Chordata upto Class level

Chordata is the largest and most familiar phylum in the animal kingdom, uniting fishes, amphibians, reptiles, birds and mammals with less familiar groups such as the lancelets (Amphioxus) and the tunicates (Ascidian). What unites this enormous diversity is not outward appearance but three fundamental features, each present at some stage of the life cycle: an elongated rod-like notochord lying below the nerve cord and above the alimentary canal, serving as a primitive internal skeleton; a dorsal, hollow, tubular nerve cord lying above the notochord, which in higher chordates enlarges anteriorly into the brain and continues posteriorly as the spinal cord; and pharyngeal gill slits, a series of openings perforating the pharynx wall that appear at some point during every chordate's development — functional as respiratory gills in aquatic forms, but present only transiently as non-functional clefts in terrestrial chordates. Chordates are additionally bilaterally symmetrical, triploblastic and coelomate with organ-system organisation, and (except in Amphioxus) possess a post-anal tail and a closed circulatory system with a ventral, myogenic heart. These shared features, and how sharply they contrast …