Home >> Science & Nature >> Animals & Science

Body Parts of a Sea Star

Sea stars or starfish of the class Asteroidea exhibit radial symmetry with five rays or legs connected to a circular body. They are related to brittle stars, sea urchins, sea cucumbers and sand dollars.

Crabs, bottom-feeding fish and sea gulls feed on sea stars. If a predator pulls off part of an arm or other part of its body, the sea star can regenerate it as long as one fifth of the sea star's body is still intact.

Locomotion and Feeding
- Suction in the tube feet provides movement and feeding. Using tube feet to grip the substrate, a star fish creeps along rocks on the sea floor in such a way that the current does not wash it away. Sea stars wrap themselves around a bivalve and use the suction in the tube feet to pull apart the shells. Then the sea star pushes its stomach out of the center of its own body and into the prey's soft body tissues while secreting enzymes that dissolve and digest the meal.
Nervous System
- Starfish do not have a central brain but they do have a complex nervous system. The esophagus is surrounded by a central nerve ring which sends out radial nerves into each arm. Their ring nerves and radial nerves coordinate the animal's equilibrium and sense of direction.
  
  They detect light through eyespots which contain a mass of pigmented epithelial ocelli cells at the end of each ray. Each ocellus is covered by a thick, transparent skin layer that is light sensitive.
Excretion and Respiration
- The tube feet and the skin contain papullae which are thin-walled projections of the body cavity. They absorb oxygen directly from the water and distribute it throughout the body by the fluid in the body cavity.
  
  Excretion is performed by the same organs which remove nitrogenous waste from the body and secrete it into the surrounding water.

Animals & Science