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
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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