Phylum hemichordata
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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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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

  • Proboscis for burrowing and feeding


Acorn worms

  • Mostly deposit feeders


Digestion

  • Mouth – esophagus – pharynx – intestine - anus


Respiration

  • Gill slits connected to pharynx

  • Cilia move water through mouth, out pharynx


Circulation

  • Closed system with blood and vessels


Nervous system

  • Similar to echinoderms – no brain

  • Nerve cords connect all regions


Reproduction

  • Dioecious = separate sexes

  • Gonads produce gametes

  • External fertilization


Tornaria larva

  • Similar appearance to echinoderm larva

  • Ciliated bands


Class Pterobranchia

  • Colonial zooid

  • Construct rigid tubes

  • Ciliated tentacles

  • U-shaped gut

  • Pharyngeal gill slits


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


Evolution across the Central American Isthmus

  • Formed 2-3 mya

  • Enough time for evolution?


Different environments?

  • Western Atlantic

    • water depth varies among islands, different climate – no upwellings, less El Nino effects, higher evaporation rate, lower productivity


Different environments?

  • Eastern Pacific

    • Deeper, upwellings bring nutrients, El Nino effects strong, stronger tidal effects


Different environments?

  • Western Atlantic: sea grass beds common

  • Not in EP


Different environments?

  • Western Atlantic: coral reefs large – hundreds of acres, higher S

  • Small outcrops in EP, lower S


Different taxa?

  • Large swimmers can still disperse around continents

  • Not smaller-bodied taxa


Genetic differences?

  • Similar mitochondrial DNA differences for all crustracea

  • Other groups – variation high


Life history differences?

  • Echinoderm, bivalve eggs smaller in open ocean EP = higher productivity


Speciation?

  • Many examples of incompatibility, but not distinct species yet


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