<p>Terrestrialization is one of the major transitions in animal evolution, and adaptation to intertidal habitats is considered an intermediate stage in this process. Gastropods have repeatedly colonized land, typically by achieving resistance to desiccation through a shell and aerial respiration via a lung. Notably, shell-less air-breathing slugs of the genus <i>Peronia</i> (Onchidiidae) inhabit rocky intertidal shores and actively move across exposed substrates during low tide, suggesting adaptations that support terrestrial activity. Here, we examined the post-embryonic development of two traits potentially involved in intertidal adaptation in <i>Peronia</i> sp.: the lung for air breathing and mucous glands associated with desiccation tolerance. We first identified the study species as <i>P. setoensis</i>, using molecular and morphological data and documented its embryogenesis. From histological observations conducted across developmental stages from hatchlings to adults, it was revealed that hatchlings lacked differentiated mucous glands, although an immature lung-like structure extended from the posterior visceral region toward the body surface, suggesting an early pneumostome formation. Subsequently, mucous gland abundance and internal lung ridge complexity increased progressively during growth. Finally, adults possessed both mucous glands densely distributed along the mantle margin and dorsal surface, as well as a well-developed posterior lung with numerous internal ridges. These results indicate that post-embryonic development of lung structure and mucous glands is closely linked to increasing terrestrial activity during periodic aerial exposure in <i>P. setoensis</i>. The stepwise development of lung and mucous glands, which may correspond to the gradual acquisition of air-breathing capacity and desiccation tolerance may reflect a developmental pathway underlying intertidal adaptation.</p>