URCEOLARIA-MITRA (VONSEIB) EPIZOIC ON POLYCELIS-TENUIS (IJIMA) - AN SEM STUDY

Dense populations of Urceolaria mitra (Cilophora, Peritrichid) were ob served on the surface of the planarian Polycelis tenuis (Platyhelminth es Turbellaria). Although the epizoite was primarily confined to the a nterior dorsal surface of the planarian, heavily infested individuals had peritrichs attached to the posterior, lateral and ciliated ventral surfaces. Unique profiles of the skeletal ring (aboral disc) in relat ion to the host surface showed that some local erosion of planarian ep ithelium occurs. Outer and inner rows of cilia extended anticlockwise around the peristomal disc. The cilia ran for one and one-eighth turns around the disc and overlapped in the form of a spiral when the proti st was extended and feeding. The cilia appeared to beat in metachronal rhythm and exhibited a wavelength and amplitude of approximately 10 m u m. Regular striations having a periodicity of 1 mu m were observed i n the periostimal membrane. A posterior or basal circlet of locomotor cilia was also observed and freeze fracture revealed an internal arran gement of microvilli, cilia and reticular membranes.