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echinoderms n.
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Echinoderms

Echinoderms

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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