The fine morphology of the osphradial sense organs of the Mollusca. 1. Gastropoda, Prosobranchia

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Abstract
The comparative ultrastructure of osphradia is investigated in 51 species of prosobranch gastropods, representative of nearly all superfamilies as well as of various habits and environments. The essential results show that the sensory epithelium of the osphradium as a whole may reflect environmental conditions, whereas the ultrastructure of osphradial cell types reflects actual taxonomic relations. Accordingly, the following taxa can be differentiated on the basis of osphradial fine structure: (i) the validity of the recently established taxon Vetigastropoda (Pleurotomarioidea, Fissurelloidea, and Trochoidea) is confirmed; (ii) the Docoglossa appear as a very isolated group; (iii) the osphradia of the Neritopsina are similar to those of the Caenogastropoda with respect to the organization of the sensory epithelium, but differ in cell types; (iv) in Valvatoidea the osphradium is indifferently elaborated, and no close relationship to other groups can be stated; (v) the Viviparoidea possess a special type of osphradium distinct from that of other taenioglossan groups; (vi) Neotaenioglossa (= Mesogastropoda partly), Heteroglossa (Cerithiopsoidea, Triphoroidea, Epitonioidea, Eulimoidea), and Stenoglossa (= Neogastropoda) represent an (unnamed) monophyletic stock which is characterized by three special cell types with a constant mutual positional relationship within the osphradial epithelium. On the basis of its structure the function of the osphradium is suggested to be chemoreceptive (also in Archaeogastropoda).