Distinct tube formed by thickened, chitinized cuticle of the penis or penial sac.

Limnodrilus hoffmeisteri.
L. hoffmeisteri 3 , penial sheaths.
L. claparedeanus 4 , photo of distal portion of penial sheath.
P. pusillus 4 , penial sheaths.

Alternative form for penial sheath : penial sheaths.