General characteristic features of phylum Chordata

  • Chordates are bilaterally symmetrical, deuterostomial, eucoelomate eumetozoans.
  • A flexible but firm supporting skeletal rod, called notochord is present in the embryo or throughout life.
  • From a purely biological viewpoint, chordates are interesting because they illustrate the broad biological principles of evolution, development and relationship.
  • All the chordates possess four outstanding and unique characteristics at some stage in their life. They are:
  1. Notochord or chorda dorsalis:
  • It is mesodermal in origin and originates from the roof of the embryonic archenteron.
  • Notochord is the prime diagnostic feature of the phylum chordata which derives its name from it.
  • Notochord is an enlarged rod like, flexible structure extending the length of the body.
  • It is situated between the nerve cord and just above the alimentary canal.
  • Structurally, it is composed of large vacuolated nochordial cells containing a gelatinous matrix and surrounded by an outer fibrous and an inner elastic sheath.
  • It is present at some stage in all the chordates. In most vertebrates, it occurs only in embryo, but remnants may persist between vertebrae which obliterate it.
  • In cephalochordates, notochord may persist even in the adults.
  1. Nerve cord or dorsal neural tube:
  • It is longitudinal, hollow or tubular structure lying just above the notochord and extending lengthwise in the body.
  • It is ectodermal in origin and derived from the dorsal neural plate of the embryo and encloses a canal, called neurocoel.
  • In the lower chordates, the nerve cord persists throughout the life.
  • In the higher chordates, it is surrounded or replaced partly or completely by a joined vertebral column.
  • In vertebrates, its anterior region is specialized to form a cerebral vesicle or brain which is enclosed by a protective bony or cartilaginous cranium.
  • The posterior part of the nerve cord becomes the spinal cord and protected within the vertebral column.
  • The central nervous system of the chordates is present in the body.
  1. Pharyngeal gill slits:
  • In all the chordates, a series of paired lateral gill slits perforate through the pharyngeal wall of the gut behind the mouth which are also termed as pharyngeal pouches.
  • In most vertebrates, they are seen only during their embryonic development.
  • In the lower chordates, fish and amphibian larvae, the pharyngeal pouches later on develop into functional gills.
  • In humans, the first pair of pouches becomes Eustachian tubes. The second pair becomes the tonsils, while the third and the fourth pair become the thymus gland and the parathyroid
  1. Post anal muscular tail:
  • The tail is the part of the body behind the cloaca or anal opening.
  • It may be present or reduced or absent in the adults.
                                                                    Schematic representation of a chordate body

Advancement of chordata over other phyla:

Living endoskeleton:

  • Only chordates possess a living endoskeleton.
  • It grows in size with the rest of the body, which permits greater freedom of movement.

Efficient respiration:

  • The gills in aquatic and the lungs in terrestrial chordates form efficient organs of respiration.
  • They provide enormous surface area for gaseous exchange.

Efficient circulation:

  • The circulatory system of chordates is well developed.
  • The blood flows freely in the respiratory organs ensuring rapid exchange of gases.

Central nervous system:

  • A growing tendency of central nervous system and the sensory system is modified accordingly.
  • The advancement of nervous and sensory organs explains the great power of adapting themselves most successfully to a variety of environments.

Major divisions of phylum chordata:

Phylum Chordata is further divided into four sub-phyla;

  • Hemichordata
  • Urochordata
  • Cephalochordata
  • Vertebrata

Among these four subphyla, Hemichordata, Urochordata and Cephalochordata are less developed chordates and collectively called Protochordates.

General characteristic features of phylum Chordata