Phylum Echinodermata
“spiny” “skin”
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Major Classes of Echinoderms
ECHINOIDEA: Includes
the sea urchins and
sand dollars
ASTEROIDEA: includes
the starfish (sea stars)
HOLOTHUROIDEA: includes
the sea cucumbers
OPHIUROIDEA: includes the
brittle stars
(below: a fossilized brittle star)
Characteristics of Echinoderms
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Coelomate and radially symmetrical
Body is usually 5-rayed (pentaradial)
Have a calcareous skeleton
Possess a water vascular system used for gas exchange,
feeding, and locomotion
Complete digestive system
Open/reduced circulatory system (no heart) with a
central ring and vessels
Simple radial nervous system (no brain)
Reproductive system consists of gonads which take up
a significant amount of space in the body cavity
Sexual reproduction with fertilization outside body
(most)
• Have ciliated, free-swimming larvae
• Abundant on most sea coasts, especially on rocky
shores
Taking a closer look at Sea Stars
• Possess 5 or more arms
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which radiate from a central
disk
Mouth is located on oral
surface (underneath)
Sea stars are found all over
the world in a variety of
shapes and colors
Most possess a simple light
sensitive eye spot at the end
of each ray (arm)
Digestion
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Sea stars have 2
stomachs that perform
different functions
1. The cardiac stomach can
2.
be pushed outside of the
mouth to engulf and
digest food, then
digested food is brought
inside to the pyloric
stomach
The pyloric stomach
further digests food and
passes it on to the
intestine and the anus
which is located on the
aboral surface (top)
This allows a sea star to
consume food that would
normally not fit into its
mouth!!!
The Water Vascular System
The water vascular system consists of a central ring and
a series of canals that eventually end in tube feet which
you can see protruding from the underside of this sea
star. These tube feet are used for movement, feeding,
and bringing in water for gas exchange.
• Here are the tube feet of the sea urchin which protrude out from
the spines.
• Sea stars and urchins alternately contract muscles that force
water into the tube feet which extends and pushes them into the
ground.
• The muscles are then relaxed in order for the tube feet to retract.
A Closer Look at Sea Urchins
• Most sea
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urchins
possess spines
of 1-3 cm but
can be found
up to 20 cm
long
Globoid shape
with no rays or
arms
Sea Urchin mouth
Sea Urchin Test (shell)
• Have ball and socket joints on the spines which allow them to
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move. If you try to touch the spines of an urchin, the
surrounding spines will converge there.
Possess a mouth with 5 calcium carbonate teeth (see above).
Also have a fleshy tongue within. This structure is termed
Aristotle’s Lantern.
Defense systems
of Echinoderms
• Spines are observed on
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many echinoderms and
can be soft or hard and
sharp. Their presence
deters predators
Pedicellariae are pincerlike structures that
protrude from the body
surface between the
spines
Pedicellariae keep the
body surface clear of
encrusting organisms
Echinoderms also
camouflage with their
natural color into their
environment
Ecological Roles of Echinoderms
• Starfish and brittle stars
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prevent the growth of
algal mats on coral reefs
Ossified skeletons of
echinoderms are major
contributors to many
limestone formations
Part of the food chain,
larvae produce food for
other creatures
The death of
echinoderms often is
followed by an increase in
seaweed growth and reef
destruction
• Sea urchin grazing reduces the rate of colonization of bare
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rock
Burrowing of sand dollars and sea cucumbers depletes the
sea floor of nutrients and encourages deeper penetration
which increases levels of oxygen and ecological tiering
Sea urchins bore into rocks, releasing nutrients
Sea cucumbers provide habitats for parasites
Echinoderms feed on dead and decaying organisms
Super Cool
Echinoderm Facts
• Sea stars are capable of
regeneration of body
parts as long as part of
the central disk remains
• The gonads of Sea
urchins and cucumbers
are considered a delicacy
in SE Asia
• Sea cucumbers will
eviscerate (shoot out parts
of their internal organs)
when they sense
extreme danger, then regrow the organs
• Sea urchins can replace
spines that are damaged
or lost