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
Namita Das
Namita Das

Namita Das is a development professional with considerable research experience in public health nutrition and teaching experience in biology and nutrition. To discuss her current profession, she has been working as a consultant in nutrition and health at the Better World Foundation in Jharkhand, India. She has research experience on tribal traditional food, nutrition, malnutrition, and health from the Cultural Research Institute, Backward Classes Welfare, and Tribal Development Department, Government of West Bengal. She has completed her graduation in Nutrition Honours with zoology and physiology from Belda College and post-graduation in Nutrition and Dietetics with a specialization in Medical Nutrition Therapy from Vidyasagar University, West Bengal, India.

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