Comparative morphological features of the caecilian inner ear with comments on the evolution of amphibian auditory structures.

Comparative fine structural studies of amphibian auditory structures in urodeles have been extended to include examination of the papilliform end-organs (amphibian, neglecta and basilar) that variably occur in species selected from three families of caecilians (Gymnophiona). The species investigated were Ichthyophis kohtaoensis (Ichthyophiidae), Dermophis mexicanus (Caeciliidae) and Typhlonectes natans (Typhlonectidae). Ichthyophis is the only form to display all three papillae; both Dermophis and Typhlonectes lack a basilar papilla but all three species show both neglecta and amphibiorum. In these forms, the amphibian papilla contained the most sensory cells with ciliary bundles organized into two proximal and distal groups polarized toward a mid-line papillar axis. The papilla neglecta contained slightly fewer sensory cells and ciliary bundles oriented predominantly posteriorly. In Ichthyophis, the basilar papilla contained the lowest sensory cell counts of any papilla. Here, basilar sensory cilia were unidirectionally polarized away from the saccule. All papillae were overlain by an essentially similar, extracellular tectorial body. When compared to auditory end-organs in the urodeles and anurans, similar conditions in caecilians are suggestive of a common ancestry for the basilar and amphibian papillae; features of the amphibiorum indicate further that it may represent part of a "displaced" papilla neglecta.