Either the movie itself or one of the guides to it claimed that the very unpleasant parasitoids in Star Trek 2 were mollusk-like. However, they looked more like arthropods to me, despite the excuse of being highly mutated.
I believe that there are also minor roles for man-eating Tridacna in old adventure movies, but I have not personally encountered such (neither the movies nor carnivorous cardiids).
Dr. David Campbell
Old Seashells
University of Alabama
Biodiversity & Systematics
Dept. Biological Sciences
Box 870345
Tuscaloosa, AL 35487-0345 USA
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That is Uncle Joe, taken in the masonic regalia of a Grand Exalted Periwinkle of the Mystic Order of Whelks-P.G. Wodehouse, Romance at Droitgate Spa