WHY IS IT NOT A "SHELL"

[dave r]

Sorry, I thought I had explained. I will try again.
Just because a creature has a shell does not make it a Mollusc, in fact many Molluscs do not have shells. It is the LIVING ANIMAL INSIDE that is the important thing. Bivalve Molluscs such as Scallops have a left and right hand shell valve, Brachiopods have a top and bottom, there is one way in which they differ. Also Brachiopods are totally sessile, they live attached to the substrate whereas most Scallops are free living and capable of moving around at will.
As I have said it is the living creature that determines what is a Mollusc, not the shell. Also extinction has nothing whatsoever to do with it. Monoplacophora are a very primitive form of molluscs but their anatomy determines them as molluscs, nothing else. Incidentally they are far from extinct just very hard to obtain as they are so small and live in abyssal depths. I have seen some for sale a few years back and was very unimpressed with them, tiny, boring things, but they were still hundreds of dollars each!
I hope this explains things a bit clearer.
Dave