- 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:
- 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.
- 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.
- 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
- 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.
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.