Echinoderms (scientific name Echinodermata) are a major group of only marine animals. The name comes from the Greek word for "spiny skin". There are about 7,000 species found usually on the sea floor in every marine habitat from the intertidal zone to the ocean depths. They have a wide variety of colours. There are at least 800 species of echinoderm on the Great Barrier Reef.

Echinoderms have radial symmetry, many having five or multiples of five arms. They have a shell, made mainly of calcium carbonate, which is covered by skin. The skin conatins cells to help support and maintenance the skeleton, pigment cells, cells to detect motion on the animal's surface, and sometimes gland cells which secrete sticky fluids or even toxins.

Digestive System

Echinoderms have a simple digestive system with a mouth, stomachs, intestineand anus. In many, the mouth is on the underside and the anus on the top surface of the animal. Sea stars can push their stomachs outside of their body and insert it into its prey allowint them to digest the food externally. This ability allows sea stars to hunt prey that are much larger than its mouth would otherwise allow.

Nervous System and Senses

Echinoderms do not have brains, they have nerves running from the mouth into each arm or along the body. They have tiny eyespots at the end of each arm which only detect light or dark. Some of their tube feet, are also sensitive to chemicals and this allows them to find the source of smells, such as food.

Circulatory System

Echinoderms have a network of fluid-filled canals that function in gas exchange, feeding and in movement. The network contains a central ring and areas which contain the tube feet which stretch along the body or arms. The tube feet poke through holes in the skeleton and can be extended or contracted. They do not have a true heart and the blood often lacks any respiratory pigment (pike haemoglobin).

Respiratory System

Echinoderms have a a poorly developed respiratory system. They use simple gills and their tube feet to take in oxygen and pass out carbon dioxide.

Reproductive System

Echinoderms are either male or female and become sexually mature after about two to three years. Most release their eggs and sperm into the water where they are fertilized. A female can release one hundred million eggs at once. Larvae devlop which eventually settle on the sea floor in their adult form.

If an arm breaks off some echinoderms, a new arm or even a new echinoderm can regrow. some sea stars and brittle stars have the ability to reproduce asexually by dividing in two halves while they are small juveniles.

Excretory System

Echinoderms have a simple excretory system with no kidneys and use diffusion to rid their bodies of nitrogenous waste which is mainly ammonia gas.

Their lifestyles vary greatly depending on which group of Echinoderms a species belongs to. Sea stars are generally predators or detritivores, eating decomposing animal and plant material.

Crinoids and some brittle stars are passive filter-feeders, absorbing suspended particles from passing water; sea urchins are grazing herbivores and sea cucumbers deposit feeders removing food particles from sand or mud.

Crabs, sharks, eels and other fish, sea birds, octopuses and larger starfish are predators of Echinoderms. Echinoderms use their skeltons, spines, toxins, and the discharge of sticky entangling threads by sea cucumbers as defence mechanisms against predators.