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Starfish
Sea Urchin
Sand Dollar
Title: Jun 4­3:41 PM (1 of 8)
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~7000 species
Ex. Starfish (sea star), sea urchin, sand dollar
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• relatively large
• marine ~ live mainly on ocean floor
• larvae have bilateral symmetry • adults have radial symmetry
• well developed coelom
• 3 germ layers
• fertilization is external
• digestion is extracellular
Title: Jun 4­2:50 PM (2 of 8)
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• 1st group to have true endoskeleton (skeleton covered with epidermis)
• internal skeleton (endoskeleton) ­provides protection and support
• hard calcified plates embedded in body wall with spiny projections
• spiny projections ~ where the phylum gets its name ­ "spiny skinned"
• unique pattern of development of egg ~ like chordates
• first opening becomes anus like in us (all others before this was mouth)
Title: Jun 5­3:16 PM (3 of 8)
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• Body is made of a central disk, arms or rays radiate outwards from it
• Has 5­20 arms
• Starfish have water vascular system that help in movement + feeding
Water Vascular System
1) Water enters opening on the dorsal surface ~ the sieve plate
2) Water then enters system of canals (ring, radial) that extend into each arm
3) Tube feet are connected to canals in arms and extend out ventral side
Title: Jun 4­3:34 PM (4 of 8)
4) Each tube foot has a bulblike structure at one end and a sucker at the other
5) Bulb part is called an ampulla (Plural ampullae)
6) System works like a hydraulic pump
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Starfish use their water vascular system to pry open their prey (ex. clams, oysters)
Food Getting:
• arms wrap around prey and attatch tube feet to shells (prey)
• eventually, the mollusk will tire and the shell will open
Digestion:
• Starfish extends its stomach into mollusk while secreting enzymes to digest the food
• Food is taken in to the stomach and stomach is pulled back into the body
• Food is then passed into digestive glands in arms where digestion is completed
Title: Jun 4­3:39 PM (5 of 8)
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Happens by diffusion across skin gills
Skin Gills ~ small projections extending from body surface
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• Wastes are eliminated by diffusion through the body surface
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• water vascular system + 100's of tube feet coordinating with one another allow the starfish to move along surfaces
Title: Jun 4­8:47 PM (6 of 8)
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• Sexes are separate
• Gametes leave through the openings in the central disk
• Fertilization is external ~ takes place in the water
• Egg ~ bilateral, free­swimming larva
• Becoming a starfish: larva attaches itself to a solid surface (ex. rock) and develops into a starfish
Regeneration: • this is possible under certain conditions
In order for regeneration to happen, a single arm must be present with a tiny part of the central disk.
People used to try and kill starfish by cutting them up. What a hard way to find out that they can regenerate! People made the starfish even more plentiful!
Title: Jun 4­8:57 PM (7 of 8)
Title: Jun 5­3:11 PM (8 of 8)