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Phylum Echinodermata
Sea Stars
Sea Urchins
Sand Dollars
Sea Cucumbers
• “Spiny Skin”
• 7,000 species
• Marine Environments
– Shallow and deep waters
• Vary in size
– From 1cm to 1m (3feet)
• Often brilliantly colored
• Radial Symmetry
– Like cnidarians and ctenophores
• Lack cephalization
• Larvae are bilaterally symmetrical
Sea Star
Brittle Star
Sea Urchin
Echinoderms are deuterostomes, which
makes them different from all of the other
Deuterostomes are coelomates whose
1. embryos have radial cleavage
2. anus forms near the blastopore
3. mesoderm arises from outpockets of the
Echinoderms Have:
• Pentaradial symmetry
– the body parts extend from the center along five spokes
• An endoskeleton composed of calcium carbonate
• Many small, movable extensions of the water-vascular
system called tube feet, which aid in
Echinoderm Classification
– Crinoidea (kri-NOID-ee-uh)
– Asteroidea (AS-tuh-ROID-ee-uh)
– Ophiuroidea (OH-fee-yoor-OID-ee-uh)
– Echinoidea (EK-uh-NOID-ee-uh)
– Holothuroidea (HOH-loh-thuh-ROID-ee-uh)
Class Crinoidea
• “lily-like”
• Examples: sea lilies, feather stars
– Five arms extend from the body and branch to
form many more arms
– Tube feet  gas exchange
– Cilia on arms direct food to the mouth
– Mouth faces up
Class Ophiuroidea
• “snake-tail”
• Examples: basket stars and brittle stars
– Long, narrow arms help them move quickly
• Can break and regenerate easily
– Live on the ocean’s floor
– Either rake in food with arms or trap it with
their tube feet or mucus between their spines
Class Echinoidea
• “spinelike,”
• Examples: sea urchins, sand dollars
– Test –compact, rigid endoskeleton
– Urchins:
• Five teeth + their muscles = Aristotle’s Lantern
• Spines can be sharp and contain venom
– Sand dollars:
• Burrows into the sand
• Use spines for locomotion and burrowing
Aristotle’s Lantern
Class Holothuroidea
• “water polyp”
• Examples: sea cucumbers
– Soft bodies because the particles that make
up their endoskeleton are small
– Use tentacles to bring food to the mouth
Sea Cucumbers
• Eviscerate their gut as a means of self
Class Asteroidea
• “starlike”
• Examples: sea stars
– Live in coastal waters around the world
– Prey on oysters, clams, and other foods
humans eat
Structure and Function
• Sea stars
typically have 5
arms, but in
some species,
there may be
as many as 24
• Two rows of
tube feet run
along the
underside of
each arm
Structure and Function
• On sea stars and sea urchins,
pedicellariae, or little pincers, help keep
the body surface free of foreign objects,
including algae and small animals that
might damage its soft tissues
Structure and Function
• Gas exchange and waste excretion take
place by diffusion through the skin gills,
Structure and Function
• Primitive nervous system
• No head = no brain
• The nervous system consists mainly of a
nerve ring that circles the mouth and a
radial nerve that runs from the nerve ring
along the length of each arm
• Together, they coordinate the movements
of the tube feet
Structure and Function
• Sea stars also have a nerve net near the
body surface that controls the movements
of the spines, pedicellariae, and skin gills.
• Eyespot on each arm responds to light
• Tube feet respond to touch and chemicals
Water-Vascular System
Madreporite-water enters through this small pore
Stone canal -connects madreporite and ring canal
Radial canal -extends down each arm and carries
water to hundreds of hollow tube feet---has valves
to ensure one way flow
Feeding and Digestion
• Most echinoderms are carnivores.
• A sea star captures a clam and attaches to both
shells with its tube feet, eventually tiring the clam’s
muscles. Once the shell is opened every so slightly,
the sea star ejects a portion of its stomach into the
clam to digest the tissue.
• The sea star ejects a portion of its stomach when it
• Waste is expelled through anus on aboral surface.
• Most echinoderms are dioecious.
• Fertilization occurs externally after the
organisms spawn.
• A free-swimming larva called a bipinnaria
results and eventually develops into an
• As long as a portion of the central ring
remains intact, a sea star can regenerate.
Which Class?