Class Aves—Birds
Approximately 9,100 species
o This class comprises the birds, which are vertebrates adapted for aerial life.
o The body is divided into head, neck, trunk, and tail.
o Their internal and external structures are modified to suit the process of flight.
o There are several exoskeletal structures, which are the feathers covering the greater part of the body, the horny scales covering the feet, the claws found at the tips of the digits, and the horny beaks covering the bony beaks. All of these structures are epidermal in origin.
o The forelimbs are modified into wings, which are used for flight. They are moved by the chest muscles, which are very well developed.
o They move on land by hind limbs.
o Many trunk and tail vertebrae, as well as some bones of the fore and hind limbs, are fused, giving compound, compact bones more suitable for flight.
o The bones are generally provided with air spaces.
o The two clavicles unite to form a furcula, and the sternum has a ventral keel in flying birds.
o The skull is produced anteriorly into upper and lower beaks carrying no teeth, and posteriorly it carries a single occipital condyle.
o The skin possesses a single gland lying dorsally at the base of the tail and known as the oil gland.
o The alimentary canal includes a crop, which is a wide sac used for temporary storage of food, and a gizzard, which is a thick-walled, muscular part of the stomach used in grinding hard food materials.
o The heart consists of four chambers, which are two auricles and two ventricles.
o There is only one aortic arch, which is the right one.
o There is a special organ of voice, the syrinx, lying at the base of the trachea.
o The lungs are connected with air sacs, which are connected with the spaces inside the bones.
o The urinary bladder is absent, and excretory materials are semisolid.
o In females, there is a single left ovary and left oviduct; the right elements are usually atrophied.
o Fertilization is internal, and the eggs are large, rich in yolk, and enclosed by hard calcareous shells.
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