Question

The body plan of the tetrapods is based on four appendages. What are the varied forms that these four appendages take, and how are they used?

Explain how amphibians represent a shift from water to land (1) in their own life cycle, and (2) within the evolutionary history of the vertebrates.

Many frogs hibernate in burrows or at the bottom of lakes during the winter. How do you expect the fat bodies in the abdominal cavity to change during hibernation?

Answer 1

Tetrapods are classified into 4 different classes (amphibia, reptilia, aves and mammals), based on various features. Each group has a very unique use for its appendages:

In Amphibia, the appendages are absent during the larval stage as in the case of a tadpole. They develop the appendages as they metamorphosize into their adult form i.e., frog. This group generally has larger hind limb and a short fore limb. There are limbless animals in this group as well (Order Apoda).

In Reptilia, the appendages persist through out their lives (except in case of snakes which lack limbs). aquatic reptiles has webs on the limbs, connecting their toes and fingers which aid in swimming as in the case of turtles. They use all 4 limbs for locomotion.

In Aves, the forelimbs have been modified in to feathered wings which enables them to fly and on land they walk with their hin limbs (biped). In aquatic birds like the penguins, the forelimds are modified into flippers to aid in swimming.

In Mammals, there is a diverse use of the appendages. Mammals have been able to adapt to various modes of life occupying land, water and air.

• Aquatic mammals like whales, dolphins and porpoises, the fore limbs are modified in to flippers and help in swimming whereas the hindlimbs are believed to have been lost during the course of evolution to give way for a more streamlined body to reduce resistence while swimming.
• In the case of terrestrial mammals, most depend on all four limbs for locomotion. This includes the ungulates, elephants and carnivores whose proportional length of forelimbs and hindlimbs are almost equal.
• The primates, whose fore limbs, due to the adaption of opposable thumbs, has the ability to manipulate things using their forelimbs. This has enabled them to adopt an arboreal mode of life. Human beings are complete bipeds and walks erect using the hindlims.
• Flying mammals (bats) has a a stretch of skin that connects their forelimbs and the torso which acts as a structure for gliding and aids in flight.