CLASS AVES (The Birds)

CLASS AVES (The Birds)
  1. This class includes feather-clad, air-breathing, warm-blooded, oviparous, bipedal flying vertebrates Body is more or less spindle-shaped and divisible into four distinct regions; head, neck, trunk and tail. Jaw bones prolonged into a toothless beak or bill.
  2. Neck is long and flexible. Tail is short and stumpy.
  3. Limbs are two pairs.
  4. Forelimbs are modified as wings for fiying Hind limbs or legs are large, and variously adapted for walking running,scratching, perching, food capturing, swimming or wading, etc.
  5. Each foot usually bears four clawed toes, of which the first or hallux is directed backwards.
  6. Exoskeleton is epidermal and horny skin is dry and devoid of glands except the oil or preen gland at the root of tail Pectoral muscles of flight are well developed
  7. Endoskeleton fully ossified, light but strong and without epiphytes.
  8. Long bones pneumatic or hollow and have no marrow Skull smooth and monocondylic, bearing a single occipital condyle. Cranium large and dome-like. Sutures indistinct.
  9. Vertebral column short. A synsacrum results by fusion of posterior thoracic, lumbar, sacral and anterior caudal vertebrae.
  10. Tailvertebrae few, compressed laterally and the last 3 or 4 fused into a ploughshare bone, pygostyle.
  11.  Sternum large, usualy with a vertical, mid-ventral keel for attachment of large flight muscles.
  12.  Both clavices and single interclavicle fused to form a V-shaped bone,called furcula or wishbone.
  13. Heart completely 4-chambered. There is neither sinus venosus nor truncus arteriosus. Only right aortic (systemic) arch persists in adult.
  14. Renal portal system vestigial. Red blood corpuscles nucleated Respiration by compact, spongy, non-distensible lungs continuous with thin-walled air-sacs.
  15. Alimentary canal contains crop (modified oesophagus) for storing and softening of food and gizzard (grinding part of stomach)
  16. for crushing it. Alimentary canal leads into a cloaca Kidneys metanephric and 3-lobed.
  17. Ureters open into cloaca. Urinary bladder absent. Birds are uricotelic, Excretory substance of urates eliminated with faeces.
  18. Brain large but smooth. Cerebrum, cerebellum and optic lobe Fertilisation internal, preceded by copulation and courtship.
  19. Females oviparous. Egg large with much yolk and hard calcareous shells greatly developed.
  20. Cranial nerves 12 pairs.
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