الفهرس 
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Abstract
Asciddians(subhylum urochordata or tunicata,class ascidacea)or sea squirts are sessile marine animals ubiquitous throughout the world.these animals have been popular subjects in developmental biology for more than a century.it was reported that this group of fouling animals were known to aristotle(384-322B.C.)in ancient greece who a=named them thalia a name still is use for a class oftunicata.