In this article we will discuss about the diagnostic features of chordata.
Phylum Chordata, the largest of the deuterostome phyla, is a diverse assemblage of marine, fresh water and terrestrial animals. The chordates are triploblastic, bilaterally symmetrical, metameric, coelomate organisms with anteroposterior and dorsoventral axes.
It includes sea squirts, amphioxus, cyclostomes, fishes, amphibians, reptilians, birds and mammals. All possess three unique chordate features a dorsal rod-like notochord, a dorsal tubular nerve cord and pharyngeal clefts or slits through the pharyngeal wall. In many chordates some of these characteristics are found only in developmental stages.
The notochord is a primitive internal skeleton consisting of an elastic rod, covered by one or two sheaths of tough connective tissue acting as a fulcrum for the segmental muscles and extends forward up to the middle of the brain where lies the hypophysis.
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It typically lies beneath the nerve cord and above the alimentary canal and is composed of large vacuolated cells. The gill slits or branchial clefts primitively serve for the passage of water from the pharynx to the outside. In vertebrates several pairs of gill slits are always functional in the adults of fully aquatic classes, and in the larvae of most amphibians.
In many aquatic forms, gills are present within this cleft. In animals equipped with lungs, branchial clefts or grooves are always found in the embryo.
In chordates, there is a single, unpaired, dorsal, hollow, fluid-filled nerve cord, without distinct ganglionic enlargements, but anteriorly differentiated into a brain in advanced forms. The dorsal origin and position of the nerve cord are regarded as related to the original mode of life of ancestral chordates.
The concentration of nerve cells to form a central nervous system takes place in relation to the direction from which the greatest stimulation comes. The dorsal position of nerve cord in chordates is regarded as the evidence that the ancestral chordates were free-swimming pelagic forms, receiving their chief stimulation from the sea surface above them, the phenomenon called neurobiotaxis.
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All vertebrates possess characteristics of chordates. Only two subphyla of chordates— Urochordata and Acrania—lack vertebral column but retains notochord and large pharynx with branchial clefts. All vertebrates are chordates but all chordates are not vertebrates.