Classical and modern studies of the labyrinth have established that the inner ear exhibits orderly sequences of changes that can be correlated with the phylogenesis of the taxa in which they occur; such sequences are evident at histological and cytological levels, as well as morphological ones. Conditions in living fishes and fishlike animals suggest that a primary evolutionary thrust, involving establishment of an effective equilibratory system, must have been essentially complete at the time of origin of the bony fishes. Concurrently with morphological changes in the labyrinth, the neuroepithelium of the vestibular receptors established patterns of polarization similar to those in higher vertebrates, but it apparently remained conservative in its architecture and patterns of innervation. Except for the ampullary crests, these receptors may have additionally been capable of responding to vibratory stimuli, but they are known to show auditory specializations only in cases where effective mechanisms for mechanical transmission have developed (e.g., clupeids, ostariophysians). It may reasonably follow that marked labyrinthine changes accompanied establishment of a columellar apparatus in early amphibians; these undoubtedly included differentiation of periotic channels and a primitive basilar papilla. Modern amphibians exhibit variations of a divergent set of labyrinthine specializations. The amphibian papilla, possibly partially homologous to the papilla neglecta, tends to be the more prominent of the two auditory receptors present; the basilar papilla, although represented in all orders, is significantly developed only in anurans. Rather elaborate morphological specializations are known to be associated with both receptors but, aside from absence of efferent innervation in the anuran basilar papilla, no major fine structural modifications have been reported to occur in the neuroepithelium of either.The combination of structures requisite to direct derivation of the avian and mammalian inner ear probably appeared quite early in reptilian stem stock. In living reptiles, the cochlear duct and its parts show several lines of modification; of particular interest are histological and cytological changes in the basilar papilla.Squamate forms (amphisbaenians, snakes, lizards) show varying stages of specilization in the supporting cells, in the morphology, polarization and innervation of hair cells, and in the tectorial membrane. Extreme changes are evident in most crocodilian sensory and supporting cells, but some neuroepithelial elements demonstrate transitional modifications. Cytological specializations closely related to those in crocodilians are evident in the avian basilar papilla. It is of interest that several of the types of change noted are similar to those which would be required to derive a mammalian organ of Corti from a primitive neuroepithelium.

Keywords:
Labyrinth, Vertebrates, Evolution, Ear, inner, Hair cells
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© 1974 S. Karger AG, Basel
1974
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