Sub-phylum Cephalochordata: Definition, Characteristics, Classification, Examples

Cephalochordata (Gr., kephale, head; chorde, cord) is a sub-phylum within the phylum Chordata. The animals in this group are small, marine invertebrates. They are commonly called lancelets or amphioxi.

Cephalochordates show many typical chordate features, including a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail.

Definition of Cephalochordata

The sub-phylum Cephalochordata comprises a group of marine animals that exhibit major chordate features, characterized by a notochord extending from the tip of the tail to the head.

Sup-phylum Cephalochordata example Branchiostoma
Figure: Branchiostoma (Amphioxus) a genus of sup-phylum Cephalochordata

Characteristics of Cephalochordata

  • Cephalochordates are small marine invertebrates, found mainly in shallow water.
  • The body is fish-like, slender, translucent, and tapered at both ends, with a post-anal tail. Due to the shape of the body, they are often called lancelets.
  • Median fins are present but lack paired fins.
  • Epidermis is single-layered.
  • Body muscles are arranged on both sides of the body. These are V-shaped and called myotomes or myomeres.
  • The rod-like notochord extends from the tip of the tail to the region beyond the brain. Hence, this group is named Cephalochordata.
  • The ventral mouth is surrounded by small tentacles.
  • The pharynx is large and comprises numerous gill slits, opening into the atrium.
  • The circulatory system is closed-type and well-developed without a specialized heart. Blood is colourless and lacks respriratoy pigment.
  • Excretion occurs by protonephridia with solenocytes.
  • Presence of a dorsal hollow nerve cord, without brain and ganglia. The nerve cord runs dorsally to the notochord and opens to the exterior through an anterior neuropore.
  • Sexes are separate. Gonads are several pairs and ductless.
  • Fertilization occurs externally.
  • Development is indirect, including a free-swimming, asymmetrical planktotrophic larva.

Examples of Cephalochordata

Branchiostoma (Amphioxus), Asymmetron.

Classification of Cephalochordata

The sub-phylum consists of only a single class Lepitocardii with two genera.

  • Class 1. Leptocardii
  • Order 1. Amphioxiformes
  • Family 1. Branchiostomatidae
    • Genus 1. Branchiostoma
    • Genus 2. Asymmetron

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