Ultrastructure and chaetotaxy of sensory receptors in the Cercariae of a species of Crepidostomum Braun, 1900 and Bunodera Railliet, 1896 (Digenea : Allocreadiidae)

Previous investigations of cercarial sensory systems have focused on chaeto taxy and ultrastructure of sensory receptors and have revealed chactotaxic patterns within families, genera. and species as well as different types of sensory receptors, However, chaetotaxic and ultrastructural observations h ave rarely been combined. We investigated the ultrastructure of cercarial s ensory receptors in conjunction with the chaetotaxy and neuromorphology in 2 allocreadiid species belonging to the genera Creepidostomum and Bunodera. Cercariae were treated with acetylthiocholine iodide and silver nitrate, a nd for scanning electron microscopy (SEM) and transmission electron microsc opy (TEM). Similar cholinergic nerve networks were revealed. Chaetotaxy was consistent with that of other allocreadiids. Seven and 6 types of receptor s were distinguished with SEM in Crepidostomium sp. and Bunodera sp.. respe ctively. Types differed in number of cilia (1 or 2), cilium length (short, moderately long, or long). presence or absence of a tegumentary collar and a domelike base, and tegumentary collar length (low. moderately low. or hig h). TEM of some types revealed unsheathed cilia, basal body. and thickened nerve collars. Some receptor types were site specific. Thus. long uniciliat ed receptors were concentrated on the dorsal surface. Other types. such as short uniciliated receptors. were widespread throughout most regions. Ultra structure and site-specificity observations suggest that most receptors are mechanoreceptors.