The lateral-line and inner-ear afferents in larval and adult urodeles.

The inner-ear and lateral-line afferents were studied in members of almost all urodele families and in two primitive representatives of anurans and gymnophionans by means of transganglionically transported horseradish peroxidase. The lateral-line projection patterns are, when present, identical in all urodeles and gymnophionans. This is in agreement with the presence of ampullary organs in all urodeles and gymnophionans which possess a lateral-line neuromast system. In contrast, even the most primitive anurans lack both ampullary organs and the dorsal projection of afferents from these organs. In urodeles the inner-ear afferents are found to enter the rhombencephalon via the octaval nerve and bifurcate in the neuropil lateral to the nucleus magnocellularis into a short ascending and a longer descending subpial fascicle. Fibers of the ascending fascicle reach the tip of the lateral recess and terminate in the eminentia granularis. Collaterals are confined to the ipsilateral lobulus lateralis and end presumably as mossy fibers. The descending fascicle ends at the obex level with only a few fibers reaching the second spinal segment. Besides extensive fiber supply to the cells of the ipsilateral ventral-zone column, collaterals are found to reach the reticular formation, nucleus cerebelli, nucleus fasciculus solitarius, intermediate nucleus and several motor nuclei. Differences in the octaval projection among urodele families are limited to variation in its size relative to that of the lateral-line projection. Only species that develop without free-living larvae differ markedly with respect to the absence of lateral-line projections and, presumably, the lateral-line nuclei of the alar plate that exists in larvae. Almost all urodeles that possess a lateral-line system as larvae retain a complete lateral-line system, including neuromasts and ampullary organs, after metamorphosis. Only Salamandra and Chioglossa lose most of their lateral-line afferents and presumably all lateral-line organs around metamorphosis. The octavolateralis projections in urodeles are found to be strikingly similar to those in lampreys, sharks, sturgeons, and, especially, gymnophionans. This points to a great conservatism of this pattern, at least among anamniotic vertebrates. However, even primitive anurans such as Ascaphus differ markedly from these patterns in that they show no ampullary organs or dorsal lateral-line projection but a dorsal projection of the inner ear. Outgroup comparison with other anamniotic vertebrates indicates that the pattern in anurans is derived from the more generalized lateral-line and inner-ear projection pattern as represented in many urodeles and gymno