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Functional and phylogenetic components in cercarial nervous systems.

Studies involving comparisons of taxa that vary in their degree of relatedness may allow the distinction of functional and phylogenetic components in cercarial sensory systems. In this study, cercariae of allocreadiids Bunodera Railliet, 1896 and Crepidostomum Braun, 1900, lecithodendriid Allassogonoporus Olivier, 1938 and opecoelid Allopodocotyle Pritchard, 1966 were compared as regards ultrastructure and chaetotaxy of sensory receptors as well as neuromorphology. Cercariae were treated with acetylthiocholine iodide and silver nitrate and some were processed for scanning and transmission electron microscopy. The types of cercarial sensory receptors differed in the presence of a tegumentary sheath, a dome-like base and a tegumentary collar, number of cilia (0, 1, 2 or more), cilium length (short, moderately long or long) and tegumentary collar length (low to moderately low, high or very high). Chaetotaxic patterns were consistent at the family level in all taxa studied. Irregular cholinergic nerve networks were identified. The present study indicates that the major categories of cercarial sensory receptors are nonciliated (including sheathed and subtegumentary types) and ciliated (including uncollared and collared types) receptors. It also allows the distinction of functional and phylogenetic components in the sensory systems of the cercariae studied. Functional components were reflected in the numbers of sensory receptors associated with each nerve region and in the ultrastructure and site-specificity of receptor types. Phylogenetic components included taxon-specific chaetotaxic patterns and receptor types.