General Characters of Class Aves

General Characters of Class Aves

Birds(AVES) are highly evolved vertebrates. Birds first appeared during Jurassic period of Mesozoic era . TheMesozoic Erais an interval of geological time from about 252 to 66 million years ago. They evolved from Ornithischian dinosaurs of Reptiliangroup. In the cretaceous period the birds became modernized. * Most birds are adapted for aerial mode of life. Hence the body shows many modifications for aerial mode of life * The science which deals with birds is calledornithology. * In the present day world 8590 species of birds are living Aves belongs to Phylum: Chordata Sub phylum: Vertebrate Group: Gnathostomata Class : Aves.

* Birds are warm blooded, bipedal verterates covered by feathers.*Birds body is divisible into head, neck, trunk and tail.* In Birds forelimbs are modified into wings (forelimb bears three cIaw less digits , wings bear feathers. They are useful for flying)* In Birds hind limbs are strong and are useful for perching and walking . They bear 4 toes each* Preen gland or oil gland present at the base tail secretes an oily substance which keeps the feathers waterproof * In Birds Skull is monocondylic single occipital condyle present * In Birds the lower jaw is made by 5 to 6 bones. * The modern birds have no teeth. * In most birdsbones are spongy and pneumatic. They are light in weight and strong. * In most birds posterior caudal vertebrae will united to form apygostyle. It acts as a rudder. During the flying it helps to change the direction of movement. * Sternum is broad with a keel. This keel is useful for the attachment of flight muscles * The clavicle and inter clavicle unite to formV shapedfurcula. * In the forelimbs carpals and metacarpals are united to form carpo-metacarpus .In hind limbs tarsal and meta-tarsals, unites to form tarso metatarsus.

* Birds eyes are well developed. They have monocular vision. In the eyecomb plate or pectenis present. It protects the retina from sunlight. It also helps in acute vision. * Birds brain is well developed. Cerebral hemispheres and cerebellum are well developed.12 pairs of cranial nerves are present

* The digestive system is well developed ,gullet is dilated into a crop which stores food. Stomach is divided into glandular proventriculus and muscular gizzard * Anus is absent. Cloaca present. The cloaca is 3 chambered. * In birds Respiratory system is well developed. The lungs are spongy and non distensible they are continued into air sacs. Nine air sacs are present which makes the body light.

* At the base of trachea a syrinx is present. * Heart is four chambered. Right arotic arch is present. * R.B.C. are oval, nucleated and biconcave. * Kidneys are metanephric and three lobed. Ureters open ino the cloaca , urinary bladder is absent. *Birds use the reptilian adaptation of excreting nitrogenous wastes - uric acid *Since uric acid has low solubility, a bird can use far less water to excrete wastes

* Sexes are separate . Sexual dimorphism is clearly exhibited. * In the female left ovary and left oviduct are present * Fertilization is internal * Birds are oviparous. They lay large cleiodic eggs with extra embryonic membranes

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