Short information about ASTERIAS (star fish)

INTRODUCTION AND DEFINATION.

Radially symmetrical metazoans which have calcified ossicles and spicules in their leathery skin (Gr. Echinos = spiny, dermates = skin). Hence they are known as spiny skinned animals. Their 6750 living species and 20,000 extinct (fossilized) species have been reported so far. All are exclusively marine.

They have as endoskleton composed of calcareous paltes and spines. Some very beautiful animals like star fishes, sea-cucumbers, brittle stars, sea-urchins and sea-illies are included in this phylum.

Asterias which is familiar to everyone and which is commonly called as star fish comes under class Asteroides of this phylum. Its specific features is central disc from which five (or more) arms originate. These are not actual fishes therefore it will be more appropriate to call them sea star instead of star fish.

Luidia, Pentaceros, Solater, Heliaster and Asterias etc. Are common genera of sea stars.

Classification

Kingdom = Animalia 

Phylum = Ecinodermata 

Class = Asteroidea 

Order = Forcipulatida 

Family = Asteriidae 

Genus = Asterias.

EXTERNAL FEATURES

(1) Shape and size - sea stars have pentamerous radial symmetry which is originated from by the bilateral symmetry of its larva. These are star-shaped and are about 25cm in diameter. There is an indistinct central disc in the body of ASTERIAS from which arise five long and tapering arms. In few animals of this class the number of arms are even more than five; for example in SOLASTER there are 7 to 14 arms.

(2) Colour - It is pink, orange, jambolin-like or yellowish in colour. It's dorsal or abnormal surface is of dark colour.

(3) Structure - The body is flattened and is distinctly divided into oral and aboral surface.

ORAL SURFACE AND ABORAL SURFACE OF STAR FISH.

The madreporite is located on the aboral surface of the sea star, which is the side opposite the mouth. Most people would consider this the "top" of the sea star.

EXPLANATION. 

Sea stars have two surfaces: the oral and the aboral. The oral is the side with the sea star's mouth and most people would think of it as the "bottom." The aboral is the opposite side, and this is where the madreporite lies.

The madreporite is the opening to the water vascular system. It takes in water that passes through the system and allows the sea star to move using its tube feet.