WHY IS IT NOT A "SHELL"

[dave r]

Hi there, I will try to answer your question even though I am no expert.
A Brachiopod shell is still a shell, the same as a sea urchin test can be called a shell. Many creatures live in what is called a shell besides Molluscs; turtles, crabs, lobsters, barnacles all have a 'shell'. The creature inside the Brachiopod shell is not a mollusc, there are very many anatomical differences that separate them. Gastropods, Bivalves, Chitons, Squid etc. all share a lot of anatomical characters that place them in the Phylum Mollusca together.
I hope this has helped.
Cheers,
Dave