What is the difference between a squid and a cuttlefish?

Hey there,
From what I can recall, i can remember 2 differences.
1) Some cuttlefish have external shells, but squids have an internalized bone like structure called 'Pen'
2) Cuttlefish are a group of animals of the Class: Cephalapoda, while squids are a type of cuttlefish.

Hope this helps :))

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Idont understand i'm sorry