Nudibranchs are mostly very colorful, and depending on the species, the color denotes danger (foul tasting) or is cryptic. Having evolved from shelled, torted ancestors, they have completely lost all vestiges of shell, and their bodies have de-torted. At the anterior end is a pair of tentacles (rhinophores), which are chemo-sensory structures, and near the posterior end are the gills (pseudobranchia).