Phylum Hemichordata Marine worms Share characteristics with echinoderms and chordates Deuterostomes...
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![Page 1: Phylum Hemichordata Marine worms Share characteristics with echinoderms and chordates Deuterostomes (radial cleavage) Pharyngeal gill slits Some have dorsal,](https://reader033.fdocuments.us/reader033/viewer/2022061514/56649c805503460f94937624/html5/thumbnails/1.jpg)
Phylum Hemichordata
• Marine worms
• Share characteristics with echinoderms and chordates
• Deuterostomes (radial cleavage)
• Pharyngeal gill slits
• Some have dorsal, hollow nerve cord
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Class Enteropneusta
• Acorn worms ~ 70 species
• Live in burrows in fine sediments
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Acorn worms
• Proboscis for burrowing and feeding
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Acorn worms
• Mostly deposit feeders
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Digestion
• Mouth – esophagus – pharynx – intestine - anus
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Respiration
• Gill slits connected to pharynx
• Cilia move water through mouth, out pharynx
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Circulation
• Closed system with blood and vessels
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Nervous system
• Similar to echinoderms – no brain
• Nerve cords connect all regions
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Reproduction
• Dioecious = separate sexes
• Gonads produce gametes
• External fertilization
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Tornaria larva
• Similar appearance to echinoderm larva
• Ciliated bands
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Class Pterobranchia
• Colonial zooid
• Construct rigid tubes
• Ciliated tentacles
• U-shaped gut
• Pharyngeal gill slits
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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
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Evolution across the Central American Isthmus
• Formed 2-3 mya
• Enough time for evolution?
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Different environments?
• Western Atlantic• water depth varies among islands, different
climate – no upwellings, less El Nino effects, higher evaporation rate, lower productivity
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Different environments?
• Eastern Pacific– Deeper, upwellings bring nutrients, El Nino
effects strong, stronger tidal effects
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Different environments?
• Western Atlantic: sea grass beds common
• Not in EP
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Different environments?
• Western Atlantic: coral reefs large – hundreds of acres, higher S
• Small outcrops in EP, lower S
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Different taxa?
• Large swimmers can still disperse around continents
• Not smaller-bodied taxa
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Genetic differences?
• Similar mitochondrial DNA differences for all crustracea
• Other groups – variation high
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Life history differences?
• Echinoderm, bivalve eggs smaller in open ocean EP = higher productivity
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Speciation?
• Many examples of incompatibility, but not distinct species yet