In all these cases the female duct bears a bursa copulatrix or receptaculum seminis.
It lodges the copulatory organ, and on its dorsal wall lies the bursa Fabricii, an organ peculiar to birds.
From this an equally slender tube proceeds, which joins its fellow of the opposite side, and the two form a thick, walled tube, which opens on to the exterior within the bursa copulatrix through which the penis protrudes.
There is some reason to believe that this complicated and variable apparatus is used for stabbing the body of another animal and that beginning as a weapon for catching prey it has become modified for hypodermic impregnation and only gradually adapted for insertion into the bursa copulatrix.
From this junction there proceeds an oviduct or " uterus " (paired or single) which before opening to the exterior expands to form a muscular protrusible pouch - the bursa copulatrix.