TUGAS MATAKULIAH KKH II

Kerjakan tugas terjemahan ini dan ditumpuk hari Jumat tgl.20 Maret 2009.

CHARACTRISTIC.
The chordates are bilaterally symmetrical, with three germ layers, a segmented body, complete digestive tract, and well-developed coelom. Three outstanding characteristics distinguish them from other animals- a single dorsal tubular nerve cord, the notochord, and gill slits in the pharynx.
These features all from in the early embryo of a chordate, and they persist, are altered, or may disappear in the adult.
The notochord is the first supporting structure of the chordate body, in the early embryo it forms above the primitive gut as a slender rod of cells containing a gelatinous matrix and is sheathed in fibrous connective tissu. In tunicates, it is present in the tail and only during the larval stages. In the lancelets and higher forms, it extends almost the length of the body. it persists throughout life as the main axial support in lancelets and lampreys, but in the fishes to mammals is later surrounded or replaced by the vertebral column.
The nerve cord forms on the dorsal surface of the early embryo soon after the gastrula stage. In folding of the ectoderm produces the hollow tubular cord that lies above the notochord. The anterior end becomes enlarged as simple "cerebral vesicle" in tunicate larvae and in lancelets, but in all vertebrates it thickens and differentiates as the brain, to becomes progressively more complex in higher forms. In tunicates the cord and vesicle degenerate to a ganglion at metamorphosis. From the lampreys onward the nerve cord later becomes surrounded ny neural arches of the vertebrae that protect it from injuy, and the brain is enclosed by a brain box, or cranium.
Paired gill pouches develop on the sides of the embryonic pharynx (digestive tract). Each is formed by an aotpocketing of endoderm in the pharynx and a corresponding inpocketing of ectoderm on the outside of the body; the intervening wall breaks through to form a gill slit. The characteristic development is seen in a shark or fish in which each slit is margined bt many slender filaments containing blood vessels, to form a gill. Water containing dissolved oxygen passes into the mouth and pharynx and out over the filaments, where the blood gives up its carbon dioxide acquires oxygen, so that the gill servesthe process of external respiration. All aquatic chordates from tunicates to amphibians respire by gills. In amphibians which transform from aquatic larvae to air-breathing adults, the gills are lost at metamorphosis. The reptiles, birds, and mammals all develop several pairs of gill pouches during early embryonic life, but they are never functional and soon close; all these animals later acquire lungs for breathing air when they hatch or are born.