Echinoderms

Members of this group are sea stars, sea urchins, sea cucumbers and sand dollars. Echinoderms have an internal skeleton called a endoskeleton – made of harden plates that makes the animal bumpy and irregular texture. Echinoderms only live in the sea.

What is an Echinoderm? Adult echinoderms have no anterior or posterior end and no head. They do usually have 2 sides – the side with a mouth (called the oral surface) and the opposite side (the aboral surface).

Echinoderms have spiny skin, an internal skeleton, a water vascular system, and tube feet. Most adult echinoderms have a 5 part radial symmetry. The larvae will usually have bilateral symmetry.

Form and Function in Echinoderms The water vascular system is filled with fluid and carries out many body functions like respiration, circulation, and movement. It opens to the outside using the madreporite. This is a sievelike structure. This will connect to a ring canal that forms a circle around the animals mouth. From the ring canal there are radial canals that extend along the arms. Attached to each radial canal is a tube foot. This is like a suction cup. This allows the animal to attach onto a surface. They have hundreds of tube feet that allow them to walk and to open prey like clams.

Feeding Sea urchins use jawlike structures to scrape algae from rocks. Sea lilies use tube feet to capture floating plankton. Sea cucumbers move across the ocean floor taking in sand and detritus. Sea stars eat mollusk like clams or mussels. They will pry the shell open and push their stomach out through their mouth, digest the mollusk in its shell and then pull its stomach and partially digested prey into its mouth.

Respiration and Circulation The tube feet provide the main surface for respiration. Some species have special gills also. They use the water vascular system for circulation.

Excretion Most echinoderms do have an anus that gets rid of digestive waste. They release cellular waste through their tube feet.

Response They do not have a well developed nervous system. They have a nerve ring around the mouth and radial nerves down each body section.

Movement Most echinoderms use tube feet to move. Sand dollars and sea urchins have moveable spines that attach to the endoskeleton. Sea stars and brittle stars use their arms to move. Sea cucumbers can crawl along the ocean floor by tube feet and muscles in the body wall.

Reproduction Echinoderms reproduce by external fertilization. Both eggs and sperm and shed into the water where fertilization will take place.

Sea Urchins and Sand Dollars These animals have large, solid plates that form a box around internal organs. Sand dollars will burrow under sand or mud. Sea urchins wedge themselves in rock crevices and defend themselves with long, sharp spines.

Brittle Stars They have slender, flexible arms. They can move quickly. They can also lose one or more arms when attacked to get away from a predator. They are filter feeders and detritivores.

Sea Cucumbers Are detritus feeders that suck up organic matter. When threatened they will spew their guts to get away and then regrow them.

Sea Stars The move by slowly creeping along the ocean floor. They are mostly carnivorous. They eat bivalves. When damaged they can grow new arms.

Sea Lilies and Feather Stars These are filter feeders that have long feathery arms. They are mostly found in tropical oceans on the ocean bottom.

Ecology of Echinoderms Sea urchins help keep the amount of algae in check. Sea stars are important predators that control clam and coral populations. A crown of thorns sea star feeds on coral on the Great Barrier Reef of Australia. It has destroyed large areas of coral.