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 (C SO) of four species of the cephalaspid genus Haminoea (Opisthobranchia, Gas tropoda), based on light and electron microscopical investigations of compl ete serially sectioned larvae. The organ, which is present in all species i nvestigated, exhibits the characteristic elements of gastropod CSOs: Two ci liary tuft cells; four to six ampullary cells showing intracellular lumina filled with bundles of tightly packed cilia, three para-ampullary cells int erspersed medially and positioned laterally; a modified thickened microvill ar border. A ciliary tuft is present in the planktotrophic developer Hamino ea cymbalum, H. exigua and the intracapsular developer H. navicula, while i t 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 fo und, Most structures, except ciliary tufts and number of ampullary cells, a re very similar among the investigated species. The CSO cells of Haminoea a re 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 homol ogized with such structures in other gastropods. The CSO's relationship to apical organs with regard to homology and the function is discussed.