Free Response

Explain why frogs are restricted to a moist environment.


A moist environment is required, as frog eggs lack a shell and dehydrate quickly in dry environments.

Describe the differences between the larval and adult stages of frogs.


The larval stage of frogs is the tadpole, which is usually a filter-feeding herbivore. Tadpoles usually have gills, a lateral line system, long-finned tails, and lack limbs. In the adult form, the gills and lateral line system disappear, and four limbs develop. The jaws grow larger, suitable for carnivorous feeding, and the digestive system transforms into the typical short gut of a predator. An eardrum and air-breathing lungs also develop.

Describe how metamorphosis changes the structures involved in gas exchange over the life cycle of animals in the clade Anura, and what evolutionary advantage this change provides.


Frogs (Anura) begin their lives as tadpoles, organisms restricted to an aquatic environment that use gills to breathe. After metamorphosis, most frogs develop lungs and lose their gills, although they will also continue to perform gas exchange through their skin. The lungs of an adult frog allow the animal to move out of the water, and become terrestrial. This limits competition between adults and tadpoles by opening new living space and food sources to the adult.