Amphioxus

• Scientific name of Amphioxus

  Branchiostoma lanceolatum
• Amphioxus subphyllum

  Cephalochordate
• 5 primitive characteristics of vertebrates found in Amphioxus

  notochord

  post-anal tail

  dorsal hollow nerve cord

  pharyngeal slits

  endostyle
• Protostome

  body cavity- schizocoelom

  spiral cleavage
• Protostome

  mouth develops from blastospore

  absence of calcified skeleton
• Deuterostome

  body cavity- enterocoelom

  radial cleavage
• Deuterostome

  mouth develops opposite blastospore

  presence of calcified skeleton
• Notochord


  a slender rod that develops from the mesoderm in all chordate

  a hydrostatic organ

  it lies in dorsal to the coelom but beneath and parallel to the central nervous system
• Pharyngeal slits


  opening (spaces) adjacent to the gills

  participates in feeding and respiratory exchange
• Endostyle


  glandular groove in the floor of the pharynx

  involved in filter feeding
• Dorsal Hollow (Tubular) Nerve Cord


  within the neurocoel

  forerunner of spinal cord
• Post-anal Tail


  extending to the anus
• Amphioxus also known as lanceolet, evolutionary link in the study of chordate evolution
• Identify the pointed structures

  A) notochord

  B) dorsal hollow nerve cord

  C) iliocolon ring

  D) intestine

  E) Postanal tail

  F) anus

  G) hindgut

  H) midgut caecum

  I) endostyle

  J) pharyngeal slits

  K) buccal cirri

  L) oral hood

  M) hatschek's pit

  N) wheel organ

  O) velum

  
• Chordate origin traced through phylogenetic reconstruction.

  challenges and limitations:

  • scant fossil
  • hypothetical relationships and description
• Hypothesis on evolution of chordates:

  1. Auricularian or Garstang's hypothesis
  2. Inversion or Annelid hypothesis
  3. Hemichordate or Enteropneust
• Auricularian or Garstang's Hypothesis


  chordate evolved from echinoderms

  involved retention of pharyngeal slits and notochord in adults

  evidences:

  • similarity in coelum formation
  • bilateral symmetry
• Inversion or Annelid Hypothesis


  protostome-like ancestor

  involved turning of body upside-down

  evidences:

  • similarity in segmentation, brain regionalization, and body plans.
• Hemichordate or Enteropneust


  exaggeration of structures from a hemichordate-like ancestor

  • Hemichordates- thought to link echinoderms and chordates

  evidences:

  • stomochord to notochord
  • pygochord to tubular nerve cord
• Oral hood


  amphioxus' organ that has something to do with mouth
• Oral/ Buccal cirri 


  tentacles found in the oral hood

  acts as a sphincter, muscular tissue/organ that allows what goes inside or outside the body
• Wheel organ


  tentacles inside oral hood for filter feeding
• Velum


  membrane found after the oral hood, also acts as a sphincter
• Velar


  tentacles found in velum
• Hatschek's pit/groove


  secretes mucus
• Pigment spot


  primitive visual sensory structure (ocelli or ocellus in vertebrates)
• Midgut/ hepatic caecum

  presumptive liver of amphioxus
• Atrium

  clear membrane (skin) containing all organs
• Iliocolon ring


  sphincter before intestine
• Atriopore


  exit of sperm/egg for external fertilization
• Anus


  exit of digested food
• Caudal end


  aka post-anal tail
• Myomeres


  lined muscles, compartmentalized

  • myosepta- covering
• Gonad


  reproductive organ that produces egg/sperm

  • ovary- large granules
  • testis- streaky/ finely granulated
• Difference of Amphioxus to vertebrates: Circulatory system

  lacks a heart and is composed of vessels and tissue channel that forms a continuous circuit.

  colorless blood.
• Difference of Amphioxus to vertebrates: Nervous system

  lacks brain (information-processing in notochord)

  vertebrates are the only chordates that have brain as part of the CNS
• Difference of Amphioxus to vertebrates: Bony Skeletal Elements

  no vertebral column and cranium
• Similarities of Amphioxus and vertebrates:

  the 5 primitive chordate characteristics, myomeres or segmented muscles, and gut structure.