Phylum CHORDATA (Gr., chorda, a string, or L., chordatus, a chord, i.e., the notochord) is the largest of the deuterostome phyla. It is the highest and the most important phylum, comprising a vast majority of animals living and extinct, such as tunicates, lancelets, lampreys, fishes, amphibians, reptiles, birds, and mammals, including man.
Characters
All chordates are bilaterally symmetrical, with 3 germ layers, a segmented body, complete alimentation, and a well-developed coelom. While groups of chordates differ widely from each other, they, however, possess three outstanding common characters that separate them from non-chordates. These three fundamental common chordate traits are as follows :
1. Notochord (Gr., noton, back; chorde, cord). It is a slender, longitudinal, stiff skeletal rod of connective tissue cells present just dorsal to the digestive tract. It is this structure that gives the phylum its name. It furnishes support to the body and is not to be confused with the nerve cord. It persists throughout life in the lower chordates, such as urochordates, cephalochordates, and cyclostomes. But in the higher chordates (fishes to mammals), it is present only in the embryos and later replaced by the vertebral column, which is made up of different vertebrae.
2. Hollow dorsal nerve cord. A hollow or tubular nerve cord is present, extending lengthwise of the body dorsal to the notochord. It arises as an infolding of the dorsal surface ectoderm of the embryo.
3. Pharyngeal gill-clefts. Paired lateral openings, commonly referred to as gill clefts or gill slits, develop on the sides of the embryonic pharynx, leading to the outside. They persist in all lower chordates and serve for aquatic pharyngeal
respiration. In higher chordates they soon close before hatching or birth as the adults of these animals develop lungs for breathing air.
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