Several mollusks that depend on symbiotic microrganisms for nutrition have a reduced gut.  At least some species of Solemya, which are chemosymbiotic, have no gut.  Many photosymbiotic cockles have reduced guts; however, Tridacna maintains both a standard gut with filter feeding and a population of symbiotic dinoflagellates.  This might partially account for its large size in contrast to Fragum and Corculum, which seem entirely dependent on their symbionts.  I don't know if anyone has looked at the gut of apparently chemosybiotic gastropods.

    Dr. David Campbell
    "Old Seashells"
    Biology Department
    Saint Mary's College of Maryland
    18952 E. Fisher Road
    St. Mary's City, MD  20686-3001 USA
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"Mollusks murmured 'Morning!'.  And salmon chanted 'Evening!'."-Frank Muir, Oh My Word!