THE PROTONEPHRIDIUM OF CATENULIDS REEVALUATED - AN ULTRASTRUCTURAL-STUDY ON STENOSTOMUM-GRANDE CHILD, 1902 (PLATYHELMINTHES, CATENULIDA)

The protonephridium of S. grande consists of a single, unbranched, tub ular element, folded on itself, and differentiated into one ascending and another descending, sectors (or canals). Both run united in parall el along the dorsal midline of the animal; the transition between asce nding/descending sectors corresponds to the anteriormost loop, bent at the level of the cerebral ganglia. The ascending canal is thin-walled , contains tufts of cilia, and bears many associated cyrtocytes. The d escending canal is much thicker; it is provided with microvilli and ha rbors a constant population of intracellular bacteria, of yet uncertai n nature. Scanning electron microscopy was helpful to characterize the cyrtocytes, which are special filtering cells: these are about 10 mu m tall with a nucleated bulb, a flat weir chamber and a tubular stalk that inserts at the ascending canal through a pore (about 1 mu m diame ter). The tips of the two cilia forming the 'flame' protrude into this opening. The filtering surfaces are paired, flat rectangles, formed b y one outer row of transverse, and another internal row, of longitudin al microvilli, both covered by a thin glycocalyx.