Nudibranch molluscs of the genus Corambe differ from most other Doridoidea by having ventral rather than dorsal anus and gills. Because of these and other features, such as separate cerebral and pleural ganglia, corambids have been considered as an archaic or enigmatic group. The first tropical eastern Pacific Corambe species is described in morphological and some histological detail. Selected organs such as circulatory and central nervous features are reconstructed from serial semithin histological slides and visualized in three dimensions using Amira software. Anatomical findings include two separate ganglia on the visceral loop and an additional ganglion on the right side of the body that is connected to the pedal ganglion. Corambe mancorensis n. sp. is dorsoventrally depressed, has an oval, fleshy notum covered with a cuticle, and has a wide posterior medial notch that can be closed completely by unique lobules. Gills are arranged in an unusual horseshoe-like manner including both phanerobranch anal (=medial) gills and corambid lateroventral gill rows, and are connected to the atrium by a complex vessel system. The three medial gills arise from a posterodorsal gill cavity within the notal notch, similar to the case in Corambe evelinae Marcus, 1958. By scanning electron microscopy a vestigial gill cavity is also detectable in C. pacifica MacFarland & O'Donoghue, 1929, but here it is situated ventrally. Our new information on adult corambids is compared with new and published ontogenetic data on phanerobranch and cryptobranch dorids, to contribute to a novel interpretation of the ontogeny of dorid mantle and gill complexes. The progenetic evolution of corambids 'recapitulates' early juvenile dorid stages - turning Haeckel's Law upside down.