By Sangeeth George and Gianni Aranoff

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Hemichordates

• By Sangeeth George and Gianni Aranoff

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General Characteristics

• Phylum hemichordata
• Hemi half
• Marine deuterostomes
• Animals that live buried in sand or mud. (Acorn
  worms)
• Molecular data suggests hemichoradates are more
  closely related to echinoderms than chordates.
  (similar larvae)
• 2 main classes
• Gas exchange occurs between blood and environment
  through gill structures. 

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Pictures

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Reproduction

• Separate sexes
• The sperm and egg are released into the water
  where fertilization takes place.
• In most species, the fertilized egg will then
  develop into larvae which settle to the bottom
  and mature into adults.
• Some species reproduce asexually.
• some acorn worms - fragmentation from trunk
• some pterobranch colonies - budding
• Quick note on response to stimuli
• body surface has a primitive receptor system w/
  scattered sensory cells
• no defined center for response to stimuli 

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Feeding Practices

• Deposit Feed consume sediment and digest the
  organic matter
• Suspension Feed collect suspended particles from
  the water.
• Food particlesare trapped on theprobiscus and
  sentto the mouth by cilia.

Hawaiian acorn worm
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Anatomy/Body

• Bilateral
• Ring of cilia around the mouth.
• Branchial opening
• Stomochord
• Dorsal nerve chord.
• Ventral nerve chord
• 3 Part body( tripartite) proboscis, collar, and
  trunk. (Coelomic cavity in each)

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Classes

• Enteropneusta (acorn worms)
• Tripartite body
• Multiple branch openings
• Slow burrowers using proboscis to burrow through
  sediment and may deposit feed or suspension feed

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More Classes

• Pterobranchia
• Only 20 living species
• form colonies in which the individuals are
  interconnected by stems, or stolons.
• Only one branchial opening
•  Almost all pterobranch species create and live
  within a network of tubes, the coenecium. These
  tubes are made up of the protein collagen,
  secreted by special glands in the proboscis.