Deuterostomes

The echinoderm-chordate line of evolution

Two major clusters of phyla

• Phylum Echinodermata
• Protochordates - Hemichordata and Chordata

General Characteristics of Deuterostomes

Development

• radial cleavage
• indeterminate development
• enterocoely
• dipleurula-type larvae

Body Plan

• tricoelomate organization
• dorsal nerve cord
• major ventral blood vessels

Deuterostome Evolution

• Sessile arm-feeding echinoderm as ancestral
• Echinoderms branch off early in deuterostome evolution
• Chordate line showed shift from arm-feeding to filter feeding through gill slits
• Within the chordate line:
  • Urochordates remained sessile as adults
  • Cephalochordates and vertebrates evolved from modifications of larval stages

Phylum Hemichordata

• Has chordate body plan:
  • pharyngeal gill slits
  • notochord-like structure supports pharynx (incomplete notochord gives the name "hemichordate")
• < 100 species, all marine
• Two major groups of hemichordates
  • Acorn worms - distinct proboscis, collar, trunk
  • Pterobranchs - "feathery wing" feeding structure; colonial and dwell in connected tubes

Phylum Chordata

• Over 50,000 species, mostly vertebrates
• Three major subphyla:
  • Urochordata
  • Cephalochordata
  • Vertebrata - vertebrae replace notochord
• Urochordata and Cephalochordata are referred to as invertebrate chordates

Characteristics of Chordata

• Notochord at some stage of development
• Dorsal hollow nerve cord
• Pharyngeal gill slits
• Contractile ventral blood vessel
• Postanal tail

Heads or Tails? Subphylum Urochordata

• "Tail chordates"
• Tunicates, ascidians, sea squirts
• About 1250 species, all marine, most sessile
• Characteristics:
  • Tadpole-like larvae, lose notochord and metamorphose into sessile adults
  • Adults have a tunic - a flexible cartilaginous exoskeleton that grows with adult
  • Large pharyngeal basket with numerous slits
  • Closed, U-shaped circulatory system that periodically reverses flow

Heads or Tails? Subphylum Cephalochordata

• "Head chordates"
• Lancelets, 25 marine species
• Eel-like chordates that retain chordate features as adults
  • Tapered at both ends, pharyngeal slits
  • Notochord extends into rostrum over the head ("head chordate")
  • Muscular tissue around connective notochord for flexing in swimming movements
  • Posterior atrial opening for excurrent water flow