The cavity within the head leads into a true buccal cavity situated within the body at the base of the foot.
At the point where the stomochord opens into the buccal cavity the nuchal skeleton bifurcates, and the two cornua thus produced pass obliquely backwards and downwards embedded in the wall of the throat, often giving rise to projecting ridges that bound a dorsal groove of the collar-gut which is in continuity with the wall of the stomochord (fig.
The buccal cavity contains a sense-organ on the ventral side called the sub-radular organ.
A gizzard is present in a few forms. The buccal cavity is sometimes armed with jaws.
This differentiation is not, however, peculiar to the Polychaetes; for in several Oligochaetes the anterior nephridia are of large size, and opening as they do into the buccal cavity clearly play a different function to those which follow.
V, Buccal cavity w, Gonad.