water. This liquid carries dissolved foods and oxygen around the body.
A starfish moves on tube feet. There are hundreds of these little flexible tubes lined up in the grooves on the underside of each arm. The outer end of each tube foot acts like a suction cup. With hundreds of tube feet pulling at any one time, the starfish is able to move along.
The common starfish eats clams and oysters. It crawls over the top of a clam, clamps on to both sides of the shell with its tube feet, and begins to pull. Of course, the clam tries to hold its shell shut, but the steady pull of the tube feet wins out. The shell opens.
The mouth of the starfish is much too small to swallow a clam. It cannot bite off pieces, either. A starfish meets this problem in a most unusual way. It turns its stomach inside out and then pushes it out through its mouth. The thin-walled stomach slides in through the open shell of the clam and clamps against the soft flesh. Digestive juices that are produced by the stomach lining then kill and digest the clam. When the dissolved food has been absorbed, the starfish pulls in its stomach and moves away. Starfish also eat any other animal food they find. This includes dead fish along the bottom.
The starfish are extremely numerous in the sea. They just about cover the bottom in some places. Some of them live in shallow water. Other species are found at great depths. Because there are so many of them, they play an important role in the food chains of the ocean community. As scavengers, they also help keep the ocean bottom free of dead fish and other debris, and thereby stimulate the growth of sea plants and animals.
Other members of the starfish phylum include sea lilies, sea urchins, and sea cucumbers. Sea lilies grow attached to the ocean bottom by long stalks and reach their feathery arms into the water to catch food.
Sea urchins are globe-shaped and covered with spines. They reached downward with a five-pointed set of jaws to pull in bits of food. Sand dollars are similar, but they are not so spiny and are very flat. They look a little like silver dollars. Sea cucumbers are long and soft and actually look something like cucumbers. In some countries they are gathered to use in soups and other dishes. The spiny body wall is eaten.