The Fine Structure of Ciliated Nerve Endings in the Cercaria of Schistosoma mansoni

The fine structure of ciliated nerve endings located in the tegument of Schistosoma mansoni cercariae is described. These include a previously undescribed multiciliate type as well as relatively simpler, uniciliate endings similar to those found in the adult. Comparison is made with similar structures seen in other animal groups. Specialized, uniciliate nerve endings in the adult of Schistosoma mansoni have been described by Morris and Threadgold (1967), Smith et al. (1969), and Silk and Spence (1969). These are generally assumed to have a sensory function. Little is known, however, about the nervous system of the cercaria of this species. Wagner (1961) reported large numbers of papillae distributed over the body and tail; Morris and Threadgold (1967) referred to the presence of a cercarial process similar to that found in the adult. The present study describes three distinct types of ciliated nerve endings in the cercaria, and their possible role as sensory receptors is discussed. MATERIALS AND METHODS Cercariae were obtained from infected snails maintained at this laboratory and fixed within 2 hr of emergence. Initial fixation was carried out at 4 C for 1 hr in cacodylate-buffered 2% glutaraldehyde, and postfixation was in Zetterquist's buffered osmium tetroxide (Glauert, 1965). Cercariae were dehydrated in a series of alcohols from 30 to 100% and finally embedded in an araldite-epon mixture. Sections were cut on an LKB 4800A ultratome, mounted on copper grids coated with formvar and carbon, and stained with uranyl acetate and lead citrate. Material was examined in a Hitachi HS8 electron microscope operating at 50 kv.