The rostellum of the stigma of violets, or of the operculum of many mosses.
(botany) A projecting part of the column in the flower of an orchid that separates the male stamen from the female gynoecium.
(biology) A retractable protruding part at the anterior end of a soft-bodied tapeworm; the scolex from which it protrudes is often armed with hooks which serve to keep it in place attached to the host.
This column stands up from the base of the flower, almost at right angles to the lip, and it bears at the top an anther, in the two hollow lobes of which are concealed the two pollen-masses, each with its caudicle terminating below in a roundish gland, concealed at first in the pouch-like rostellum at the front of the column.
These complex organs have apparently arisen by the increase in depth and differentiation of an accessory sucker such as is borne on the phyllidia of the former group. Lastly, the scolex of the more familiar Taeniidae (Tetracotylea) carries a rostellum encircled with hooks and four cup-shaped suckers the margins of which do not project beyond the surface of the body.
In one genus (Polypocephalus) the place of a rostellum is taken by a crown of retractile tentacles.
A peculiar modification of this type of scolex occurs in the Echinobothridae, in which the axial part of the organ (the rostellum) is elongated and provided with several rows of hooks, whilst the phyllidia have partially fused.