Chaetotaxy and ultrastructure of sensory receptors in the cercaria of a species of Allassogonoporus Olivier, 1938 (Digenea : Lecithodendriidae)

A standard procedure that combines chaetotaxic, ultrastructural and neuromo rphological observations has recently provided a new perspective to the stu dy of cercarial sensory systems. In the present work, we aimed to extend th e use of this combination of techniques to investigate the chaetotaxy of Al lassogonoporus sp. in conjunction with the ultrastructure of sensory recept ors and neuromorphology. Five nerve regions were distinguished. A conspicuo us bilobed cerebral ganglion was observed at the level of the pharynx. The chaetotaxic pattern was generally consistent with that of other lecithodend riids. Four types of receptors were distinguished with scanning electron mi croscopy. These types differed in cilium length (short, moderately long or long) and tegumentary collar length (moderately low or high). Internal ultr astructure of receptor type IIAL revealed an unsheathed cilium, a closed ba sal body, septate extracellular junctional complexes and thickened nerve co llars. Some receptor types were site-specific. Long uniciliated receptors w ere found mainly on the dorsal surface, whereas short uniciliated receptors were widespread across the tegument. Ultrastructure and site-specificity o bservations suggest that most sensory receptors are mechanoreceptors, proba bly reflecting the important role mechanoreception plays in host finding.