The aquatic habit of members of the Pipidae seems to have resulted in the retention of a number of neotenic features, and these are evident in the inner ear as well as in other systems of organs.
From a comparative study of the morphological details of the inner ear of adults of Hemipipa carvalhoi, Hymenochirus curtipes and Xenopus laevis, juvenile Pipa pipa and larval stages of Hemipipa, Xenopus and Rana fuscigula, it appears that the arrangement in Hemipipa is the least specialized, while the inner ear of Xenopus closely resembles that of amphibious Anura.
Attention is drawn to Witschi's conclusions (1947–1956) that in larval forms of Rana and Xenopus the lungs act as sound receptors, associated with which is the temporary development of certain auxiliary bronchial structures. In the present observations these structures are interpreted otherwise, and no definite conclusions are reached as to their participation in the auditory mechanism.
Variations in the inner ear of the four species of Pipidae studied appear to strengthen opinions regarding the somewhat unsatisfactory classification of the family. Hemipipa and Pipa fall naturally into the subfamily Pipinae, but the marked differences between Hymenochirus and Xenopus, not only in regard to the inner ear, but also in other anatomical features, suggest that these two genera should be relegated to separate subfamilies.