All chordates possess three fundamental distinct features they are :
(i) Presence of elongated rod like notochord below the nerve cord and above the alimentary canal.
- It serves as a primitive internal skeleton.
- It may persist throughout life in lancelets and lampreys.
- In adult vertebrates, it may be partially or completely replaced by backbone or vertebral column.
(ii) A dorsal hollow or tubular fluid filled nerve cord lies above the notochord and below the dorsal body wall.
- It serves to integrate and co-ordinate the body functions.
- In higher chordates, the anterior end of the nerve cord gets enlarged to form the brain and the posterior part becomes the spinal cord, protected inside the vertebral column.
(iii) Presence of pharyngeal gill slits or clefts in all chordates at some stage of their life cycle.
- Gill slits or clefts that perforates the walls of pharynx and appears during the development of chordate.
- In aquatic organisms, pharyngeal gill slits are vascular, lamellar and form the gills for respiration.
- In terrestrial organisms - traces of non-functional gill clefts appear during embryonic developmental stages and disappear later.
- Chordates are bilaterally symmetrical, triploblastic, coelomates
- Organization : organ system level.
- They have post anal tail, closed circulatory system with a ventral myogenic heart except in Amphioxus.
