To determine the correct order of development of structures during the metamorphosis of a toad, we need to understand the life cycle of amphibians, particularly toads. Toads undergo a complex metamorphosis that includes several distinct stages, starting from the egg to the adult form. Correct Answer: B. 15243 Step-by-Step Explanation:

1. External Gills (1):
2. When toad eggs hatch, they develop into tadpoles. In the early stages of tadpole development, they possess external gills. These gills are essential for respiration in the aquatic environment, allowing the tadpole to extract oxygen from the water.
3. Why it comes first: External gills are the first respiratory structures to develop because tadpoles are fully aquatic and rely on these gills for survival in their early life stages.
4. Internal Gills (5):
5. As tadpoles grow, they begin to develop internal gills. These structures are more efficient for gas exchange and are a transitional form as the tadpole prepares for life on land.
6. Why it comes second: Internal gills develop after external gills as the tadpole matures, but before the limbs develop, as they still require a means of respiration while they are still primarily aquatic.
7. Forelimbs (2):
8. The next significant change is the development of forelimbs. This occurs as the tadpole begins to undergo metamorphosis, preparing for its transition to a terrestrial lifestyle.
9. Why it comes third: Forelimbs develop after the gills because the tadpole needs to be able to swim effectively before it can transition to land. The forelimbs are crucial for movement and support once the tadpole starts to leave the water.
10. Hindlimbs (4):
11. Following the development of forelimbs, hindlimbs begin to form. This is a critical stage in metamorphosis, as hindlimbs are essential for jumping and moving on land.
12. Why it comes fourth: Hindlimbs develop after forelimbs because they are part of the final adaptations for terrestrial life. The hindlimbs are typically larger and stronger, allowing for effective locomotion.
13. Mouth (3):
14. Finally, the mouth undergoes significant changes during metamorphosis. The tadpole's mouth is adapted for herbivorous feeding (eating algae), but as it metamorphoses into a toad, the mouth changes to accommodate a carnivorous diet (eating insects).
15. Why it comes last: The mouth's adaptation occurs last because it is closely linked to the dietary needs of the adult toad, which only becomes relevant after the limbs have developed and the tadpole is ready to leave the water.

Why Other Options Are Incorrect:

• Option A (12345): This order suggests that external gills are followed directly by internal gills, forelimbs, hindlimbs, and then the mouth. While it correctly places external gills first, it incorrectly places the mouth last, which is not the correct sequence of development.
• Option C (21345): This option incorrectly places internal gills first, which is not accurate as external gills develop first in tadpoles. The sequence of forelimbs and hindlimbs is also incorrect in this option.
• Option D (53412): This option suggests that hindlimbs develop before forelimbs and that the mouth develops before any limbs, which is not consistent with the biological development of toads. The sequence is entirely out of order.

Revision Summary:

• Toad metamorphosis begins with external gills for aquatic respiration.
• Internal gills develop next as the tadpole matures.
• Forelimbs appear before hindlimbs, aiding in the transition to land.
• Hindlimbs develop last, crucial for movement on land.
• The mouth adapts last to fit the adult diet, completing the metamorphosis.

Understanding the sequence of these developmental stages is crucial for grasping the life cycle of amphibians and their adaptations to different environments.