The Cephalic Sensory Organ in Pelagic and Intracapsular Larvae of the Primitive Opisthobranch Genus Haminoea (Mollusca: Gastropoda)

This study provides a detailed description of the cephalic sensory organ (CSO) of four species of the cephalaspid genus Haminoea (Opisthobranchia, Gastropoda), based on light and electron microscopical investigations of complete serially sectioned larvae. The organ, which is present in all species investigated, exhibits the characteristic elements of gastropod CSOs: Two ciliary tuft cells; four to six ampullary cells showing intracellular lumina filled with bundles of tightly packed cilia; three para-ampullary cells interspersed medially and positioned laterally; a modified thickened microvillar border. A ciliary tuft is present in the planktotrophic developer Haminoea cymbalum, H. exigua and the intracapsular developer H. navicula, while it is lacking in the poecilogonous developer H. callidegenita. The number of ampullary cells ranges from 4 ( H. cymbalum, H. exigua) to 6 ( H. navicula, H. callidegenita). In general no clear correlation of development mode (e.g. planktotrophic vs. intracapsular) and organization of the organ can be found. Most structures, except ciliary tufts and number of ampullary cells, are very similar among the investigated species. The CSO cells of Haminoea are compared with those of other gastropods in the literature. The organ as a whole as well as its cellular components of Haminoea can clearly be homologized with such structures in other gastropods. The CSO's relationship to apical organs with regard to homology and the function is discussed.