Q.Fish and frog can respire in water. Can they respire through their skin? If yes, why do they have gills?

Ichthyophis is a caecilian, a limbless burrowing amphibian, so it belongs to Class Amphibia.

Ichthyophis has an elongated, snake- or earthworm-like body with no limbs, and a series of ring-like skin folds (annuli) that make it look segmented, like an earthworm. It burrows in moist soil and leads a largely fossorial or semi-aquatic life. Despite its limbless, worm-like appearance, it is not a worm, fish, or reptile: it has moist, glandular skin (not the dry, scaled skin of reptiles), it breathes partly through its skin and lungs (not gills as in fishes), and it lays eggs that hatch into aquatic larvae with gills, …

The presence of gills in the frog's tadpole stage is evidence, per the biogenetic law, that frogs (amphibians) evolved from gill-bearing, fish-like ancestors, even though the adult frog itself is air-breathing.

A frog's life cycle includes an aquatic larval stage (the tadpole), which resembles a fish in many ways — it has a fish-like tail for swimming and respires using external, and later internal, gills, just as fish do throughout their lives. As the tadpole undergoes metamorphosis into the adult frog, the gills are lost and lungs (along with skin respiration) take over as the adult respiratory organs. This transient reappearance of a gill stage during the frog's own individual development (ontogeny) is interpreted, under the biogenetic law proposed by Ernst Haeckel ('ontogeny recapitulates phylogeny'), as a trace of the evolutionary history (phylogeny) of the amphibian lineage — amphibians are believed to have evolved from gill-bearing, aquatic, fish-like ancestors, and the tadpole's gill stage is a developmental echo of that ancestry. It does not mean adult frogs will regain gills in the fu …

Ichthyophis is a limbless, worm-like amphibian belonging to the order Apoda (Gymnophiona/caecilians), unlike Rana, Hyla, and Salamander which all have limbs.

Class Amphibia is divided into three orders based on body form:

• Anura (Salientia): tailless, with well-developed hind limbs for jumping — e.g., Rana (frog), Hyla (tree frog). Both have limbs.
• Urodela (Caudata): possess a tail and well-developed limbs, e.g., Salamander. Has limbs. …