CLASS ASTEROIDEA (aster, star + oeides, in the form of)
The class Asteroidea contains sea stars. It has about 1,500 species. They live on hard substrates in marine environments. Some species also live in sandy or muddy substrates. Sea stars are brightly colored with red, orange, blue or gray shades. Asterias is an orange sea star. It is common along the Atlantic coast of North America. It is frequently studied in introductory zoology laboratories.
Sea stars usually have live arms. These arms radiate from a central disk. The oral opening or moult is present in the middle of one side of the central disk. It is normally oriented downwards. Mouth has movable oral spines around it. Movable and fixed spines project from in skeleton. They make the aboral surface rough. Thin folds of the body wall extend between ossicles. These are called dermal branchiae. These are involved in gas exchang
The aboral surface of some sea stars has numerous pincher like structures called dicellarieae. Pedicellarieae clean and protect the body surface. Pedicellariae may be nached on a movable spine. Or they may be fused with the skeletal ossicles. Water vascular system.A series of ossicles are present in the arms. They form an ambulacral groove.
Ambularal runs along the length of the oral surface of each arm. The ambulacral groove covers tie radial canal. Paired rows of tube feet protrude through the body wall on the sides of the ambulacral groove. Tube feet of sea stars move in a stepping Sea star moves by alternate extension, attachment, and contraction of tube feet. Nervous system coordinates the tube feet. Therefore, all feet move the sea star in the rection. However, the tube feet do not move collectively. The suction disks of t are effective structures for attachment. Thus sea stars maintain their position. n move from place to place even in strong wave.
Sea stars feed on snails, bivalves, crustaceans, polychaetes, corals and detritus. The ni out h pens into a short esophagus. Esophagus opens into a large stomach. Stomach fills most o the coelom of the central disk. Stomach is divided into two regions.
(a) Cardiac stomach: It is the larger part of stomach. It receives ingested food. It joins the smaller pyloric stomach.
(b) Pyloric stomach: It is also called aboral stomach.
The pyloric stomach gives rise to a duct. This duct connects to the pyloric cecae. Pyloric cecae are secretary and absorptive structures. Two pyloric cecae extend into each arm. Stomach opens into a short intestine. Intestine opens into rectal cecae and anus. Anus is nontu ctional. It opens at the aboral surface of the central disk.
Mechanism of digestion
1. Ingestion: Some sea stars ingest whole prey. The preys indigested extracellularly within the stomach. Undigested material is expelled through the mouth. Many sea stars feed bivalves. They open the valves of the mollusc. The sea star wraps itself around the hi alve’s ventral margin.
2 . Digestion: Tube feet attach to the outside of the shell of the body wallmuscles open the valves. When the valves arc opened about 0.1 mm, oelomic pressure is increased in the body. This pressure ejects the cardiac portion of the sea star’s stomach into the bivalve shell. Digestive enzymes are released, and partial digestion occurs in the bivalve of the shell. This digestion weakens the Madreporme bivalve’s adductor muscles.
Thus the shell opens up completely. Partially digested tissues are taken stomach into the pyloric portion of Gonad the stomach and pyloric cecae for further digestion and absorption. The sea star retracts the stomach after feeding and initial digestion with the help of stomach retractor muscles.
Respiration and excretion
Gases, nutrients, and metabolic wastes are transported to the coelom by diffusion and ciliated cells. Gas exchange and excretion occur by diffusion. This diffusion occurs through dermal branchiae, tube feet and other membranous structures. The hemal system of sea star consists of strands of tissues. These tissues encircle the mouth near the ring canal. They extend aborally near the stone canal and run into the arms near radial canals.
The nervous system of sea stars consists of a nerve ring and radial nerves.
1. The nerve ring encircles the mouth.
2. The radial nerves extend into each arm. These nerves lie within the ambulacral groove. These are present towards oral side of the radial canal. Radial nerves coordinate the functions of tube feet.
3. Another nerve net is present within the body wall.
Most sensory receptors are distributed over the surface of the body and tube feet. Sea star shows response to light, chemicals and various mechanical stimuli. They have specialized photoreceptors at the tips of their arms. These are actually tube feet that lack suction cups. F ese tube feet have a pigment spot surrounding a group of ocelli.
REGE ERATION, REPRODUCTION, AND DEVELOPMENT
1. Regneration: Sea stars have excellent of powers of regeneration. They can reg nerate any part of a broken arm. In some species, the body is broken. The broken am with a portion of the central disk regenerates. It forms complete animal. Re eneration is a slow process. It takes a year for complete regeneration.
2. Asexual reproduction: The central disk divides during asexual reproduction. Each regenerates to form complete organism.
3. Sexual reproduction
Sea stars are dioecious. But sexes are indistinguishable externally. Two gonads are preset in each arm. These gonads enlarge and fill an arm during the reproductive period. Gonophores open between the bases of each arm.
External fertilization takes place in them. But gametes cannot survive long in the ocean. Therefore, maturation of gametes and spawning is coordinated. The photoperiod and temperature coordinate sexual activity. Additionally, one individual release gametes with spawing pheromones. This pheromone induced the other sea stars to spawn. Therefore it inc eases the chance of fertilization.
Their embryos are planktonic (floating). Embryo uses cilia for swimming. Following larva are formed during development:
1. Bipinnaria larva: After gastrulation, bands of cilia are termed. Finally, bipinnaria larva is formed. It has bilateral symmetry.
2. Brachiolaria larva: Bipinnaria larva feeds on planktonic protists. It develops larval arms and change into brachiolaria larva.
3. Juvenile sea star: Brachiolaria larva settles on the substrate. It is metamorphoses into a juvenile sea star.