General Characteristics of Cephalochordata.

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🐟 General Characteristics of Cephalochordata (Lancelets / Amphioxus)

  • Small, fish-like marine organisms found in shallow tropical and temperate oceans.
  • Adapted for both burrowing and swimming.
  • Lack true head and paired appendages; possess dorsal, ventral, and caudal fins.
  • Body wall includes:
    • One-cell-thick epidermis
    • Underlying dermis, connective tissue, and segmental muscles
    • No exoskeleton

🧠 Key Body Features

  • Notochord: Extends the entire body length from anterior to posterior
  • Enterocoelic coelom, reduced in the pharyngeal region
  • Long alimentary canal with ciliary feeding
  • Gills for respiration
  • Closed circulatory system, lacking heart and respiratory pigments
  • Well-developed hepatic portal system
  • Excretion via paired protonephridia with solenocytes
  • Poorly developed brain, no distinct sensory organs
  • No gonoducts; external fertilization with separate sexes

🔬 Chordate Features in Cephalochordates

  • Dorsal tubular nerve cord
  • Notochord persists throughout life
  • Pharyngeal gill slits
  • Post-anal tail
  • Liver diverticulum and hepatic portal system

🧭 Primitive & Specialized Traits

Primitive Traits Specialized Traits
No heart, kidneys, or paired limbs Elaborated pharynx supported by oral hood and atrium
Lack distinct sense organs Ciliary feeding specialization
Body plan resembles invertebrates (e.g., mollusc-like appearance) Efficient filter feeding system

🧬 Classification

  • Subphylum: Cephalochordata
  • Class: Leptocardia
  • Genera: Branchiostoma (Amphioxus), Asymmetron

🤝 Affinities with Other Groups

Similarities with Urochordata:

  • Presence of pharyngeal gill slits, endostyle, atrium, and ciliary feeding.

Differences:

  • Cephalochordates lack a test and heart; their notochord and nerve cord persist into adulthood.

Invertebrate-Like Features:

  • Flame cells (like helminths), nephridia (like annelids), slug-like body resembling molluscs


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