Larval organs in Euthyneura (Mollucca)

Larvae play a central role in the phylogenetic discussion on relationships of marine invertebrates. Especially the structure of larval excretory organs (i.e. protonephridia) and apical sensory organs are used as arguments in these discussions. Despite the use of these characters in argumentation the knowledge is still incomplete. The application of electron microscopy helps to answer questions of homology on the cellular level. Therefore the protonephridia and the cephalic sensory organs of selected species (primitive opisthobranch gastropods and primitive pulmonates) have been investigated by transmission electron microscopy. The opisthobranch protonephridia are paired and consist each of one terminal cell which is provided with an ciliary flame and builds up the ultrafiltration apparatus. The nephrocyst as part of the protonephridium is provided with a duct bearing microvilli which surround the ciliary flame. The excretory duct, the cells of which are ciliated too, opens with a pore to the outside. There is no special pore cell, but the pore is built up by the surrounding epithelial cells of the lateral body wall. Known findings of nephrocysts in literature now can be interpreted as findings of protonephridia. This shows a common distribution of protonephridia within opistobranch gastropods. The comparative investigation of pulmonate and opisthobranch cephalic sensory organs showed three types of sensory cells: ciliary tuft cells, ampullary cells and para-ampullary cells. The ciliary tuft cells resemble the apical organ of other marine invertebrate larvae and is suggested to be a remnant of this structure. The ampullary cells are classified as a synapomorphic structure of higher gastropods reaching down to the level of caenogastropods .