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Echinoderms
The spiny skinned animals
Introduction
• Echinodermata are all marine, triploblastic
unsegmented coelomates
• Phylum has 3 unique features:
– pentagonal symmetry (bilateral in larvae)
– calcite spicules embedded in the skin, often
partly fused
– Tube feet (podia)
An unhurried phylum..
• No echinoderm moves fast, apart from a
very few deep sea holothurids which swim
actively
• Crinoids are sessile, the others crawl at a
rate of mm / minute
• During one Antarctic marine survey a
starfish was tagged. A year later the same
animal was in the same exact spot, having
apparently done nothing at all!
5-radial layout
• Many organ systems in the echinoderms follow
the same basic structure as the water-vascular
and nervous systems: a 5-radial circum-oral
ring
• These rings give rise to 5 radial branches
(canals in the case of the WVS)
• A few asteroids have 7, 10, 11 arms - in which
case 7,10, 11 radial branches
Phylum Echinodermata
• Echinoderms means “prickly skin”
• Animals of this phylum include the starfish,
sand dollar, sea cucumber and sea urchin.
• Almost all live in the ocean
• All adult echinoderms have radial
symmetry.
BODY STRUCTURE
• All echinoderms have tube feet - these are
hollow structures used for moving and
grasping.
• The tube feet are found on the ventral side
of the echinoderm’s arm.
• Each tube foot looks like a tiny medicine
dropper with a suction cup at the bottom.
How echinoderms use their tube
feet.
1 Water enters an echinoderm through a hole on the
top of its body.
2 The water then travels through canals in the arms
to the tube feet.
3 Muscles allow water to move into and out of the
tube feet.
4 This pumping action enables the suction cups to
grasp and release objects and move the
echinoderm along
How a starfish eats
1 The tube feet help the starfish capture its food.
2 A star fish will wrap its arms around a clam
attaching the arms to each side of the clam’s shell.
3 Eventually the tube feet will pull the shells apart.
4 The star fish then forces its stomach out of its
body and into the clam shell.
5 The stomach digests the clam within the shell.
Surface features
• Echinoderm skin has several distinctive sets
of organs protruding from their skin:
– Tube feet (podia)
– Spines
– Pedicillaria
Gonads
• Lie as 10 (2N) paired structures at the
base of ambulacral grooves.
• Sexes are separate, and discharge
gametes into the sea water
• Gonads can be large - echinoid gonads
almost fill the test, and can be eaten as a
delicacy.
Sadly...
• Of the 13 classes of echinoderms known, 7
are extinct.
• Echinoderms were dominant forms in
Carboniferous seas, but have suffered a
long-term decline in phyletic richness
Echinoderms
• The spiny skinned animals include:
–
–
–
–
–
Class Crinoidea - the crinoids
Class Asteroidea - starfish (sea stars)
Class Ophiuroidea - brittle stars, basket star
Class Echinoidea - sea urchin, sand dollar
Class Holothuroidea - sea cucumbers
Crinoids
•
•
•
•
Filter feeders
some are sessile
some are motile
They were very
common in the
Paleozoic
• Their bodies are often
found in limestone
deposits
Crinoid Arm
Star fish (sea star)
•
•
•
•
Carnivores
motile
move on tube feet
endoskeleton made of
calcareous plates (ie.
Calcium carbonate)
• breathes through
dermal “skin gills”
Star fish
• The water vascular system’s opening is
called a madreporite. It opens into a
vertical stone canal. The stone canal
empties into a radial canal. The radial canal
then goes out to the arms in radial canals.
The radial canals then feed water to the tube
feet.
Starfish arm
• Each arm contains a digestive gland and
gonads
• The top of the tube feet are called ampulla
Starfish
• The eye of the starfish is at the end of the
arms. (It is often red coloured)
• The anus of the starfish is on the top (aboral
side)
• The mouth piece of the starfish is called
“Aristotle’s Lantern”.
Tube Feet and Mouth
Sunstar
Brittle Stars
• These are perhaps the fastest of the
echinoderms.
• Most of them are filter feeders or detritus
eaters
Basket Star
Brittle Star
Echinoidea
• Lack arms
• eat algae or are
detritus eaters
• usually have spines
• are protected by
pedicellarae
Sea Heart
• Sea Hearts are found
in the mud of muddy
beaches.
Sea Cucumbers
• Detritus eaters
• do not have skeletal
parts
• will eviscerate if they
are scared