Useful information on the external morphology of sea star

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It is commonly called Sea Star or Star fish.

Systematic Position:

  1. Phylum - Echinodermata
  2. Sub-Phylum - Eleuthrozoca
  3. Class - Asteroidea
  4. Genus - Asterias

Habitat:

Asterias is a free living marine echinoderm. It crawls slowly in shallow water to very great depth. It prefers rocky areas where locomotion and concealment are easier. It lives at the bottom. The bottom dwellings are called benthozoic.

Habits:

Sea star is a carnivorous animal. It is nocturnal in habit and it creeps slowly in the sea bottom. It possesses a good power of autotomy and regeneration. The sea star is diocious fertilization occurs in sea. Development includes bilaterally symmetrical larvae.

External Morphology:

Asterias has a glittering star-shaped body with penta-radial symmetry. But the larval body shows bilateral symmetry. Its colour varies from orange to purplish. The body consists of a central disc from which radiates out five elongated and tapering arms. The body is flattened with distinct oral and aboral surfaces.

Oral Surface:

The lower surface of the body is termed as oral surface. It is flat. At the center of the ventral surface, there is a pentagonal aperture called mouth or actinostome. The mouth is surrounded by a delicate membrane the peristome and is guarded by five groups of oral spines.fr each angle of mouth extends an oral groove called the ambulacral groove which run along the ventral surface of the corresponding arms. Two or three rows of calcerous spines are located on the either side of ambulacral groove, these are called ambulacral spines. Each ambulacral groove contains short tubular retractile projections called podia or tube feet. The tube feet serve as organ of locomotion, food capture and respiration.

The tip of the arm bears small, median, non-retractile, hollow terminal tentacles. At the end of each ambulacral groove, there is a small photosensitive eye spot. The eye is made up of several ocelli.

Aboral Surface:

The upper convex surface is called aboral surface. It is covered with a large number of calcerous spines. The spines are supported by calcareous ossicles burid in the skin. Between ossicles, there are a number of minute dermal pores, projects a small contractile process called dermal pranchia or popula (gills) which are respiratory and excretory organs. Around between spines there are microscopic pincer shaped bodies called pedicellariae. These are grasping organs used for cleaning and protecting the body. The aboral central axis bears circular anus and madreporite. The madreporite is a flat, sub-circular plate on the ventral surface of the disc at the base of two arms. These two arms are called bivium and the remaining three are trivium. The madreporite is a sieve like plates, with numerous pores, which leads into water vascular system inside the body. The water vascular system is a network of hydraulic canals branching into tube feet that function in locomotion, feeding and gas exchange.

Pedicellariae:

The pedicellariae of sea star is minute jaw like structures. These are found on both surfaces. These are of two types: stalked and sessile. But only stalked type occurs in Asterias. Each pedicel aria consists of a movable stalk bearing two articulating calcareous serrated (toothed) valves and articulating with a third calcareous plate, the basal piece, such pedicellariae containing 3 calcareous plates are termed forcipulate. The valves open and close with the help of one pair of abductor muscles and two pairs of abductor muscles respectively. On the basis of deposition of valves, they are of two types. Straight type and other is closed type. In the straight type, the jaws are straight and they remain parallel through out their length like a forcep. In closed type, the basal ends of two jaws are curved and they curved each other and work like a scissor. Pedicellariae help in food capture and removal of debris and minute organisms like larvae. They interfere with respiration. They also function as an organ of offence and defence.


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