In this worm its environment determines form of the mouth, Two choices:-"Depending on the environment in which Pristionchus grows, it develops either a short wide mouth or a long narrow one. The wide-mouthed variant, which has a single, characteristic tooth, is suitable for carrying out predatory attacks. The narrow version, in contrast, is mainly used for grazing on bacterial food sources. The developmental path taken by a Pristionchus larva is not decided by its genes but by the environment. When the animals were starved or when too many worms crowded the Petri dish, the researchers observed the increased development of the wide-mouthed variant."- Read more at: http://phys.org/news/2013-11-worm-mouths-devious-evolutionary-path.html#jCp