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28–4 Echinoderms

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What Is an Echinoderm?

Echinoderms are characterized by spiny

skin, an internal skeleton, a water vascular

system, and suction-cuplike structures

called tube feet. Most adult echinoderms

exhibit five-part radial symmetry.

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The skin of echinoderms is stretched over an endoskeleton that is formed of hardened plates of calcium carbonate.

Adult echinoderms typically have no anterior or posterior end and lack cephalization.

The side in which the mouth is located is called the oral surface, and the opposite side is called the aboral surface.


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Echinoderms are deuterostomes, animals in which

the blastopore develops into an anus.

Sea stars, sea urchins, and sand dollars are all

echinoderms.


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Form and Function in Echinoderms

Echinoderms have a system of internal

tubes called a water vascular system.

The water vascular system carries out

many essential body functions, including

respiration, circulation, and movement.


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Echinoderm Anatomy


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The water vascular system opens to the outside

through a sievelike structure called a madreporite.


madreporite

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A tube foot is attached to each radial canal.


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Tube feet act together to create enormous force

allowing echinoderms to “walk,” and pull open shelled

prey.


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Feeding

Echinoderms have several methods of feeding.

• Sea urchins use five-part jawlike structures to scrape algae from rocks.

• Sea lilies use tube feet along their arms to capture floating plankton.

• Sea cucumbers move across the ocean floor, taking in sand and detritus.


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Sea stars usually feed on mollusks such as clams

and mussels.


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The sea star starts digesting the mollusk in its own

shell.

The sea star pulls its stomach and the partially

digested prey into its mouth.


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Respiration and Circulation

In most echinoderms, the thin-walled tissue of the

tube feet provides the main surface for respiration.

In some species, small outgrowths called skin gills

also function in gas exchange.

Circulation of needed materials and wastes takes

place throughout the water vascular system.


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Excretion

In most echinoderms, digestive wastes are released as

feces through the anus.

Nitrogen-containing cellular wastes are excreted primarily

in the form of ammonia.

This waste is passed into surrounding water through the

thin walled tissues of tube feet and skin gills.


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Response

Most echinoderms have a nerve ring that

surrounds the mouth, and radial nerves that

connect the ring with the body sections.

Most echinoderms have scattered sensory cells

that detect light, gravity, and chemicals released

by potential prey.


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Movement

Most echinoderms move using tube feet.

Sand dollars and sea urchins have movable spines

attached to the endoskeleton.

Sea stars and brittle stars use their arms for

locomotion.

Sea cucumbers crawl by using both tube feet and

the muscles of their body wall.


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Reproduction

Echinoderms reproduce by external fertilization.

Both sperm and eggs are shed into open water,

where fertilization takes place.

The larvae swim around for some time.

Larvae then swim to the ocean bottom and

develop into adults.


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Groups of Echinoderms


Classes of echinoderms include sea

urchins and sand dollars; brittle stars;

sea cucumbers; sea stars; sea lilies

and feather stars.

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Sea Urchins and Sand Dollars

Sea urchins and sand dollars have large, solid

plates that form a box around their internal organs.

Many are detritivores or grazers that eat large

quantities of algae.


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Sand dollars often burrow under layers of sand or

mud to protect themselves.

Some sea urchins wedge themselves in rock crevices

during the day.


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Brittle Stars

Brittle stars are common in many parts of the sea,

especially on coral reefs.

They are filter feeders and detritivores.


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Brittle stars have slender, flexible arms.

They shed one or more arms when attacked.

The detached arm keeps moving, distracting the

predator while the brittle star escapes.


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Sea Cucumbers

Most sea cucumbers are detritus feeders.

Herds roam across the deep-sea floor sucking up

organic matter and the remains of other animals

and plants.

Sea cucumbers look like warty, moving pickles.


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Sea Stars

Most sea stars are carnivorous, preying on bivalves.

If a sea star is pulled into pieces, each piece will grow into a new animal, as long as it contains a portion of the central part of the body.

Sea stars move by creeping slowly along the ocean floor.


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Sea Lilies and Feather Stars

Sea lilies and feather stars are common in tropical

oceans.

Many modern feather stars live on coral reefs,

where they perch on top of rocks and use their

tube feet to catch floating plankton.


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Ecology of Echinoderms


A sudden rise or fall in the number of echinoderms

can cause major changes to populations of other

marine organisms.

Sea urchins help control the distribution of algae and

other forms of marine life.

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Sea stars are important predators that help control

the numbers of other organisms.

The crown-of-thorns sea star has destroyed

extensive areas of coral in the Great Barrier Reef of

Australia.


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