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| | Starfish (G. Asteroida - Star) – is a group of invertebrates of erinaceous type. Approximately there are 1600 contemporary species. In fossil condition they are known from Ordovician era.
Biology
Starfish is star-shaped or pentagonal erinaceous type. Sizes fluctuate between 2cm to 1m., though majority of them are of 12-24cm. Many types are brightly painted.
From flat center or slightly elevated disk of starfish extend 5(rarely-up to 40) beams or arms. When arms are contracted, body’s form is nearly pentagonal. Arms are outgrown-digestive organs and also serve as sprouts of genitals; spinal bones are arranged longitudinally. Skin is equipped with orderly arranged skeletal plates and is armed with barbs, needles and sometimes with pedicellaria.
Ambulacra are in open strias which are drawn from lower part of beam up to mouth. Legs represent flexible tubular projection, as a rule, with suckers at the end and are moved by pressure of water in internal canals and ampoules of ambucrum system. Mouth is at the center of abdominal area. Anus is located on spine; madrepore plate is in the spinal side.
Digestive system is noted to be peculiar. Above the mouth inside the disk two air pockets are arranged; from top (pyloric) extend 10 digestive (liver) sprouts, each two for each hand. Lower stomach can splay by abducting and partially digesting food.
Sensory organs; red eye spots are at the end of arms and tactile edges of skin.
Starfish have strong ability to regenerate; restoring torn away arms. Besides, a part of starfish with a big piece of central disc is capable of becoming a full-fledged starfish. Particularly remarkable is Linckia type which can develop a new organ instead of torn away arm.
Starfish’s lifetime is up to 35 years
Life and feeding
Bottom-dwelling animals, crawl with the help of ambulacra, and are often provided with suckers. Dwell universally in oceans and in seas (except still areas) up to 8.5km depth.
Majority of starfish are predators feeding mainly on molluscs, barnacles, polychaete and other invertebrates. Some feed on plankton and detritus;
There are types hunting with pedicellaria.
After finding dimyary mollusc, starfish clasps it with arms, stick to that with ambulacrums and on account of muscle effort opens shucks. Lower stomach splays up penetrates shell obduces soft parts of mollusk and digests without swallowing. Liquefied feeding then sucked inwards to the body.
Several starfish cause damage by destroying commercial fishing (oysters and mussels); Others eat corals.
Breeding and evolution
Starfish are predominantly dioecious. Usually they have two gonads each in each arm, opening at the base of beam in mating season. They breed spawning eggs and sperm in water. After insemination appears free-swimming larva which is usually stuck to a nearby elder starfish and under influence of its pheromone metamorphosis occurs. Some types-hermaphrodites hatch fry in a special emunctories or in cavity.
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