Page 210 - Biology_F5

Comparative studies of natural groups of organisms

Head
Tail Trunk

Nostril
FOR ONLINE READING ONLY
Short leg Ear Mouth
Toe with claw

Figure 3.70: Generalised structure of a lizard
Class Aves flight. They have large eyes to body ratio;
The class Aves includes all birds. It is an and large eyes give birds good and keen
extremely distinctive and successful class. eyesight important in flight. Birds can
Aves are bipedal feathered and warm- migrate during harsh conditions, since
blooded (homoeothermous) animals, as they have wings that enable them to move
they are able to maintain a constant body fast. Birds are widely spread all over the
temperature. They have unique “one-way” world; some are found in very cold snowy
breathing system. They have light, yet environments and others in dry and hot
strong hollow bones, forming a skeleton environments.
in which many bones are fused or lost and
have powerful flight muscles. Birds have Structure of a pigeon
evolved specific adaptations to enable them The body is spindle shaped and the size
fly. They have fused hollow bones making varies from 20 cm to 25 cm. The body is
birds to have light weight. They also have divided into the head, neck, trunk, and
a large keel for attaching flight muscles tail. Most parts of the body are covered by
and large chest muscles used for flight. feathers. The head is small and rounded
Birds also have their fore limbs modified (Figure 3.71). It is anteriorly pointed into a
for flight and have feathers which are used short beak. On the lateral side of the head,
for flying. They have also developed long there is a pair of prominent eyes. The ear
flight feathers on the wings and tail to help comprises small apertures on the posterior
birds attain balance and steer. Birds have side of the eyes. Each aperture remains
developed air sacs connected to the lungs covered by a special group of feathers called
enabling them to extract more oxygen to auricular feathers. Each aperture leads to a
release more energy to power flight. Also canal called external auditory meatus, which
they have a four chambered heart that is closed below by the tympanic membrane.
enables them get more oxygen and avoid The trunk is the greatest and widest part of
mixing oxygenated and deoxygenated the body. It is boat-shaped and bears a pair
blood. Birds have well developed brain of wings and a pair of legs. The entire foot
enabling them to have quick reaction during is covered with horny epidermal scales. At

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