Synthesis of paucimannose N-glycans by Caenorhabditis elegans requires prior actions of UDP-N-acetyl-D-glucosamine : α-3-D-mannoside β 1 , 2N-acetylglucosaminyltransferase I , α 3 , 6-mannosidase II and a specific membrane-bound β-N-acetylglucosaminidase