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Phylum Hemichordata

Phylum Hemichordata. Marine worms Share characteristics with echinoderms and chordates Deuterostomes (radial cleavage) Pharyngeal gill slits Some have dorsal, hollow nerve cord. Class Enteropneusta. Acorn worms ~ 70 species Live in burrows in fine sediments. Acorn worms.

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Phylum Hemichordata

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  1. Phylum Hemichordata • Marine worms • Share characteristics with echinoderms and chordates • Deuterostomes (radial cleavage) • Pharyngeal gill slits • Some have dorsal, hollow nerve cord

  2. Class Enteropneusta • Acorn worms ~ 70 species • Live in burrows in fine sediments

  3. Acorn worms • Proboscis for burrowing and feeding

  4. Acorn worms • Mostly deposit feeders

  5. Digestion • Mouth – esophagus – pharynx – intestine - anus

  6. Respiration • Gill slits connected to pharynx • Cilia move water through mouth, out pharynx

  7. Circulation • Closed system with blood and vessels

  8. Nervous system • Similar to echinoderms – no brain • Nerve cords connect all regions

  9. Reproduction • Dioecious = separate sexes • Gonads produce gametes • External fertilization

  10. Tornaria larva • Similar appearance to echinoderm larva • Ciliated bands

  11. Class Pterobranchia • Colonial zooid • Construct rigid tubes • Ciliated tentacles • U-shaped gut • Pharyngeal gill slits

  12. New topic: invertebrate evolution • Original prediction for larva type: • Ancestral condition = external fertilization, non-feeding larvae • More recently evolved = internal fertilization, feeding larvae • Both conditions are scattered across phyla

  13. Evolution across the Central American Isthmus • Formed 2-3 mya • Enough time for evolution?

  14. Different environments? • Western Atlantic • water depth varies among islands, different climate – no upwellings, less El Nino effects, higher evaporation rate, lower productivity

  15. Different environments? • Eastern Pacific • Deeper, upwellings bring nutrients, El Nino effects strong, stronger tidal effects

  16. Different environments? • Western Atlantic: sea grass beds common • Not in EP

  17. Different environments? • Western Atlantic: coral reefs large – hundreds of acres, higher S • Small outcrops in EP, lower S

  18. Different taxa? • Large swimmers can still disperse around continents • Not smaller-bodied taxa

  19. Genetic differences? • Similar mitochondrial DNA differences for all crustracea • Other groups – variation high

  20. Life history differences? • Echinoderm, bivalve eggs smaller in open ocean EP = higher productivity

  21. Speciation? • Many examples of incompatibility, but not distinct species yet

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