What type of freshwater organism has a two-part shell?

Mollusks are the correct classification for freshwater organisms that possess a two-part shell, commonly known as a bivalve shell. Bivalves are a group within the Mollusca phylum that includes clams, oysters, and mussels. These organisms have a shell composed of two hinged parts, which they can open and close. The two halves are generally made of calcium carbonate and serve both protection and support for the soft body of the organism inside.

In contrast, cnidarians such as jellyfish and corals are distinguished by their unique body structures and lack any type of shell. Arthropods, which include insects, crustaceans, and arachnids, have exoskeletons made of chitin rather than bivalve shells. Fish, being vertebrates, have a completely different anatomy, often featuring scales and fins but no shells at all. Hence, the defining characteristic of a two-part shell firmly places mollusks as the answer to the question.