Starfish - Sea Stars Description

Anatomy

Most starfish or sea stars are pentamerous, having five arms that radiate from a central disk. The sea star is covered with tough skin and sharp, calcareous spines. At the base of the spines are pincer-like structures called pedicellaria that protect the starfish from small animals or algae that might settle on the body surface.

A network of calcareous plates beneath the skin form an external skeleton. These plates are joined by connective tissue and muscle which gives the sea stars their flexibility.

Underside of starfish

Sea stars have a complete digestive system with a mouth at the centre of their underside and an anus on their upper surface.

There is a groove down the centre of the underside of each arm that contains two rows of tubed feet with a suction cap at the end of each arm that can tell only light from dark. Each radial part has a set of internal organs.

Close up of tube feet of the starfish

General Features

Sea stars are often brightly coloured, usually red, orange to violet hues and unusual colours like blue and green but some are pale as well as patterns are sometimes intricate.The underside of a sea star is often lighter colour than the upper surface.

Starfish lifting one arm

Sea stars can regenerate lost arms and even a new body if enough of the arm remains.

Sea stars are sensitive to touch, light, temperature, orientation and water conditions. The tube feet, spines and pedicellariae are sensitive to touch while the eyespots are light sensitive.

Each arm on a sea star has a short sensory tentacle at the end that responds to chemicals and vibrations in the water. A sea star often lifts the end of an arm to perceive light and movement.



Website created by Jenny Stevenson - June 2006.