In the course of evolution the ancestral lineages of various animals succeeded in the transition from water to land, thereby meeting the various requirements of this step independently. As most aquatic and terrestrial animals mainly rely on their olfactory sense for orientation and foraging as well as intra- and interspecific communication, one of the most important abilities they had to keep was the ability to sense chemicals in their surroundings. It is well known how terrestrial animals such as vertebrates and insects sense and compute environmental odors from a molecular and an information integration perspective. However, to conclude what the ancestral state of olfaction was, it is necessary to compare those findings with species that never left water. In hermit crab species, crustaceans provide the opportunity to study the current state of a primary aquatic nose in direct comparison to the results of 20 million years of terrestrial olfaction. Terrestrial hermit crabs developed a fully functional aerial sense of smell based on a similar set of variant ionotropic receptors like their aquatic relatives. Differences in the antennal transcriptomes of aquatic and terrestrial hermit crabs are rather small, implying that it required small genetic adaptations to transfer the ancestral aquatic organ to air. Crucial support is presumably given by glandular structures which produce and excrete a mucosal substance to keep the aesthetasc surface moist and the microbial colonization at bay.