Invert Chordates

• Chordates


  Deuterostomes, the blastopore becomes the anus
• Amphioxus


  • Invertebrate chordate
  • Evolutionary step between invertebrates & vertebrates
  • No head = absence of special sense organs
• 5 Distinct Chordate Characteristics

  • Notochord: stiff fluid of tissue running length of body
  • Dorsal Tubular Nerve Cord
  • Pharyngeal Pouches/Gill Slits
  • Post-anal Tail
  • Endostyle (ancestral) & Thyroid Gland (derived)
• Similarities Between Chordates & Nonchordates

  • Bilateral symmetry
  • Anterior-posterior axis
  • Coelom
  • Metamerism
  • Cephalization
• Similarities Between Chordates & Deuterostomes

  • Radial cleavage
  • Anus from blastopore
  • Mouth from secondary origin
  • Organ system development
• Urochordata (sea squirts, tunicates)

  • Mobile larvae, sessile adult
  • Mantle = tunic/outer covering
  • Incurrent/excurrent siphon & ciliary tract keeps food off gills
  • Solitary or compound, will share tunic but have own incurrent/excurrent siphon
  • Adult has notochord & endostyle, larvae has all 5 chordate characteristics
• Urochordata Circulatory

  • Ventral heart drives blood one direction, few beats, reverse direction
• Urochordata Respiratory

  • Body wall diffusion
• Urochordata Nervous

  • 1 nerve ganglion — nerve plexus
  • Not well developed sensory organs
• Urochordata Reproduction

  • Monoecious, external fertilization
  • Free swimming larvae - attachment - metamorphose - tunicate
• Cephalochordata (lancelets)

  • All 5 chordate characteristics
  • Atriopore = excurrent siphon
• Cephalochordata Nutrition

  • Ciliary tracts in gut to move food to hepatic cecum (intracellular digestion, homologous to pancreas)
• Cephalochordata Circulatory

  • Closed, no heart
  • Ventral aorta = pumping organ
  • No respiratory pigments
• Cephalochordata Respiratory

  • Body wall diffusion
• Cephalochordata Nervous

  • Ocellus
  • Anterior portion of NC is enlarged (homologous to brain)
• Cephalochordata Reproduction

  • Dioecious, external fertilization
  • Gametes leave through atriopore
• Musculoskeletal Modification

  • Endoskeleton of cartilage or bone
  • V muscles — W muscles (myomeres for 3 sites of attachment)
  • Neural spines = attachment for segmented muscles
• Bone/Protective Structures

  • Homeostasis for mineral storage, deposition site
  • Keratinized structures
  • Ancestral fish have bony scales
• Digestion: more organs for a more varied diet
• Excretion: paired kidneys for waste
• Nervous

  • Neural crest = ectodermal cells along neural tube in embryo secrete cranium, skeletal, sensory organs, & pharynx
  • Ectodermal placodes = ectodermal thickenings that give rise to olfactory epithelium, lens of the eye, & inner ear epithelium
• Ancestral Vertebrates

  • Ostracoderm = jawless fish-like vertebrate from Paleozoic with organ system development
  • Pikia: jawless fish-like vertebrate, ribbon shaped, notochord like amphioxus but has myomeres
  • Haikouella lanceolata: Small, fish-like, notochord, pharynx, dorsal nerve cord, no cranium
• Gnathostomes

  • Jaws arose once, homologous to first 2 gill arches in fish
  • Have 4 copies of all hox genes (amphioxus has 1 copy)
• Ammocete


  • Larval lampreys
  • Ancestral vertebrate body
• 1. Notochord: stiff fluid of tissue running length of body 
  2. above gut, below nerve cord
  3. body support & site of muscle attachment
  4. first part of endoskeleton present in embryo 
• 1. Dorsal Tubular Nerve Cord 
  2. enfolding of ectodermal cells in embryo 
  3. INVERTS = ventral to digestive tract
  4. VERTS = dorsal to digestive tract, will become spinal cord 
  5. dorsal to notochord 
•  

  1. Pharyngeal Pouches/Gill Slits 
  2. Inpocketing of ectoderm & endoderm 
  3. TETRAPODS = give rise to Eustacian tube, middle ear cavity, tonsils, parathyroid 
  4. high blood flow for feeding & repsiration 
• 1. Endostyle (ancestral) & Thyroid Gland (derived)
  2. Endostyle = mucus secreting to trap food 
  3. Homologous to hormone secreting thyroid gland