Ultrastructure and chaetotaxy of sensory receptors in the cercaria of a species of Allopodocotyle Pritchard, 1966 (Digenea : Opecoelidae)

Previous investigations of sensory systems in opecoelid cercariae have focu sed on chaetotaxy and ultrastructure of sensory receptors. They revealed ch aetotaxic patterns within family, genus, and species as well as different r eceptors. Chaetotaxic and ultrastructural observations have rarely been com bined. We investigated the ultrastructure of cercarial sensory receptors in conjunction with chaetotaxy and neuromorphology in a species of allopodoco tyle. Cercariae were treated with acetylthiocholine iodide and silver nitra te, and some were processed for light, scanning (SEM), and transmission (TE M) electron microscopy. Five nerve regions were distinguished. Chaetotaxy t vas consistent with that of other opecoelids. Five types of receptors were distinguished with SEM. Types differed in number of cilium-like structures (one or more), length of cilium-like structure (short, moderately long, or long;), presence or absence of a tegumentary collar, and length of tegument ary collar (low, moderately low, or very high). Internal ultrastructure of some types revealed unsheathed cilium-like structures, basal body, and thic kened nerve collars. Possible subtegumentary and sheathed receptors are int roduced. Some receptor types were site-specific. For example, receptors wit h multiple cilium-like structures were concentrated on cephalic region wher eas receptors with short cilium-like structure were widespread throughout m ost regions. Ultrastructure and site-specificity observations suggest that most receptors are mechanoreceptors.