The saccular lung, found in snakes, appears to have no role in breathing. It seems likely that it's a 'vestigial' organ from a time when the ancestors of snakesplugin-autotooltip__plain plugin-autotooltip_bigSnake evolution

Current evidence suggests that snakes started to evolve less than 150 million years ago, but the debate about the evolutionary origins of snakes have not yet been definitively resolved, despite more than 100 years of research.

There are two competing theories. The first suggests that snakes evolved from land-based lizards which burrowed - eventually losing their fore-limbs and then the hind-limbs. had two working lungs. Modern snakes have only one 'working' lung - which is suggested to be an evolutionary trait as a result of their lack of body space.

Some aquatic snakes use the saccular lung to adjust buoyancy whilst swimming, but terrestrial snakes seem to have no obvious use for it.

One current theory is that it might have a function as a 'cantilever' to help maintain body shape. See: Herpetologica Vol. 59, No. 1, pp. 52-57