What is the molluscan osphradium? A reconsideration of homology

The relationships among the living classes of molluscs have remained contentious, in part because of a lack of consistent morphological characters that unequivocally unite clusters of taxa within this extremely disparate phylum. The osphradium, a chemosensory organ on or near the gills, is a putative potential synapomorphy of Mollusca. Although the osphradium regularly appears on illustrations of hypothetical unifying molluscan bauplans, the homology of these putative sensory structures has been debated in most classes. Considered examination of the evidence for homology based on development, ultrastructure, neural architecture, and function, demonstrates a lack of support for inferred homology among the varied structures described as 'osphradia'. Neuroanatomical features are increasingly recognized as important for resolving deep phylogenetic divergences. The construction of morphological character sets for phylogenetic inference is dependent on robust determination of homology. Naming a structure does not underwrite its relationship to any other sensory organ. A fundamental assumption that one clade is 'primitive' may have biased the interpretation of anatomical results by early, and even some contemporary researchers, leaving the equality among important characters in doubt. Although these various sensory structures are no doubt real anatomical features, we question the assumption that there is a single molluscan osphradium and propose that at least two distinct classes of epithelial sensory structures have been identified as the molluscan osphradium. The complex suite of characters that describe the many varied sense organs in the mantle cavities of molluscs, may yet provide a foundation for neurophylogenetic insights to molluscan evolution.