The Latin word Avis means bird. Vertebrate animals that have left water and land to take to the skies are called birds. Their bodies are adapted for flight and they have evolved as flying machines. The physical characteristics of the birds resemble those of archosaurs of the Mesozoic era. The total number of species is 10,000. 21 classes and 706 species of Avis class have been identified in Bangladesh. Dr. Siddiq Publications