Sensory cell/gland cell complexes associated with the pallial tentacles of the bivalve <i>Lima Hians</i> (Gmelin), with a note on specialized cilia on the pallial curtains

Abstract

Abstract The numerous brilliant orange tentacles with their characteristically ruffed appearance borne by the middle folds of the mantle margins of Lima hians possess at least three types of ciliated sensory receptor. The most commonly occurring type consists of a dense tuft of 35-40 stiff, non-motile cilia borne by a cluster of 4-6 primary sensory neurons; it is thought to function as a multidirectional mechanoreceptor. Each cluster of sensory neurons is invariably associated with a pair of gland cells which release a sticky acrid substance when the tentacles are autotomized. Associated with these sensory cell/gland cell complexes in some, but not all, cases is a monociliary receptor and an additional unpaired gland cell; the function of the gland cell is unknown. The monociliary receptor consists of a single, long, stiff kinocilium the base of which is surrounded by a corolla of nine short, club shaped stereocilia. A complex basal apparatus connects the basal body underlying the kinocilium with the stereocilia. It is suggested that this second type of receptor functions as a vibration receptor. The third type of receptor consists of 17-20 non-motile cilia arranged in a circle and borne by a single sensory neuron which, in its distal region, envelopes a gland cell; the latter probably secretes mucus. The receptor is thought to serve as a chemo- and/or mechanoreceptor. Each of the epithelial cells covering the tentacles encloses a large, fluid filled vacuole with the cytoplasm and cell organelles restricted to a narrow zone underlying the plasma membrane. Each cell functions as a readily deformable constant volume unit capable of adjusting to the marked changes in form and volume which the tentacles may undergo. The distal regions of the outer faces of the enlarged inner folds (the pallial curtains) of the mantle margins bear dense patches of short (2 μm), stiff, non-motile cilia. The matrix of the apical regions of these cilia is electron dense and this matrix is interpreted as serving to reinforce and strengthen the apical region. The cilia are thought to function as an anti-slip device when the outer faces of the pallial curtains are opposed. It is also suggested that the electron dense material at the tips of the cilia may so bind the peripheral and central tubules in these regions that intrinsic movement of the cilia by the sliding of the peripheral tubules relative to one another is prevented, although limited passive bending of the cilia could still occur. A similar, but less extensive, electron dense matrix was observed at the tips of the stiff, non-motile, sensory cilia.