Answer» - Aves are commonly known as birds. The characteristic feature of Aves :
- They have feathers and the ability to fly except for flightless birds (Eg: Ostrich, Kiwi, Penguin).
- The forelimbs are modified into wings, and the hind limbs are adapted for walking, running, swimming and resting.
- The skin is dry and devoid of glands except the oil gland or preen gland.
- The exoskeleton consists : epidermal feathers, scales, claws on legs and horny covering on the beak.
- The endoskeleton is fully ossified (bony) and the long bones are hollow with air cavities (pneumatic bones).
- The pectoral muscles of flight are well developed. (Muscles are pectoralis major and pectoralis minor).
- Respiration is by compact, elastic, spongy lungs that are continuous with air sacs
- The heart is four chambered.
- Aves are homeothermic.
- Migration and parental care is well marked.
- Urinary bladder is absent.
- Sexes are separate with well-marked sexual dimorphism.
- In males, the testes are paired but in females, only the left ovary is well developed while the right ovary is withered.
- All birds are oviparous.
- Eggs are megalecithal and, cleidoic.
- Fertilization is internal. Eg: Corvus (Crow), Columba (Pigeon).
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