I dredge up from the depths that the three varieties of Schistosomes (mansoni, japonicum, haematobium) are differentiated in part by the flat disc-like end part, which is called the operculum. Never saw it used in any other context, or questioned where the word came from -- just another medical Latinism. "Pitcher-lid" is a very appropriate description.

(Schistosomes aren't very well-known in the US, but world-wide they cause a huge number of infections and are truly a public health problem. The life cycle is ridiculously intricate, involving as I recall snails to propogate one stage. In between, humans get "swimmer's itch" from bathing in schistosome-infested waters.) (Did I leave out anything material, Bill?)