. Phylum: Chordata (G. Chorda –string ) Chordata is the largest phylum with most familiar group of animals, such as fishes, amphibians, reptiles, birds and mammals and less known forms such as lancelets (Amphioxus) and tunicates (Ascidian). All chordates possess three fundamental distinct features at some stage of their life cycle (Figure .
), they are: Muscle segments Notochord Mouth Pharyngeal slits or clefts Anus Muscular, post-anal tail Dorsal, hollow nerve cord Figure . A Typical Chordate . 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. . 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. . Presence of pharyngeal gill slits or clefts in all chordates at some stage of their lifecycle.
It is a series of gill slits or clefts that perforates the walls of pharynx and appears during the development of every Figure . Example of Hemichordata - Balanoglossus XI Std Biology-Zoology Chapter- chordate. In aquatic forms, pharyngeal gill slits are vascular, lamellar and form the gills for respiration. In terrestrial chordates, traces of non-functional gill clefts appear during embryonic developmental stages and disappear later.
Besides the above said features, chordates are bilaterally symmetrical, triploblastic, coelomates with organ system level of organisation; they possess post anal tail, closed circulatory system with a ventral myogenic heart except in Amphioxus. List the three features common to all chordates at sometime in their life. . .
Subphylum: Urochordata or Tunicata (G. Oura – A tail; L . Chord – cord ) They are exclusively marine and are commonly called sea squirts . Mostly sessile, some pelagic or free swimming, exist as