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Echinoderms

Echinoderms. “spiny skinned” Ex: starfish, sea urchins, sea cucumbers, sand dollars. Starfish body plan. Central disk with radiating arms (usually 5) Pentaradial symmetry (no head, mouth is on underside of disk) Rigid body due to calcareous plates and spines

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Echinoderms

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  1. Echinoderms “spiny skinned” Ex: starfish, sea urchins, sea cucumbers, sand dollars

  2. Starfish body plan • Central disk with radiating arms (usually 5) • Pentaradial symmetry (no head, mouth is on underside of disk) • Rigid body due to calcareous plates and spines • Endoskeleton (embedded in flesh)

  3. LocomotionWater vascular system • Water enters via sieve plate, then drawn down a tube to ring canal (encircles disk) • 5 radial canals arise from ring canal connected to tube feet • Tube feet end in ampulla which contracts, thus forcing water into the tube feet and extending the foot. The foot then contracts and pulls the animal forward

  4. Digestion • Move slowly, prey on clams or oysters • Pulls shell apart by pulling in feet in turn until clam is exhausted and relaxes its muscles • Extends stomach into clam inside out and takes in material • Doesn’t need an intestine because all material is digested by the 5 pairs of digestive glands ( 1 pair per arm)

  5. Respiration • Large coelom between body wall and digestive tract are lined by cilia (these open into skin gills which diffuse oxygen in and carbon dioxide out) • Skin gills are protected by spines and pedicillaria (small pincers)

  6. Excretion • By ameboid cells in the coelomic fluid. Engulf nitrogenous waste then escape through the skin gills

  7. Nervous system • Simple because the organism is slow • Ring nerve circles the mouth and branches into 5 radial nerves • Poorly developed sense organs • Sensory cells (touch) are all over the surface • Eyespots at the tip of each arm

  8. Reproduction • Ovaries or testes lie in each arm • Eggs and sperm are released into sea (broadcast method) • Has a free swimming larva (which has cilia and has bilateral symmetry, not radial like the adults) • Regeneration – one arm can turn into a new animal as long as it has most of the central disk

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