Search

The reduction in the visual system of the Gymnophiona has been noted by many authors. Adult caecilian amphibians eyes are small, covered by skin and sometimes bone, with retinal cells that contain only rod opsins and co-option of structures usually associated with the visual system used for novel sensory structures and functions. Here, we provide a description of the eye of Rhinatrema bivittatum, the first of any rhinatrematid caecilian, and consider its significance for our understanding of the nature of the eye in the last common ancestor of living caecilians. Characters discovered include zonule fibres, an aqueous humour and the remnants of an attachment that could be a protractor lentis muscle.

The morphology of the tectum mesencephali and the medial pallium is studied in species representing the six families of caecilians (Amphibia: Gymnophiona) in order to determine whether differences in brain morphology are related to function, phylogenetic history, or life history strategies. In general, the caecilian tectum is characterized by simplification in having little to no lamination and few migrated cells. The degree of morphological complexity differs between species and between brain regions. Our data suggest that changes in brain morphology are due to a mosaic of different influences. We did not find a strict correlation between visual system reduction and tectal morphologies. However, phylogenetic effects exist. The greatest degree of morphological complexity is found in members of the Rhinatrematidae, a family that is considered basal to the lineage. Thus, simplification of brain morphology in caecilians must be considered a secondary or derived rather than a primitive feature. Direct development and miniaturization are correlated with the greatest simplification in the tectum mesencephali and medial pallium. There is a relationship between differences in brain morphology and heterochrony in caecilians, as in other amphibians. J. Morphol. 231:11-27, 1997. © 1997 Wiley-Liss, Inc.

Descriptions of the trunk musculature of six species representing sex genera and five families of caecilians reveal considerable variation, which may be useful in future systematic studies. The muscle units of the external muscular sheath (M. dorsalis trunci, M. subvertebralis) of caecilians are homologous with, and closely similar in position to, those of salamanders. The major difference in trunk musculature is the presence in caecilians of an additional muscle layer ventral to the M. subvertebralis. This muscle may be a neomorphic derivative from either the M. subvertebralis or the M. transversus. Unlike burrowing reptiles, which have ball-and-socket intervertebral joints, caecilians have retained the primitive amphicoelous centrum and compensate for stresses associated with burrowing by the presence of intercentral ligaments and interlocking basapophyses and subcentral keels. Association of Uraeotyphlus with the Ichthyophiidae and the validity of the Rhinatrematidae are supported by data from the trunk musculature.