Enteric pouch of the umbrella, in the left-hand figure, points to the cavity uniting neighbouring pouches near the margin of the umbrella and giving origin to TCa, the tentacular canal.
At the base of the head dorsally are a pair of flat tentacular lobes from the edges of which the cephalic filaments or captacula arise.
The vestibule of the mouth is the space bounded by the oral hood; this arises by secondary downgrowth of lid-like folds over the true oral aperture, and is provided with a fringe of tentacular cirri, each of which is supported by a solid skeletal axis.
Thus in the recently discovered arctic genus Prosorhynchus the muscular and glandular extremity is protrusible, but in the allied Gasterostomum this organ is represented by a sucker with fimbriated or tentacular margins.
A pair of cerebral ganglia lie on the dorsal side of the oesophagus: they innervate the proboscis or head and its tentacular lobes and captacula.