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Amphibian lungs

The lungs of most frogs and other amphibians are simple balloon-like structures, with gas exchange limited to the outer surface area of the lung. This is not a very efficient arrangement, but amphibians have low metabolic demands and also frequently supplement their oxygen supply by diffusion across the moist outer skin of their bodies. Unlike mammals, which use a breathing system driven by negative pressure, amphibians employ positive pressure. Note that the majority of salamander species are lungless salamanders and conduct respiration through their skin and the tissues lining their mouth.

Invertebrate lungs

Some invertebrates have "lungs" that serve a similar respiratory purpose but are not evolutionarily related to vertebrate lungs. Some arachnids have structures called "book lungs" used for atmospheric gas exchange. The Coconut crab uses structures called branchiostegal lungs to breathe air and indeed will drown in water, hence it breathes on land and holds its breath underwater. The Pulmonata are an order of snails and slugs that have developed "lungs".