Presumptive ciliated receptors associated with the fibrillar glands of the spines of the echinoderm Amphipholis squamata.

The ultrastructure of the cuticle, fibrillar glands and stäbchen (see Reichensperger 1908) of the arm spines of the brittle star Amphipholis squamata are described. Stäbchen are shown to be cuticular projections into which pass one or more uniciliate cytoplasmic processes. The proximal tips of the cilia protrude into the external environment to a variable extent. Evidence is presented for considering these ciliated endings to be the terminations of sensory neurites. Stäbchen are, in consequence, presumed to have a receptor function. Groups of stäbchen are spatially associated with fibrillar gland openings on the sides of spines while the bases of fibrillar gland cells are in contact with axons from the spine nerve which are laden with 45-270 nm diameter, electron-dense, membrane-bounded granules. The possible functional significance of these relationships is discussed in the context of a hypothesis in which fibrillar gland secretions are considered to act as food-catching mucus.