Echinoderms Phylum Echinodermata -sea stars, sand dollars, sea urchins & sea cucumber -marine -deuterostomes -more closely related to chordates, than to other invertebrates -no head or any other sign of cephalization -have endoskeleton composed of calcium carbonate plates called ossicles -adults are radially symmetrical with five-part body plan (pentaradial symmetry) -larvae are bilaterally symmetrical -most forms have a water vascular system with tube feet for locomotion, respiration and excretion -date back to Cambrian period, over 500 million years ago -echinoderm means spiny skin 1
-about 7,000 species Classification Class Crinoidea -mouth faces forward & surrounded by many arms -sea lilies are sessile as adults while feather stars are not -sea lilies, feather stars -about 600 species Sea Lily Feather Stars Class Ophiuroidea -usually have five slender, delicate arms or rays 2
-gather food off ocean floor with arms and tube feet -brittle stars, basket stars -Ophiuroidea means snake-tail -about 2000 species Brittle Star Class Echinoidea -means spine-like -sea urchins (bottom dwellers) & sand dollars (seacoast dwellers) -internal organs are enclosed within a compact, rigid endoskeleton called a test -spherical body, oval or diskshaped -arms lacking but five-part body plan still apparent -about 900 species Class Holothuroidea -means water-polyp -sea cucumbers 3
Sea Cucumber -uses tentacles to sweep up sediment and water; then stuffs its tentacles into its mouth and cleans the food off them -elongated, thickened body with tentacles around mouth -about 1500 species Class Asteroidea -means starlike -sea stars -body usually has five arms & double rows of tube feet on each arm -mouth directed downward -about 1500 species 4
Structure & Function of Echinoderms The sea star will be used to demonstrate some of the details of echinoderm structures and function. External Structure Oral surface - the side of the body where the mouth is located. Aboral surface - the side of the body that is opposite from the mouth. Pedicellariae - tiny pincers located on the body of a sea star. Used to help keep the body surface free of foreign objects, including algae and small animals that might grow on the sea star or damage its soft tissues. Skin gill 5
Water-Vascular System Sea stars have a network of water-filled canals that are connected to their tube feet. Madreporite a sieve-like plate on the Aboral surface where water enters the water-vascular system. Stone canal - a tube that connects the Madreporite to the ring canal, which encircles the mouth. Water passes through the Madreporite, then the stone canal into the ring canal. 6
Radial canal - a tube that extends from the ring canal to the end of each arm. They carry water to the hundreds of hollow tube feet. Valves prevent water from flowing back into the radial canals from the tube feet. Ampulla - a bulblike sac located on the upper end of each tube foot. Contractions of muscles around the ampullae forces water into the tube feet, causing them to extend. Small muscles can also raise the center of each tube foot s disklike end, creating suction. These muscle contractions allow sea stars to climb and capture prey. 7
Feeding & Digestion Cardiac stomach a stomach that a sea star can turn inside out through its mouth when it feeds. It transfers food to the pyloric stomach. Crown-of-Thorns Sea Star feeding on coral 8
Pyloric stomach - connected to a pair of digestive glands in each arm it, along with the cardiac stomach and digestive glands, breaks down food by secreting enzymes. Other Body Systems Skin Gills hollow tubes that project from the coelom, lining to the exterior, which allow for waste excretion and gas exchange by diffusion. Nerve Ring the majority of nerve tissue that makes up a nervous system in the sea star and encircles the mouth. Radial Nerve a nerve that runs from the nerve ring along the length of each arm. 9
Reproduction & Development Separate sexes External fertilization (1 female releases 200 millions eggs per year) Bipinnaria a bilaterally symmetrical, free swimming larva. After about two months the larva settles to the sea bottom, and metamorphosis begins. During metamorphosis the larva develops into a pentaradially symmetrical adult. Regeneration Regenerate arms from the central disk. Very slow as long as a year. Defensive mechanism can shed an arm when grabbed by a predator. Some species split their body in half & regenerate missing pieces. 10