The Scaphopoda are marine infaunal carnivores that feed on foraminiferans and other microorganisms selected and manipulated by their unique feeding tentacles or captacula. Their tusk-like shell is open at both ends; the burrowing foot and captacula protrude anteriorly, while respiratory currents pass through the posterior opening. Although the scaphopods comprise one of the smallest molluskan classes in terms of species diversity, they have a worldwide distribution ranging from intertidal to depths in excess of 6000 m. Despite detailed monographic work from the late nineteenth and early twentieth centuries, however, the biology of scaphopods is today among the least understood of mollusks. To some extent, this is related to a significant departure from more typical molluscan organization inferred from studies of the larger classes. For example, the mantle cavity is elongate and restricted in space, and several associated organs are lost, including the osphradium and ctenidia; the latter is associated with the loss of auricles from the scaphopod heart. There is also little record of ontogeny in the adult shell, as the older portion is periodically decollated to maintain the posterior aperture for passage of inhalant and exhalant currents. This and other constraints on scaphopod diversification are reflected in the range limits of shell shape within the class. In contrast, the "Dentalium" egg has been a model system for experimental embryology, particularly in studies of cell lineage and early morphogenesis. Later larval development and most organogenesis is nonetheless poorly known, although recent studies have considerably enhanced our knowledge in this area. Global biogeographic patterns of scaphopod diversity have been studied only preliminarily. A general decrease in diversity with depth is broken by small diversity peaks at bathyal depths. There is also evidence for a marked latitudinal diversity gradient in the world's oceans, near equatorial in the Pacific but at approximately 20 degrees N in the Atlantic. Scaphopods have a wide diversity of ecto- and endo-symbiotic associations with other organisms, including commensal bacteria and ciliates, mutualistic anemones and corals, and parasitic algae and platyhelminths. Other documented associations include predation by naticid gastropods and ratfish, and the inhabitation of empty scaphopod shells by a variety of sipunculans and hermit crabs. Phylogeny within the class is still unresolved, although significant progress has been made recently in documenting morphological variation among families, genera, and species for application in a cladistic context. The evolutionary relationships of Scaphopoda within Mollusca are farther from resolution, with every conchiferan class a proposed sister group among modern analyses. Molecular avenues should provide significant progress in scaphopod phylogenetics, as indicated by recent comparative sequence analysis and gene expression patterns. Scaphopods comprise the most recent class of mollusks to appear in the fossil record,...

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Reynolds, P. D. 1997. The phylogeny and classification of Scaphopoda (Mollusca): an assessment of current resolution and cladistic reanalysis.-Zool. Scr. 26; 13-2 1.The first cladistic analysis of phylogeny in the class Scaphopoda (Steiner 1992a(Steiner ,1996 examined relationships among family and selected sub-family taxa using morphological data. A preferred/ consensus tree of relationships illustrated monophyly of the orders Dentaliida and Gadilida, partial resolution among dentaliid families, and complete resolution among gadilid taxa. However, several alternative replications of the analysis, including use of a revised data matrix, did not produce the reported tree number or level of resolution; in all cases, monophyly of the Dentaliida was not supported by strict consensus of resultant parsimonious trees. Reanalysis, using unordered characters and outgroup rooting, only clearly resolves monophyly of the Gadilida and the sister relationship of the Entalinidae with the remaining gadilid families. These analyses emphasize the need for more comparative data and thorough parsimony analysis in scaphopod cladistic phylogenetics, as relationships in this class are still some way from resolution.

Phylogenetic relationships among families in the molluscan class Scaphopoda were analysed using morphological characters and cladistic parsimony methods. A maximum parsimony analysis of 34 discrete characters, treated as unordered and equally weighted, from nine ingroup terminal taxa produced a single most parsimonious tree; supplementary analyses of tree length frequency distribution and Bremer support indices indicate a strong phylogenetic signal from the data and moderate to minimally supported clades. The traditional major division of the class, the orders Dentaliida and Gadilida, is supported as both taxa are confirmed as monophyletic clades. Within the Dentaliida, two clades are recognized, the first comprised of the families Dentaliidae and Fustiariidae, the second of the Rhabdidae and Calliodentaliidae; together, these groups comprise a third clade, which has the Gadilinidae as sister. Within the Gadilida, a nested series of relationships is found among [Entalinidae,[Pulsellidae, wemersoniellidae, Gadilidae]]] . These results lend cladistic support to earlier hypotheses of shared common ancestry for some families, but are at variance with other previous hypotheses of evolution in the Scaphopoda. Furthermore, analysis of constituent Gadilinidae representatives provide evidence for paraphyly of this family. The relationships supported here'provide a working hypothesis that the development of new characters and greater breadth of taxonomic sampling can test, with a suggested primary goal of establishing monophyly at the family level. 0 1999 The Linnean Society of London ADDITIONAL

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