End Show Slide 1 of 37 Copyright Pearson Prentice Hall 28–4 Echinoderms

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28–4 Echinoderms


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What are the distinguishing features of echinoderms?

What Is an Echinoderm?


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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. Sea stars, sea urchins, and sand dollars are all echinoderms.


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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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What functions are carried out by the water vascular system of echinoderms?

Form and Function in Echinoderms


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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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What are the different classes of echinoderms?

Groups of Echinoderms


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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 (Class Echinoidea)

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.

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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Sea urchins


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Sand dollar


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Brittle Stars (Class Ophiuroidea)

Brittle stars are common in many parts of the sea, especially on coral reefs.

They are filter feeders and detritivores. 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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Brittle stars


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Sea Cucumbers (Class Holothuroidea)

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 cucumber


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Sea Stars (Class Asteroidea)

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 star


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Sea Lilies and Feather Stars (Class Crinoidea)

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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Sea lily


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Feather star


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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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Rown of thorns sea star


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28–4


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The body plan of echinoderms is unusual in that adult echinoderms

a. lack cephalization.

b. lack a nervous system.

c. lack cephalization and cannot move.

d. lack radial symmetry.


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The larvae of echinoderms are

a. not symmetrical.

b. radially symmetrical.

c. bilaterally symmetrical.

d. spherically symmetrical.


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The water vascular system of echinoderms opens to the environment through the

a. tube feet.

b. siphon.

c. madreporite.

d. mouth.


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The crown-of-thorns sea star is a major threat to

a. marine algae.

b. coral reefs.

c. sea cucumbers.

d. bottom-dwelling fish.


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All of the following are echnioderms EXCEPT

a. sea stars.

b. horseshoe crabs.

c. sea cucumbers.

d. sand dollars and sea urchins.

END OF SECTION

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End Show Slide 1 of 37 Copyright Pearson Prentice Hall 28–4 Echinoderms