Presence of Haversian canal is the characteristic feature of mammalian bones. Each Haversian canal contains artery, vein, lymph vessel, nerve and some bone cells. The Haversian canals are interconnected by transverse channels, called, Volkmans canals. Ostracoderma as well as some other groups of fishes have both a cellular and cellular bone. In amphibians and reptiles, bone is often lamellar and cellular with osteocytes present. Pneumatic bones are found in birds i.e., they contain air sacs in place of bone marrow, which help them in flying.