The cephalic sensory organ in veliger larvae of pulmonates (Gastropoda: Mollusca).

The apical area of larvae of four primitive pulmonate species was investigated by means of serial ultrathin and light microscope sections. Cephalic sensory organs (CSOs) were found in the larvae of Onchidium cf. branchiferum (Onchidiidae) and Laemodonta octanfracta (Ellobiidae), while no trace of the organ was present in the larvae of Ovatella myosotis (Ellobiidae) or Williamia radiata (Siphonariidae). TEM investigation revealed very similar CSOs in O. cf. branchiferum and L. octanfracta, with characteristic putative sensory cell types: ampullary cells with an internal ampulla containing densely packed cilia, para-ampullary cells with external cilia parallel to the surface, and ciliary tuft cells, bearing short ciliary tufts. The epithelium covering the organ has a thick microvillar border with microvilli laterally bearing a pair of electron-dense accumulations and a glycocalyx with interspersed flat plaque-like elements. While homologues of all major elements of the CSO can be found in other gastropod taxa, for example caenogastropods and opisthobranchs, the homology of the ampullary cell with similar cells in nongastropods appears unlikely. The CSO of L. octanfracta is associated with an additional structure, an epithelial external protrusion, lying ventral to the CSO. The absence of the organ in W. radiata weakens hypotheses on the organ's function of examining settlement conditions and velar control.