External Features of Starfish

Asterias is exclusively marine, bottom dwelling or benthonic animal. Asterias has a radially symmetrical and pentamerous body. The body consists of a central, pentagonal central disc from which radiate out five elongated, tapering, symmetrical spaced projections, the rays or arms. The size varies from 10-20 cm in diameter though some forms may be much smaller or longer. The colour is variable having shades of yellow, orange, brown and purple. The body has two surfaces, the upper convex and much darker side is called the aboral or abactinal surface. Oral Surface: The side of body, which in natural condition remains towards the substratum and contains the mouth or oral opening, is flat and of dark orange to purplish colour, is called oral or actinal surface. The oral surface bears the following structures: 1. Mouth 2. Ambulacral Grooves 3. Tube Feet or Podia 4. Ambulacral Spines 5. Sense Organs Aboral Surface The Aboral surface bears the following structures: 1. Anus2. Madreporite3. Spines4. Papulae or Gills5. Pedicellariae