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Echinoderms
CHAPTER 14
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Phylum Echinodermata: Diversity and Characteristics
Characteristics
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All members have a calcareous skeleton
Spiny endoskeleton consists of plates
Unique water-vascular system
radial symmetry in adults
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Phylum Echinodermata: Diversity and Characteristics
Diversity
• Likely descended from bilateral ancestors
– Larvae are bilateral
• Perhaps evolved radiality as an adaptation to sessile
existence
• Body plan is derived from crinoid-like ancestors
transformed into free-moving descendants
• Lack ability to osmoregulate
– Restricts them to marine environments
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Phylum Echinodermata: Diversity and Characteristics
• None are parasitic
• Asteroids or sea stars
– Mostly predators
• Ophiuroids or brittle stars
– Move by bending their jointed muscular arms
– May be scavengers, browsers, or commensal
• Holothurians or sea cucumbers
– Mostly suspension or deposit feeders
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Phylum Echinodermata: Diversity and Characteristics
• Echinoids or sea urchins
– Found on hard bottoms while sand dollars prefer sand
substrate
– Feed on detritus
• Crinoids
– Sessile and flower-like as young and detach as adults
– Suspension feeders
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Phylum Echinodermata: Diversity and Characteristics
Ecology, Economics, and Research
• Due to spiny structure, echinoderms are not often
preyed upon
• A few fish and otters are adapted to feed on sea
urchins
• Sea stars feed on molluscs, crustaceans, and other
invertebrates
– May damage oyster beds
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Class Asteroidea
Form and Function
• External Features
– Have a central disc with tapering arms extending outward
– Body is flattened and flexible, with a pigmented and
ciliated epidermis
– Mouth is on the underside or oral surface
– Usually there are 5 arms but there may be more
• rows of tube feet
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Class Asteroidea
• Water-Vascular System
– This system is another coelomic compartment and is
unique to echinoderms
– Functions in locomotion, food-gathering, respiration, and
excretion
– Opens to outside at madreporite on aboral side
– Madreporite leads to stone canal, which joins ring
(circular) canal that encircles the mouth
– Radial canals diverge from ring canal and extend into each
ray
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Class Asteroidea
• Feeding and Digestive System
– Mouth leads through a short esophagus to large central
stomach
– Lower part of stomach can be everted through the mouth
during feeding
– Anus is inconspicuous
– Consume a wide range of food
• Sea urchins
• Molluscs
– Sea stars pull valves apart and evert stomach through crack
• Small particles carried by tube feet to mouth
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Class Asteroidea
• Reproductive System, Regeneration, and Autonomy
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Sexes separate in most sexes
Pair of gonads
Fertilization is external
Eggs and sperm are shed into the water in early summer
Regenerate lost parts
• Cast off injured arms and regenerate new ones
• An arm can regenerate a new sea star if at least onefifth of central disc is present
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Class Asteroidea
Sea Daisies
• Diversity and Characteristics
– Small, disc-shaped animals discovered in deep water off
New Zealand
– Described in 1986, only two species are known
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Class Ophiuroidea
Form and Function
• Largest in number of species
– Over 2000 extant species
• Arms of brittle stars are slender and distinct from the
central disc
• Tube feet lack suckers
• Biology
– Brittle stars are secretive and live on hard or sandy bottoms
where little light penetrates
• Often under rocks or in kelp holdfasts
– Browse on food or suspension feed
– Basket stars perch on corals and extend branched arms to
capture plankton
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Class Echinoidea
• Diversity
– Approximately 950 species of living echinoids
– Sea urchins lack arms but their tests show five-part
symmetry
– Most sea urchins have radial symmetry and long spines
– Sand dollars and heart urchins (irregular echinoids) have
become bilateral with short spines
– Echinoids occur from intertidal regions to deep ocean
• Reproduction
– Sexes separate
– Gametes are shed into sea for external fertilization
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Class Holothuroidea
• Diversity
– Approximately 1150 species of holothuroids
– As common name suggests, these animals resemble
cucumbers
– Greatly elongated in the oral-aboral axis
– Ossicles are greatly reduced and body is soft
– Some species crawl on the ocean bottom, others are found
under rocks or burrow
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•
Class
Holothuroidea
Reproduction
– Sexes are separate in most
– Fertilization is external
– A few brood their young inside the body or on the body surface
• Biology
– Sea cucumbers use ventral tube feet and muscular body waves to
move
– Some trap particles on the mucus of tentacles, ingesting food particles
in pharynx
– Others graze sea bottom with tentacles
– Cast out part of viscera when irritated
• Must regenerate these tissues
• Organs of Cuvier are expelled in direction of an enemy
– Sticky and have toxins
– One small fish, Carapus, uses the cloaca and respiratory tree of a sea
cucumber for shelter
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Class Crinoidea
• Diversity
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Crinoids include both sea lilies and feather stars
Far more numerous in fossil record
Unique in being attached for most of their life
Sea lilies have a flower-shaped body at tip of a stalk
Feather stars have long, many-branched arms
• Adults are free-moving but may be sessile
– Many crinoids are deep-water species
– Feather stars are found in more shallow water
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