Linked from
The 10 pages that link to Mollusk, each with the reason it gives.
AnimalBroader topic: Mollusks display striking variation in body form within one animal phylum.
InvertebrateBroader topic: Mollusks range from shelled grazers to highly capable predators.
ShellfishBroader topic: Clams, oysters, mussels, scallops, and many other shellfish are mollusks.
ProtostomeBroader topic: Mollusks demonstrate how different body plans evolved within Protostomia.