Phylum Echinodermata
Biology 11
General characteristics
 Spiny
 Radial symmetry
 Water vascular system
 Endoskeleton
Endoskeleton
 Hard, spiny, or bumpy endoskeleton covered with a thin epidermis.
 Endoskeleton composed of calcium carbonate (limestone).
 Some spines are modified into pedicellariae for protection or self‐cleaning.
 Can be either rigid (as when we try to pull one off of a rock) or very flexible (when righting itself), since endoskeleton is made of plates joined by muscles which can contract or relax.
Radial Symmetry
 Enables sessile & slow‐moving animals to sense potential food, predators, and other aspects of their environment from all directions
Echinoderm Diversity
 1. Class Asteroidea:
 Ex. Sea stars
 Most species have 5 or more arms
 2. Class Ophiuroidea:
 Ex. Brittle Star & basket star
 Compact size; move quickly
 Arms used for motility
Echinoderm Diversity
 3. Class Crinoidea:
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Ex. Sea lilies & feather star
Few living species remain
Sea lilies are attached to the ocean floor
Feather stars begin attached but break away and swim freely
 4. Class Echinoidea:
 Ex. Sea urchin & sand dollar
 Bodies shaped like spheres or disks; no arms
 Solid, internal shell
 Special feeding structures called Aristotle’s lantern: made up of five sharp, triangular tooth‐likes structures surrounding the mouth
Echinoderm Diversity
 5. Class Holothuroidea:
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Ex. Sea cucumber
No arms; elongated body
Bilaterally segmented
Lie and move on one side only
Sea Stars (Class Asteroidea)
Sea Star
General Characteristics
 Radial symmetry
 Have a dorsal and ventral side, but no anterior and posterior ends
 Five part body plan
 NO cephalization (i.e. no brain)
Sea Star
Movement and Support
 Movement carried out by WATER VASCULAR SYSTEM
 WVS: A network of fluid‐filled tubes that enable the sea star to move
 Along the underside of each of the sea star’s arms are many pairs of TUBE FEET
 Tube feet: external structures of the water vascular system
 On top of each tube foot is an AMPULLA
 Ampulla: muscular sac that helps force water through the WVS
 Muscles of the ampullae contract, pushing water into the tube foot causing it to extend
 When the foot attaches to a surface, it contracts, pushing the water back into the ampullae and pulling the sea star forward
Sea Star
Water Vascular System (WVS)
 System enables them to move, exchange gases, capture food, and excrete wastes
 Water enters the madreporite  ring canal 
long radial canals  hollow tube foot
Sea Star
Digestion
 All echinoderms have a mouth, stomach and intestines.
 Sea stars are carnivorous
 Feed on mollusks by extruding their stomach through their mouth onto the food.  Tube feet aid in prying apart shells of mollusks.  Powerful enzymes are secreted by a pair of digestive glands in each arm
 The stomach then absorbs the liquid food. (No intestine needed). Sea Star
Circulation of Nutrients
 There is a heart‐like sac under the madreporite that pushes fluid nearly identical to seawater through the coelem, bathing all organs
 The fluid is kept in motion by beating cilia.
 Skin gills and tube feet bring in oxygen to this fluid.
Sea Star
Excretion
 Wastes are flushed from body through the tube feet or skin gills.
 A sea star also has an anus, located in the center of the dorsal side.
Sea Star
Nervous System
 No brain or even fused ganglia, only a central nerve ring that surrounds the mouth
 Nerves extend from the nerve ring down each ray (below radial canals) into a nerve net that is sensory and coordinates movement
 No sensory organs, only cells that detect light & touch
 Have eyespots at the end of each arm that allow them to detect light, and chemical receptors on their tube feet
Sea Star
Reproduction
 Sea stars are separately sexed.
 Two ovaries or testes lie in the corners of each arm and open directly to the exterior
 Spawning by one can result in those nearby to also spawn
Phylum Echinodermata Includes…
 Sea Stars
Phylum Echinodermata Includes…
 Brittle Stars
 Fragile: rays will break off when picked up.
Phylum Echinodermata Includes…
 Sea Urchins
and Sand Dollars
 Covered in spines.
 No rays
Phylum Echinodermata Includes…
 Sea Cucumbers
 Leathery covering allows flexibility.
 When threatened, may expel tubes from anus or release internal organs to confuse predator.
Phylum Echinodermata Includes…
 Sea lilies and Feather stars
 Feathery arms allow it to swim.
Phylum Echinodermata Includes…
 Sea daisies
 Tube feet located around edge of disk.
 Less than 1 cm in diameter
 Discovered in 1986