sea stars, sea urchins, and sea cucumbers. echinoderm larvae are bilaterally symmetrical; adult...

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  • Sea Stars, Sea Urchins, and Sea Cucumbers

  • Echinoderm larvae are bilaterally symmetrical; adult echinoderm have pentamerous radial symmetry (symmetry based on five parts) .They have no head.Echinoderms have an oral and aboral surface.They have a complete digestive tract and an internal skeleton (endoskeleton).Although the skeleton can look like it is external, as in the spines of sea urchins, the endoskeleton is covered by a thin layer of ciliated tissue.The spines and pointed bumps give many echinoderms a spiny appearance.

  • All echinoderms have a water vascular system which is a network of water-filled canals.Tube feet are muscular extensions of these canals.The tube feet extend when filled with water and by the action of muscular sacs called ampullae, that extend inside the body opposite the tube feet.Tube feet often end in a sucker and are used for attachment and locomotion.

  • Sometimes called starfishes.Most species have five arms that radiate from a central disk. Some of them have more than five can be up to 50.Hundreds of tube feet protrude from the oral surface along radiating channels on each arm called ambulacral grooves.Sea stars can move in any direction, usually slowly, by reaching out their tube feet and pulling themselves along.The endoskeleton consists of interconnected calcium carbonate plates that form a flexible framework. This allows the arms to be flexible.Most sea stars are predators of bivalves, snails, barnacles, and other slow moving animals.

  • Brittle stars have a star-shaped body.The five arms are long and very flexible and branch out from a small central disk.They have swift, snake-like movements of the arms.Brittle stars use the tube feet for eating particulate organic matter off the bottom of the water. Brittle stars are widely distributed, but are difficult to see because they often hide under rocks and corals. They also cover themselves with mud or sand.

  • The endoskeleton forms a round, rigid, shell-like test with movable spines and pedicellariae.Locomotion is achieved by the spines, jointed to sockets in the test, and the sucker-tipped tube feet.The mouth is located on the bottom and the anus on the top.They graze on attached or drifting seaweed or seagrasses. They have an intricate system of jaws and muscles called Aristotles lantern that is used to bite off algae and other bits of food from the bottom. Sea urchins are a common sight on rocky shores. They are also the most important grazers in the kelp communities.

  • Sea urchin populations sometimes explode.These explosions have had devastating impacts on kelp communities. Sea urchins normally feed on drift kelp.During population explosions, or plagues, the urchins eat live kelps and other seaweed.When the sea urchins eat the holdfasts or stipes, the kelps break loose, float away, and die.The urchins may completely clear large areas, which are then known as urchin barrens.

  • Sand dollars live in soft bottoms by having flattened bodies and short spines. They are deposit feeders that use their tube feet and sometimes strands of mucus to pick up organic particles.Sand dollars are covered with short, fine spines with which they move and burrow in the sand. Large, flattened tube feet protrude from the five double rows of holes on the upper survace and serve as gills.

  • Smaller, mucus-covered tube feet, on both surfaces of the test, collect small organic particles and pass them to the ciliated grooves that sweep them to the mouth to be eaten. Sand Dollars form dense beds on sandy bottoms along the California coast. They lie partially buried in the sand, angled into the current.

  • Sea cucumbers are cylinder-shaped invertabrate animals that live in seas worldwide. They are found in a variety of sea floor habitats, from warm tropical waters to cold deep sea trenches.They have a life span of about 5 to 10 years. The sea cucumber is elongated, leathery and muscular; spines are contained with the skin. These echinoderms have no arms, but do have five-part symmetry. Surrounding the mouth are 8 to 30 tentacles (modified tube feet). Five double rows of tube feet (with tiny suction cups) run along the body; they are used for crawling along the sea bed or anchoring to a rock.

  • A sea cucumber breathes by pumping sea water in and out of an internal organ called a respiratory tree.Sea cucumbers have no brain. The biggest sea cucumber, the tiger's tail sea cucumber (Holothuria thomasi), is about 2 m long - most sea cucumbers are much smaller than this. Sea cucumbers eat decaying matter that floats in the water or is in the sand. Sea turtles, crustaceans, many fish, and people eat sea cucumbers.

  • Sea cucumbers can expel most of their internal organs to confuse predators - they later regrows the organs. Some sea cucumbers' bodies contain toxins that can deter attackers. Eggs and sperm are broadcast (released) into the water (the sexes are separate). When an egg is fertilized and hatches, the tiny planktonic larva drifts with the ocean currents. It will eventually settle onto the sea floor and develop into an adult.

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