Kingdom Animalia | Phylum Chordata | Part 1 | Introduction - in English - for Class 11 | NEET
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Phylum: Chordata(G. Chorda —string)
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Chordata is the largest phylum with familiar group of animals, such as fishes,
amphibians, reptiles, birds and mammals.
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And less known animals, such as lancelets (Amphioxus) and tunicates
(Ascidian).
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All chordates possess three fundamental distinct features at some stage
of their life cycle, they are:
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1. Presence of elongated rod like notochord above the alimentary canal and
below the nerve cord.
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It serves as a primitive internal skeleton.
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It may persist throughout life in lancelets and lampreys.
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In adult vertebrates, notochord may be partially or completely replaced
by backbone or vertebral column.
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2. A dorsal hollow or tubular nerve cord lies below the dorsal body wall
and above the notochord.
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Nerve cord serves to integrate and co-ordinate the body functions.
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In higher chordates, the anterior end of the nerve cord becomes the brain
and the posterior part becomes the spinal cord, which is protected inside the
vertebral column.
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3. In chordates, presence of pharyngeal gill slits or clefts at any stage of their lifecycle.
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Pharyngeal gill slits - A series of gill slits or clefts that perforates
the walls of pharynx and appears during the development of every chordate.
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In aquatic forms, pharyngeal gill slits are lamellar, vascular and form
the gills for respiration.
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In terrestrial chordates, traces of non-functional gill clefts appear
during embryonic developmental stages and it will disappear or disintegrate later.
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Besides the above said features, Chordates are bilaterally symmetrical,
triploblastic (three germ layers), coelomates, organ system level of organization
seen.
• They possess post anal tail, Closed circulatory system - a ventral myogenic heart except in Amphioxus (lancelet).
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