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Download Acorn worms
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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
